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LECTURE NOTES Course : PST 102; PLANT FORMS AND FUNCTIONS (2 Credits /Compulsory) Course Duration : 15hrs Teaching and 45hrs Practical Lecturer: BELLO, Omolaran Bashir Ph.D, M.Sc. (Ilorin), B.Sc. (Ibadan), OND (Computer Studies) Course: PST 204 Plant Morphology (2 Credits /Compulsory) Course Duration: 15hrs Teaching and 45hrs Practical E-mail : [email protected], obbello [email protected] [email protected] Office Location: Department of Biological Sciences Consultation Hours; 2.30-4.00 pm Monday-Thursdays. 1. FUNCTION OF CELLS AND CELLULAR ORGANELLES The Cell The cell is the unit of biological function or structures or structures and of reproduction. I A cell is also the basic and fundamental unit of life. All living things are composed of cells. Some microscopic organisms, such as bacteria and protozoa, are unicellular, meaning consist of a single cell. Plants, animals, and fungi are multicellular and composed of many cells working in concert. Cells carry out thousands of biochemical reactions each minute and reproduce new cells that perpetuate life. Reasons for Cell Study The chromosomes carrying the gene resides in the cell Mitosis & Meiosis which are primarily responsible for continuity occur within the cell It is the first level or stage in the organization of life. Plant Cell 1 Cells fall into one of two categories: Prokaryotic or Eukaryotic cells. Prokaryotic cell is found only in bacteria and archaebacteria. Eukaryotic cells contain numerous compartments, or organelles, within each cell are found in plants, animals, fungi, and all other life forms. Plants are multicellular eukaryote with the ability to photosynthesize. Structure of a plant leaf Plant Cell Organelles 2 A plant Cell (i) Chloroplasts An examination of leaves, stems, and other types of plant tissue reveals the presence of tiny green, spherical structures called chloroplasts, visible here in the cells of an onion root. Chloroplasts are essential to the process of photosynthesis, in which captured sunlight is combined with water and carbon dioxide in the presence of the chlorophyll molecule to produce oxygen and sugars that can be used by animals. Without the process of photosynthesis, the atmosphere would not contain enough oxygen to support animal life. (ii) Cell wall Cell wall is the most important feature distinguishing the cells of plants from those of animals. In plants this wall protects the cellular contents and limits cell size. It also has important structural and physiological roles in the life of the plant, being involved in transport, absorption, and secretion. A plant's cell wall is composed of several chemicals, of which cellulose (made up of molecules of the sugar glucose) is the most important. Cellulose molecules are united into fibrils, which form the structural framework of the wall. Other important constituents of many cell walls are lignins, which add rigidity, and waxes, such as cutin and suberin, which reduce water loss from cells. Many plant cells produce both a primary cell wall, while the cell is growing, and a 3 secondary cell wall, laid down inside the primary wall after growth has ceased. Plasmodesmata penetrate primary and secondary cell walls, providing pathways for transporting substances. (iii) Protoplast Within the cell wall are the living contents of the cell, called the protoplast. These contents are bounded by a cell membrane composed of a phospholipid bi-layer. The protoplast contains the cytoplasm, which in turn contains various membrane-bound organelles and vacuoles and the nucleus, which is the hereditary unit of the cell. (iv) Vacoules Vacuoles are membrane-bound cavities filled with cell sap, which is made up mostly of water containing various dissolved sugars, salts, and other chemicals. (iv) Plastids Plastids are types of organelles, structures that carry out specialized functions in the cell. Three kinds of plastids are important here. Chloroplasts contain chlorophylls and carotenoid pigments; they are the site of photosynthesis, the process in which light energy from the sun is fixed as chemical energy in the bonds of various carbon compounds. Leucoplasts, which contain no pigments, are involved in the synthesis of starch, oils, and proteins. Chromoplasts manufacture carotenoids. (vi) Mitochondria Whereas plastids are involved in various ways in storing energy, another class of organelles, the mitochondria, are the sites of cellular respiration. This process involves the transfer of chemical energy from carbon-containing compounds to adenosine triphosphate, or ATP, the chief energy source for cells. The transfer takes place in three stages: glycolysis (in which acids are produced from carbohydrates); the Krebs cycle, also called the citric acid cycle; and electron transfer. Like plastids, mitochondria are bounded by two membranes, of which the inner one is extensively folded; the folds serve as the surfaces on which the respiratory reactions take place. (vii) Ribosomes Two other important cellular contents are the ribosomes, the sites at which amino acids are linked together to form proteins, (viii) Golgi Apparatus The Golgi apparatus, which plays a role in the secretion of materials from cells. (ix) Endoplasmic Reticulum Endoplasmic reticulum is a complex membrane system that runs through much of the cytoplasm and appears to function as a communication system; various kinds of cellular substances are channeled through it from place to place. Ribosomes are often connected to the endoplasmic reticulum, which is continuous with the double membrane surrounding the nucleus of the cell. (x) Nucleus 4 The nucleus controls the ongoing functions of the cell by specifying which proteins are produced. It also stores and passes on genetic information to future generations of cells during cell division. Nucleus and its components 5 A typical Animal cell Differences between plant and animal cell S/N Features Plant cells 1. Size Plant cells are relatively large in size Animal cells Animal cells are smaller in size compared to plant cells 2. Cell Wall Present in Plant cell Absent in animal cell 3. Chloroplast Present in Plant cell Absent in animal cell 4. Lysosomes Reduced number of lysosomes in plant cells Large number of lysosomes in animal cell 5. Centrosome Centrosome is absent in plant cells Centrosome is present in animal cells 6. Plastids Present in plant cells Absent in animal cells 6 7. Vacuoles Vacuoles are more conspicuous Vacuoles are less conspicuous 8. Phagocytosis Plant cells cannot be phagocytic Animal cells can be Phagocytic 9. Centrioles Cells of higher plants lack centrioles Centrioles are present 10. Plasmodesmata Plasmodesmata is present Plasmodesmata is absent 2. CELL DIVISION Cell division is the process by which a parent cell divides into two or more daughter cells. In Eukaryotes, there are two distinct type of cell division: a vegetative division, whereby each daughter cell is genetically identical to the parent cell (mitosis), and a reductive cell division, whereby the number of chromosomes in the daughter cells is reduced by half, to produce haploid gametes (meiosis). Most cells divide at some time during their lives. Organisms rely on cell division for reproduction, growth, and repair and replacement of damaged or worn-out cells. There are hormones in an organism’s body that sends signals to the cells to prepare for division when it is needed. 7 Cell Division Cells are classified into two categories: simple, non-nucleated prokaryotic cells, and complex, nucleated eukaryotic cells. By dint of their structural differences, eukaryotic and prokaryotic cells do not divide in the same way. Also, the pattern of cell division that transforms eukaryotic stem cells into gametes (sperm cells in males or ova – egg cells – in females) is different from that of the somatic cell division in the cells of the body. The primary concern of cell division is the maintenance of the original cell's genome. Before division can occur, the genomic information that is stored in chromosomes must be replicated, and the duplicated genome must be separated cleanly between cells. A great deal of cellular infrastructure is involved in keeping genomic information consistent between "generations". 1. MITOSIS Mitosis is the portion of the cell cycle when the cells nucleus is replicated and divided into two identical nuclei containing genetically identical material. Mitosis results in two cells that are genetically identical, a necessary condition for the normal functioning of virtually all cells. Mitosis is vital for growth; for repair and replacement of damaged or worn out cells; and for asexual reproduction, or reproduction without eggs and sperm. Mitosis forms somatic cells, which are also referred to as the cells of the body (having a 2 n or diploid number of chromosomes). This is the type of cell division all of the cells in the body do except for those responsible for “sex cell” production. Mitosis maintains the cell's original ploidy level (for example, one diploid 2n cell producing two diploid 2n cells; one haploid n cell producing two haploid n cells; etc.). There are four (4) main stages in mitosis: i. Prophase This is the first stage of mitosis. In this stage the sister chromatids also condense to a visible form. The DNA is arranged and condensed into chromosomes. The nuclear envelope and break up, exposing the chromosomes. The spindle fibers begin to form the centrioles. You can see how the DNA is arranged and condensed into chromosomes in the picture. Prophase stage 8 The nuclear envelope also breaks up, exposing the chromosomes. The spindle fibers begin to form, extending from the centrioles. These are made up of microtubules and attach to the centromere of the sister chromatids. The centrioles slowly migrate to opposite sides of the cell. ii. Metaphase This is when the chromosomes are lined up along the metaphase or equatorial plate, an imaginary line in the center of the cell. The chromosomes are moved here with the help of the spindle fibers and the centrioles. iii. Anaphase The centromere of each chromosome are pulled apart by the spindle fibers, causing the sister chromatids to separate, creating two daughter chromosomes. One of the daughter chromosomes is pulled to one side of the cell, while the other is pulled to the opposite pole. This process is critical, because it ensures that the soon to be daughter cells will each have full, identical sets of chromosomes, also being identical to the parent cell. iv. Telophase The new nuclei begin to form around the new sets of chromosomes, at each end of the cell. The chromosomes also begin to unravel, back into their loose form. By the end of this phase, the spindle fibers are also disassembled. At the same time, cytokinesis begins, and the cell is pinched” into two new cells. As mitosis comes to an end, the two new nuclei must end up in two new cells; this is where cytokinesis comes in. 9 Cytokinesis is when the cell’s cytoplasm divides into two, making two new cells called daughter cells. Each of these new cells receives one of the new nuclei, making the daughter cells genetically identical to the parent cell. Animal cells and plant cells complete mitosis and cytokinesis differently. Significance of Mitosis (i) Genetic stability: Mitosis produces two nuclei which have the same number of chromosomes as the parent cell. Daughter cells are genetically identical to the parent cell and no variation in genetic information can therefore be introduced during mitosis. (ii) Growth: The number of cells within an organism increases by mitosis and this is the basis of growth in multicellular organisms. (iii) Cell replacement: Replacement of cells and tissues also involve mitosis. Cells are constantly dying and being replaced, an obvious example being in the skin. (iv) Regeneration: Some animals are able to regenerate whole parts of the body such as legs in crustacean and arm in starfish. Production of new cells involves mitosis. (v) Asexual reproduction: Mitosis is the basis of asexual reproduction, the production of new individuals of a species by one parent organism. Many organisms undergo asexual reproduction. Differences between mitosis in animal and plant cells Plant Cell Animal Cell 10 No centriole present. Centrioles present. No asters form. Aster form. Cell division involves formation of cell plate. Cell division involves furrowing and cleavages of cytoplasm. Occurs mainly at meristems. Occurs in tissues throughout the body. MEIOSIS Meiosis is the formation of gametes, performed by reproductive cells only. This will result in a reduction of the chromosome number, forming haploid cells (n). So, unlike mitosis, which produces two daughter cells with identical chromosomes, meiosis produces 4 daughter cells, each with half the number of chromosomes that are not identical to each other. In a process called meiosis, the homologous pairs of chromosomes are separated and end up in 4 daughter cells. This happens in two main states, Meiosis I and Meiosis II. The pairs separate during meiosis I, and meiosis II is when the sister chromatids separate, much like in mitosis. Meiosis I Meiosis I is a 4 step process. The start of this is much like mitosis, the cells have gone through the G1, S and G2 phases, and thus the DNA has been replicated and is in the form of sister chromatids, connected at the centromere. The main difference here is that the homologous pairs of chromosomes pair up and then proceed as follows: Prophase I It is the longest phase of meiosis. During prophase I, DNA is exchanged between homologous chromosomes in a process called homologous recombination. This often results in chromosomal crossover. The new combinations of DNA created during crossover are a significant source of genetic variation, and may result in beneficial new combinations of alleles. The paired and replicated chromosomes are called bivalents or tetrads, which have two chromosomes and four chromatids, with one chromosome coming from each parent. The process of pairing the homologous chromosomes is called synapsis. At this stage, non-sister chromatids may cross-over at points called chiasmata (plural; singular chiasma).The prophase I has six (6) sub-stages: 11 (a).. Leptotene: The first stage of prophase I is the leptotene stage, also known as leptonema, from Greek words meaning "thin threads". In this stage of prophase I, individual chromosomes—each consisting of two sister chromatids—change from the diffuse state they exist in during the cell's period of growth and gene expression, and condense into visible strands within the nucleus. However the two sister chromatids are still so tightly bound that they are indistinguishable from one another. During leptotene, lateral elements of the synaptonemal complex assemble. Leptotene is of very short duration and progressive condensation and coiling of chromosome fibers takes place. Leptotene (b) Zygotene: The Zygotene stage, also known as zygonema, from Greek words meaning "paired threads" occurs as the chromosomes approximately line up with each other into homologous chromosome pairs. This is called the bouquet stage because of the way the telomeres cluster at one end of the nucleus. At this stage, the synapsis (pairing/coming together) of homologous chromosomes takes place, facilitated by assembly of central element of the synaptonemal complex. Pairing is brought about in a zipper-like fashion and may start at the centromere (procentric), at the chromosome ends (proterminal), or at any other portion (intermediate). Individuals of a pair are equal in length and in position of the centromere. Thus pairing is highly specific and exact. The paired chromosomes are called bivalent or tetrad chromosomes. 12 Zygotene stage (a) Pachytene: The pachytene stage, also known as pachynema, from Greek words meaning "thick threads” is the stage when chromosomal crossover (crossing over) occurs. Non-sister chromatids of homologous chromosomes may exchange segments over regions of homology. Sex chromosomes, however, are not wholly identical, and only exchange information over a small region of homology. At the sites where exchange happens, chiasmata form. The exchange of information between the non-sister chromatids results in a recombination of information; each chromosome has the complete set of information it had before, and there are no gaps formed as a result of the process. Because the chromosomes cannot be distinguished in the synaptonemal complex, the actual act of crossing over is not perceivable through the microscope, and chiasmata are not visible until the next stage. 13 Pachytene stage Diplotene: During the Diplotene stage, also known as diplonema, from Greek words meaning "two threads" the synaptonemal complex degrades and homologous chromosomes separate from one another a little. The chromosomes themselves uncoil a bit, allowing some transcription of DNA. However, the homologous chromosomes of each bivalent remain tightly bound at chiasmata, the regions where crossing-over occurred. The chiasmata remain on the chromosomes until they are severed in anaphase I. In human fetal oogenesis all developing oocytes develop to this stage and stop before birth. This suspended state is referred to as the dictyotene stage and remains so until puberty. Diplotene Stage Diakinesis: Chromosomes condense further during the Diakinesis stage, from Greek words meaning "moving through". This is the first point in meiosis where the four parts of the tetrads are actually visible. Sites of crossing over entangle together, effectively overlapping, making 14 chiasmata clearly visible. Other than this observation, the rest of the stage closely resembles prometaphase of mitosis; the nucleoli disappear, the nuclear membrane disintegrates into vesicles, and the meiotic spindle begins to form . Diakinesis Metaphase I This is where the homologous pairs are lined up next to each other, along the equatorial plate. Homologous pairs move together along the metaphase plate: As kinetochore microtubules from both centrioles attach to their respective kinetochore, the homologous chromosomes align along an equatorial plane that bisects the spindle, due to continuous counterbalancing forces exerted on the bivalents by the microtubules emanating from the two kinetochore of homologous chromosomes. The physical basis of the independent assortment of chromosomes is the random orientation of each bivalent along the metaphase plate, with respect to the orientation of the other bivalents along the same equatorial line. Anaphase I The homologous pairs are now separated, due to the spindle fibers pulling them apart, from the centromere. Each chromosome still has two sister chromatids. Kinetochore (bipolar spindles) microtubules shorten, severing the recombination nodules and pulling homologous chromosomes apart. Since each chromosome has only one functional unit of a pair of kinetochore, whole chromosomes are pulled toward opposing poles, forming two haploid sets. Each chromosome still contains a pair of sister chromatids. During this time disjunction occurs, which is one of the processes leading to genetic diversity as each chromosome can end up in either of the daughter cells. Non kinetochore microtubules lengthen, pushing the centrioles farther apart. The cell elongates in preparation for division down the center. Telophase I The nuclear membrane may or may not reform, depending on the species, but in any case, cytokinesis does occur, resulting in two new cells, each with the haploid number of chromosomes, which are still in the form of sister chromatids. The first meiotic division effectively ends when the chromosomes arrive at the poles. Each daughter cell now has half the number of chromosomes but each chromosome consists of a pair of chromatids. The microtubules that make up the spindle network disappear, and a new nuclear membrane 15 surrounds each haploid set. The chromosomes uncoil back into chromatin. Cytokinesis, the pinching of the cell membrane in animal cells or the formation of the cell wall in plant cells, occurs, completing the creation of two daughter cells. Sister chromatids remain attached during telophase I. TELOPHASE I The Stages of Meiosis I Meiosis II In the second meiotic division the cell moves directly into prophase II, skipping the interphase replication of DNA. This is what follows after telophase I and cytokinesis. The daughter cells from meiosis I are what go into this phase. They divide again, but this time occurring much the same as mitosis. The only difference is that there are n number of chromosomes rather than 2n, and we end up with a total of4 daughter cells rather than2.Again, since there were two daughter cells produced in meiosis I, and each of them divide again, the result of meiosis is 4 daughter cells, each with n number of chromosomes, which are not identical to the original parent cell or each other. This is done through 4 more steps: prophase II, metaphase II, anaphase II and telophase II, followed by the final cytokinesis. 16 Prophase II During Prophase II, nuclear envelopes (if they formed during Telophase I) dissolve, and spindle fibers reform. All else is as in Prophase of mitosis. In prophase II we see the disappearance of the nucleoli and the nuclear envelope again as well as the shortening and thickening of the chromatids. Centrioles move to the Polar Regions and arrange spindle fibers for the second meiotic division. Indeed Meiosis II is very similar to mitosis. The Prophase II Stage Metaphase II In metaphase II, the centromeres contain two kinetochore that attach to spindle fibers from the centrosomes (centrioles) at each pole. The new equatorial metaphase plate is rotated by 90 degrees when compared to meiosis I, perpendicular to the previous plate. Anaphase II During Anaphase II, the centromeres split and the former chromatids (now chromosomes) are segregated into opposite sides of the cell. The centromeres are cleaved, allowing microtubules attached to the kinetochore to pull the sister chromatids apart. The sister chromatids by convention are now called sister chromosomes as they move toward opposing poles. The Metaphase II and Anaphase II stage Telophase II 17 Telophase II is identical to telophase I. Cytokinesis separates the cells. Telophase II completes meiosis. The spindle fibers disappear and a new nuclear membrane forms around each new group of chromosomes to form four haploid cells. The Telophase II stage DIAGRAMMATICAL SUMMARY OF THE STAGES IN MEIOSIS II 18 Significance of Meiosis (i).Sexual Reproduction: Meiosis occurs in all organisms carrying out sexual reproduction. During fertilization the nuclei of the gamete cells fuse. Each gamete has one set of chromosomes (is haploid n). The product of fusion is a zygote which has two sets of chromosomes 9the haploid condition 2n).If meiosis does not occur fusion in gametes would result in the doubling of the chromosomes foe each successive sexually reproduced generation. (ii) Genetic Variation: meiosis also produces opportunities foe new combinations of genes to occur in the gametes. This leads to genetic variation in the offspring produced by fusion of the gametes. Comparison of mitosis and meiosis Mitosis 1. It occurs in the vegetative or somatic cells. 2. After this division two daughter cells are produced. 3. Daughter cells process the same chromosome number as that of parent cell. 4. It is of short duration. 5. Daughter cells are similar to parent cell genetically. 6. There is no crossing over in mitosis. 7. With the splitting of centromere, chromatids are pulled apart towards the respective poles. Each chromatid behaves as an independent chromosome. 8. This is an equational division. 9. Homologous chromosomes are not arranged in pairs in the equatorial plate. 10. Genetic variation does not occur between daughter cells. 11. DNA synthesis is completed in the interphase. 12. In metaphase, the centromeres are lined up on the equatorial plane and the arms extend into the cytoplasm. 13. Mitosis maintains ploidy level. Meiosis 1. It occurs in the reproductive cells. The testes in males and the ovaries in females, in males, each of the meiotic divisions result in four equally sized haploid cells that mature into functional sperm cells. In females, the meiotic divisions are uneven, resulting in three tiny cells called polar bodies and one large egg that can be fertilized. 2. Four daughter cells are produced after meiotic division. 3. Chromosome number of daughter cells is reduced to half. 4. It is of long duration. The complete process is divided in to two divisions. 5. Daughter cells have genetic differences from the parent cell. 6. Crossing over takes place between the non-sister chromatids. 7. Whole chromosome moves apart in anaphase I of meiosis because there is no splitting of centromere. 8. In meiosis, the first division is reductional and the second one is equational. 9. Homologous chromosomes are arranged in pairs in the equatorial plate. 19 10. Exchange between maternal and paternal chromosomal segments does not render the daughter cell identical. 11. DNA synthesis is not completed in the interphase. 12. In metaphase I, the centromeres of the homologous chromosomes lie towards the two opposite poles and their arms extend towards the equator. 13. Meiosis reduce ploidy level Comparison of Mitosis and Meiosis Stages Stages Mitosis Prophase Homologous chromosomes remain separate. No formation of chiasmata. No crossing over. Meiosis Homologous chromosomes pair up. Chiasmata form. Crossing over may occur. Metaphase Pairs of chromatids line up on the equator of the spindle. Pairs of chromosomes line up on the equator. Anaphase Centromeres divide. Chromatids separate. Centromeres do not divide. Whole chromosomes separate. Separating chromosomes and their chromatids may not be identical due to crossing over. Separating chromatids are identical. Telophase Same numbers of chromosomes present in daughter cells as parents cells. Both homologous chromosomes present in daughter cells if diploid. Half the number of chromosomes present in daughter cells. Only one of each pair of homologous chromosomes present in daughter cell. 3. HEREDITY Heredity is the passing of traits to offspring (from its parent or ancestors). This is the process by which an offspring cell or organism acquires or becomes predisposed to the characteristics of its parent cell or organism. Through heredity, variations exhibited by individual can accumulate and cause some species to evolve. The study of heredity in biology is called genetics. Types of Allele There are mainly two types of allele namely; i. Dominant 20 This is the relationship between alleles of a gene, in which the phenotypic effect of one allele masks the phenotypic effect another allele at the same locus Intermediate (also called "codominant"). Codominant – a phenomenon in which a single gene has more than one dominant allele. ii. Recessive A recessive gene is an allele that causes a phenotype(visible or detectable characteristics) that is only seen in a homozygous genotype(an organism that has two copies of the same allele) and never in a heterozygous genotype. 4. NUTRITION IN PLANTS Nutrition can be defined as the process by which an organism obtains food which is used to provide energy and materials for its life sustaining activities. Thus the term nutrition includes the means by which an organism obtains its food and also the process by which the nutrients in the food are broken down to simpler molecules for utilization by the body. Modes of nutrition The chemical substances that provide nourishment to living organisms are called nutrients. Depending on the mode of nutrition the organism are classified as autotrophs and heterotrophs Autotrophs Organisms which utilize carbon dioxide as their sole source of carbon for the formation of organic food by the process of photosynthesis are called autotrophs. (Self nourishing). In addition to carbon dioxide, autotrophs require water and several in organic ions. If the autotrophs prepare their own food by utilizing chemical energy they are called Chemoautotrophs. Photosynthesis A green plant differs from other living things in their mode of nutrition, because of their ability to utilize light energy in order to manufacture their food. This is accomplished through a process called photosynthesis. This ability to manufacture food is important, not only to plant but to animals which depends directly or indirectly on plants for food. Photosynthesis therefore is the process whereby green plants manufacture feeding materials from carbon dioxide and water using light energy and chlorophyll , this can represented by the following equation. Sunlight 6Co2 + 6H20 → C6H12O6 + 6O2 Chlorophyll (Glucose) (Oxygen) From the chemical equation above water combines with carbon dioxide in the presence of sunlight within the chlorophyll of leaves to manufacture food (Glucose) and oxygen is librated as by product. Photosynthesis takes place in the chlorophyll alls of leaves and stem and other green part of plants. The cells have chloroplast containing chlorophyll chloroplast are present in guard cells of stomata, green stems, green floral plants (e.g sepals, petals, stigma) and developing fruits. The main photosynthetic organs are the leaves and the main photosynthesing material 21 required for photosynthesis are water from the soil and carbon dioxide from the atmosphere. These are conveyed to the chlorophyllous cells where high energy containing compounds like carbonhydrate and oxygen. The carbohydrate is distributed to all part of the plant while the oxygen I given off through the stomata into the atmosphere. Chemistry of Photosynthesis Photosynthesis occurs in two stages namely • Light reaction 2. Dark reaction. • Light Reaction This occurs during the day or in the presence of sunlight. The light energy is captured by the chlorophyll and electrons are emitted (released). The energy so trap is used to split water molecules into hydrogen ion (H+) and hydroxyl ion (OH), this splitting of water into hydrogen ion (H+) and hydroxyion (OH-) is called photolysis of water. Sunlight Chlorophyll 2H20 ---------------→ 2OH + 2H+ The importance of this stage is to transfer the light energy to chemical energy of ALP (Adenosine triphosphate) and make reduce NADP (Nicotinamide Adenosine de nucleon tide phosphate. In the dark, this energy will be converted to chemical energy in organic compounds. Photolysis of water can also be represented as. 4H2O Water light → Chlorophyll 4H+ + hydrogen ion 4OH hydroxyl ion The hydrogen ion is converted to water 4OH – 2H2O + O2 during this process oxygen is given out as by produce. A compounds co. enzymes or NADP is reduced by hydrogen ion to NADP and ATP is formed. Dark Reaction: This occurs at night or in the absence of light together with the energy provided by the ATP, the reduced compound NADP leads to the breakdown of carbon dioxide. This is controlled by specific enzymes whereby 3 carbon compound (CH2O) or sugar is formed. 4H+ + Co2 Enzyme → CH2 O + H20. CH2o is the carbon structure from which simple sugar fat & oil, protein etc are formed during the dark reaction. 22 Materials and Conditions Necessary for Photosynthesis (i) Carbon dioxide: Carbon dioxide from the atmosphere diffuses into intercellular spaces through the stomata of leaves then into the Mesophyii all containing chloroplasts. (ii) Water and Mineral Salt: these are derived from the soil, they pass into the root of plant through the root hairs by a process called OSMOSIS, water and mineral salts are conducted by Xylem through the stem and finally to the Mesophyii cells containing chloroplants. (iii) Sunlight: is obtained from solar energy. This is trapped by Chlorophyii of the leaves to split water molecules into hydrogen ions and hydro xylem ion (photolysis). (iv) Optinum temperature: temperature is derived from solar energy and from chemical reaction within the leaves during which heat is generalized temperature ratume is essential for the proper functioning of enzymes during photosynthesis. (v) Chlorophyll: this is the green pigment found in the spongy Mesophyii of leaves. This is where food is help to trappe solar energy converting it to chemical energy. (vi) Enzymes: this is also an internal condition needed for photosynthesis. It acts as a catalyst that ensure that reaction is completed. Uses of Photosynthesis The main produce formed during photosynthesis is simple sugar notably Glucose. The simple sugar formed is partly used by the plant and excess of heat. It is converted to starch immediately for storage the starch is then transported to other part of the plant through the phloem vessels. This process is known as Translocation. Importance of Photosynthesis • Production of food • Purification of the atmosphere • Release of oxygen into the environments oxygen needed for respiration is release in the environment during photosynthesis. • Building block for other substances such as protein, fat and oil. Nutrient Elements • The primary macronutrients: nitrogen (N), phosphorus (P), potassium (K) • The three secondary macronutrients: calcium (Ca), sulphur (S), magnesium (Mg) • The micronutrients/trace minerals: Silicon (Si), boron (B), chlorine (Cl), manganese (Mn), iron (Fe), zinc (Zn), copper (Cu), molybdenum (Mo), nickel (Ni), selenium (Se), and sodium (Na) Three fundamental ways plants uptake nutrients through the root: (1) Simple Diffusion, occurs when a nonpolar molecule, such as O2, CO2, and NH3 that follow a concentration gradient, can passively move through the lipid bilayer membrane without the use of transport proteins. (2) Facilitated Diffusion, is the rapid movement of solutes or ions following a concentration gradient, facilitated by transport proteins. 23 (3) Active transport, is the active transport of ions or molecules against a concentration gradient that requires an energy source, usually ATP, to pump the ions or molecules through the membrane. Functions of Different Nutrients a. Macronutrients Nitrogen is a major component of proteins, hormones, chlorophyll, vitamins and enzymes essential for plant life. Nitrogen metabolism is a major factor in stem and leaf growth (vegetative growth). Too much can delay flowering and fruiting. Deficiencies can reduce yields, cause yellowing of the leaves and stunt growth. Phosphorus is necessary for seed germination, photosynthesis, protein formation and almost all aspects of growth and metabolism in plants. It is essential for flower and fruit formation. Low pH (<4) results in phosphate being chemically locked up in organic soils. Deficiency symptoms are purple stems and leaves; maturity and growth are retarded. Yields of fruit and flowers are poor. Premature drop of fruits and flowers may often occur. Phosphorus must be applied close to the plant's roots in order for the plant to utilize it. Large applications of phosphorus without adequate levels of zinc can cause a zinc deficiency. Potassium is necessary for formation of sugars, starches, carbohydrates, protein synthesis and cell division in roots and other parts of the plant. It helps to adjust water balance, improves stem rigidity and cold hardiness, enhances flavor and color on fruit and vegetable crops, increases the oil content of fruits and is important for leafy crops. Deficiencies result in low yields, mottled, spotted or curled leaves, scorched or burned look to leaves. Sulfur is a structural component of amino acids, proteins, vitamins and enzymes and is essential to produce chlorophyll. It imparts flavor to many vegetables. Deficiencies show as light green leaves. Sulfur is readily lost by leaching from soils and should be applied with a nutrient formula. Some water supplies may contain Sulfur. Magnesium is a critical structural component of the chlorophyll molecule and is necessary for functioning of plant enzymes to produce carbohydrates, sugars and fats. It is used for fruit and nut formation and essential for germination of seeds. Deficient plants appear chlorotic, show yellowing between veins of older leaves; leaves may droop. Magnesium is leached by watering and must be supplied when feeding. It can be applied as a foliar spray to correct deficiencies. Calcium activates enzymes, is a structural component of cell walls, influences water movement in cells and is necessary for cell growth and division. Some plants must have calcium to take up nitrogen and other minerals. Calcium is easily leached. Calcium, once deposited in plant tissue, is immobile (non-translocatable) so there must be a constant supply for growth. Deficiency causes stunting of new growth in stems, flowers and roots. Symptoms range from distorted new growth to black spots on leaves and fruit. Yellow leaf margins may also appear. 24 b. Micronutrients Iron is necessary for many enzyme functions and as a catalyst for the synthesis of chlorophyll. It is essential for the young growing parts of plants. Deficiencies are pale leaf color of young leaves followed by yellowing of leaves and large veins. Iron is lost by leaching and is held in the lower portions of the soil structure. Under conditions of high pH (alkaline) iron is rendered unavailable to plants. When soils are alkaline, iron may be abundant but unavailable. Applications of an acid nutrient formula containing iron chelates, held in soluble form, should correct the problem. Manganese is involved in enzyme activity for photosynthesis, respiration, and nitrogen metabolism. Deficiency in young leaves may show a network of green veins on a light green background similar to an iron deficiency. In the advanced stages the light green parts become white, and leaves are shed. Brownish, black, or grayish spots may appear next to the veins. In neutral or alkaline soils plants often show deficiency symptoms. In highly acid soils, manganese may be available to the extent that it results in toxicity. Boron is necessary for cell wall formation, membrane integrity, calcium uptake and may aid in the translocation of sugars. Boron affects at least 16 functions in plants. These functions include flowering, pollen germination, fruiting, cell division, water relationships and the movement of hormones. Boron must be available throughout the life of the plant. It is not translocated and is easily leached from soils. Deficiencies kill terminal buds leaving a rosette effect on the plant. Leaves are thick, curled and brittle. Fruits, tubers and roots are discolored, cracked and flecked with brown spots. Zinc is a component of enzymes or a functional cofactor of a large number of enzymes including auxins (plant growth hormones). It is essential to carbohydrate metabolism, protein synthesis and internodal elongation (stem growth). Deficient plants have mottled leaves with irregular chlorotic areas. Zinc deficiency leads to iron deficiency causing similar symptoms. Deficiency occurs on eroded soils and is least available at a pH range of 5.5 - 7.0. Lowering the pH can render zinc more available to the point of toxicity. Copper is concentrated in roots of plants and plays a part in nitrogen metabolism. It is a component of several enzymes and may be part of the enzyme systems that use carbohydrates and proteins. Deficiencies cause die back of the shoot tips, and terminal leaves develop brown spots. Copper is bound tightly in organic matter and may be deficient in highly organic soils. It is not readily lost from soil but may often be unavailable. Too much copper can cause toxicity. Molybdenum is a structural component of the enzyme that reduces nitrates to ammonia. Without it, the synthesis of proteins is blocked and plant growth ceases. Root nodule (nitrogen fixing) bacteria also require it. Seeds may not form completely, and nitrogen deficiency may occur if plants are lacking molybdenum. Deficiency signs are pale green leaves with rolled or cupped margins. Chlorine is involved in osmosis (movement of water or solutes in cells), the ionic balance necessary for plants to take up mineral elements and in photosynthesis. Deficiency symptoms 25 include wilting, stubby roots, chlorosis (yellowing) and bronzing. Odors in some plants may be decreased. Chloride, the ionic form of chlorine used by plants, is usually found in soluble forms and is lost by leaching. Some plants may show signs of toxicity if levels are too high. Nickel has just recently won the status as an essential trace element for plants according to the Agricultural Research Service Plant, Soil and Nutrition Laboratory in Ithaca, NY. It is required for the enzyme urease to break down urea to liberate the nitrogen into a usable form for plants. Nickel is required for iron absorption. Seeds need nickel in order to germinate. Plants grown without additional nickel will gradually reach a deficient level at about the time they mature and begin reproductive growth. If nickel is deficient plants may fail to produce viable seeds. Sodium is involved in osmotic (water movement) and ionic balance in plants. Cobalt is required for nitrogen fixation in legumes and in root nodules of non-legumes. The demand for cobalt is much higher for nitrogen fixation than for ammonium nutrition. Deficient levels could result in nitrogen deficiency symptoms. Silicon is found as a component of cell walls. Plants with supplies of soluble silicon produce stronger, tougher cell walls making them a mechanical barrier to piercing and sucking insects. This significantly enhances plant heat and drought tolerance. Foliar sprays of silicon have also shown benefits reducing populations of aphids on field crops. Tests have also found that silicon can be deposited by the plants at the site of infection by fungus to combat the penetration of the cell walls by the attacking fungus. Improved leaf erectness, stem strength and prevention or depression of iron and manganese toxicity, have all been noted as effects from silicon. Essential Elements in Plants Element Form available Macronutrients Carbon CO2 Major functions Major components of plant, organic components Oxygen O2 Major components of plants organic compound Hydrogen H2O Major components of plants organic compound. Nitrogen NO-3 or NH+4 Component of nucleic acids, proteins, hormone chlorophyll, co enzymes. Potassium K+ Cofactors that function in protein photosynthesis. major solute functioning in water balance operation of stomata. Calcium Ca2+ Important in formation of stability of the cell walls and in the maintenance of a membrane 26 structure and permeability activates some enzymes regulate many response of cell to stimuli. Magnesium Mg2+ Phosphorus H2PO-4,HPO2-4 Components of nucleic acid . Phosphorus ATP, Several coenzymes. Sulphur SO42- Components of protein and co enzymes. Micronutrients Chlorine Cl- Require for water splitting step of photosynthesis functions in water balances. Iron Fe2+, Fe2+ Components of cytochromes, activates some enzymes. Manganese Mn2+ Active in formation of amino acids. Activate some enzymes : required for water splitting step of photosynthesis. Boron H2BO3 Cofactor in chlorophyll synthesis, may be involved in carbohydrate transport and nucleic acid synthesis; role in cell wall formation Zinc Zn2+ Active in formation of chlorophyll , activate some enzymes Copper Cu+ or Cu2+ Component of any redox and lignin biosynthetic enzymes Nickel Ni2+ Cofactor for an enzyme functioning in nitrogen metabolism Molybdenum MO2- Essential for symbiotic relationship with nitrogen fixing bacteria ; cofactor in nitrate reduction Components of chlorophyll: activate many enzymes. 27 5. RESPIRATION IN PLANTS Respiration is the exchange of oxygen and carbon dioxide between the atmosphere and the body cells. It can also be defined as the metabolic processes by which living cells breakdown carbohydrates, amino acids, and fats to produce energy in the form of Adenosine Triphosphate (ATP). Cellular Respiration Cellular Respiration, process in which cells produce the energy they need to survive. In cellular respiration, cells use oxygen to break down the sugar glucose and store its energy in molecules of adenosine triphosphate (ATP). Cellular respiration is critical for the survival of most organisms because the energy in glucose cannot be used by cells until it is stored in ATP. Cells use ATP to power virtually all of their activities—to grow, divide, replace worn out cell parts, and execute many other tasks. Cellular respiration occurs within a cell constantly, day and night, and if it ceases, the cell—and ultimately the organism—dies. Cellular Respiration Two critical ingredients required for cellular respiration are glucose and oxygen. The glucose used in cellular respiration enters cells in a variety of ways. Plants, algae, and certain bacteria make their own glucose through photosynthesis, the process by which plants use light to convert carbon dioxide and water into sugar. Regardless of how they obtain it, cells must have a steady supply of glucose so that ATP production is continuous. Oxygen is present in the air, and also is found dissolved in water and diffuses into cells plants. 28 Chemical Reactions and Metabolic Pathways To understand cellular respiration, it is necessary to understand the nature of chemical reactions. Chemical reactions can occur outside of living organisms—the rusting of a car, for example, is a chemical reaction—or they can occur within organisms, where they are termed biochemical reactions. In a chemical or biochemical reaction, the bonds between atoms that hold molecules together break apart, and the atoms rearrange to form new molecules. Water molecules, for example, are composed of hydrogen and oxygen atoms, and under certain conditions, the bonds between these atoms can break and reform to yield separate molecules of hydrogen and oxygen gas. In living organisms, most biochemical reactions occur with the help of enzymes, specialized proteins designed to carry out specific reactions. All biochemical reactions release energy in the form of heat as they occur. Cells carry out biochemical reactions to create needed molecules—such as proteins or starch—or to destroy these molecules once they are no longer needed. If certain molecules are built or destroyed in a single biochemical reaction, the reaction may release too much heat, which could incinerate the cell. To control the release of heat, cells build up and break down most molecules in a linked series of small reactions that release only a little bit of heat at a time. The series of linked biochemical reactions is called a metabolic pathway. Cellular respiration is one of the most important metabolic pathways found in cells. This enzyme-assisted, step-by-step process not only protects the cell from lethal temperature increases but also provides the cell with a mechanism of transferring the energy of glucose to ATP in a controlled manner. How Cellular Respiration Works The process of cellular respiration occurs in four stages: glycolysis; the transition stage; the Krebs cycle, also known as the citric acid cycle; and the electron transport chain. Each stage accomplishes different tasks. Glucose is the primary fuel used in glycolysis, the first stage of cellular respiration. This all-important molecule is found in the cell’s cytoplasm, the gel-like substance that fills the cell. Glucose consists of 6 carbon, 12 hydrogen, and 6 oxygen atoms bonded together, along with electrons (negatively charged atomic particles) associated with each atom. Of these components, only the hydrogen atoms and certain electrons participate directly in glycolysis. 29 30 Glycolytic pathways In glycolysis, glucose is broken down with the help of enzymes and other molecules found in the cytoplasm. Enzymes first attach two phosphate groups to glucose to make it more reactive. A phosphate group is a cluster of one phosphorus and four oxygen atoms. The addition of the two phosphate groups prepares glucose for the action of another enzyme. This enzyme splits glucose in half to produce two three-carbon molecules, each with one phosphate group attached. In the next step, an enzyme removes one hydrogen atom and two electrons from each threecarbon molecule. Both hydrogen atoms are modified to hydrogen ions, positively charged particles. A hydrogen ion and two electrons from each three-carbon molecule are transferred as a unit to a large molecule called nicotine amide adenine dinucleotide (NAD+) to form two molecules of NADH. The hydrogen ions and electrons stored in each molecule of NADH are used to make ATP in later stages of cellular respiration. In the final steps of glycolysis, two hydrogen atoms are removed from each three-carbon compound. These hydrogen atoms bond to free-floating oxygen atoms in the cytoplasm to form water. This step prepares the two three-carbon compounds for action by the next enzyme in the pathway. This enzyme removes the phosphate group from each three-carbon compound. Each phosphate group then bonds to a single molecule of adenosine triphosphate (ADP). ADP is composed of three carbon-based rings and a tail of two phosphate groups. The addition of the third phosphate group to the tail forms ATP. In this step, two new ATP molecules are produced. When cells require energy, another enzyme breaks off the third phosphate group, releasing energy that powers the cell. The removal of the third phosphate from ATP converts ATP back to ADP, which is used again in cellular respiration to make more ATP. When the two three-carbon compounds are separated from the phosphate groups, the three-carbon compounds are converted to two molecules of pyruvate, each composed of three carbon, three oxygen, and three hydrogen atoms. When glycolysis is complete, important products used in the next steps of cellular respiration have been produced: two molecules of NADH and two molecules of pyruvate. The two ATP molecules gained in glycolysis are used for reactions in the cell that require energy. The pyruvate molecules move from the cytoplasm to special structures in the cell called mitochondria, where the remaining steps of cellular respiration are carried out. Each mitochondrion contains a membrane that is folded back and forth many times. This extensive membrane is studded with hundreds of thousands of enzymes that direct cellular respiration. The numerous enzymes enable great quantities of ATP to be produced simultaneously in one mitochondrion. Without mitochondria or a similar structure, most cells could not generate enough ATP to survive. The transition stage is a short biochemical pathway that links glycolysis with the Krebs cycle. In this brief stage, enzymes transfer hydrogen’s and electrons from the two pyruvate molecules to two molecules of NAD+ to form two more molecules of NADH. Another enzyme breaks off one carbon and two oxygen atoms from each pyruvate molecule. These atoms combine to form carbon dioxide, the primary waste product of cellular respiration, which diffuses out of the cell. As a result of these reactions, each pyruvate molecule is transformed into a two-carbon compound called an acetyl group. The two acetyl groups unite with two molecules of coenzyme 31 A to form two acetyl coenzyme A molecules. The acetyl coenzyme A molecules are the molecules that enter the Krebs cycle. Krebs cycle: The oxidation of pyretic acid into co2 and water is called Krebs Cycle. This circle is also citric acid cycle because the cycle begins with the formation citric acid. The citric acid is a carboxylic acid containing 3cooh groups. Hence this cycle is also called as tricarboxylic acid cycle or TCA cycle. The cycle occurs only in the presence of oxygen. Hence it is an aerobic process. It takes place in the mitochondria. 32 During the Krebs cycle, the acetyl coenzyme A molecules are processed. As this complex pathway progresses, six molecules of NADH are formed. Additional carbon dioxide is created, and this process releases energy that is used to build two molecules of ATP from a pool of ADP and phosphate groups in the mitochondria. Hydrogen’s and electrons then are transferred to a 33 molecule of flaking adenine dinucleotide (FAD++)to form FADH2, a molecule like NADH that temporarily stores hydrogen and electrons for later use. By the end of the Krebs cycle, most of the usable energy from the original glucose molecule has been transferred to ten molecules of NADH (two from glycolysis, two from the transition stage, and six from the Krebs cycle); two molecules of FADH2; and four molecules of ATP, two of which were formed in glycolysis. Electron Transport Chain . The reactions of the electron transport chain occur in several closely spaced molecules embedded in the mitochondrial membrane. Acting like specialized delivery trucks, the NADH and FADH2molecules dump off their load of electrons and hydrogen ions near these electron transport chain molecules. The first molecule in the chain has an attraction for electrons and grabs them, but the molecule next to it in the chain has an even stronger attraction and grabs the electrons away from the first molecule. The electrons are passed down the chain in this manner, until they reach oxygen, the final molecule in the chain. Oxygen has a stronger appetite for electrons than any molecule in the chain, and the electrons therefore are held by oxygen. They are joined by the hydrogen ions that were dropped off by NADH and FADH2 at the beginning of the electron transport chain. The combination of the electrons, hydrogen ions, and oxygen forms water, used by the cell in other biochemical reactions. As NADH and FADH2 release hydrogen and electrons in the electron transport chain, they are converted back to NAD+ and FAD++, respectively, providing the cell with a steady supply of these molecules so that cellular respiration can be carried out over and over again. As the electrons flow down the electron transport chain, they release a veritable windfall of energy that is used by an enzyme to make more ATP, again from the pool of ADP and phosphate groups in the mitochondria. In most cells, the electron transport chain produces 32 molecules of ATP. Together with the two ATP molecules gained in glycolysis and the four generated in the Krebs cycle, cellular respiration produces a grand total of 38 molecules of ATP for every molecule of glucose processed. Glucose molecules enter the cell by the hundreds of thousands. They are processed simultaneously to generate millions of ATP molecules every second. Some of the ATP molecules remain in the mitochondria to supply it with needed energy, but the majority stream from the mitochondria to the cytoplasm, where they fuel the cell’s activities. It is estimated that a single human brain cell uses a staggering 10 million ATP molecules per second to carry out its tasks. Although glucose is the primary fuel for cellular respiration, cells can rely on other molecules to produce ATP. The cellular respiration pathway is connected to other metabolic pathways that can donate molecules to cellular respiration at different steps along the way. For example, glycerol, a breakdown product of fat, can enter the cellular respiration pathway in the middle of glycolysis. Another product of fat digestion, fatty acids, can enter at the transition stage. Glycerol is modified in glycolysis to pyruvate, and fatty acids are modified to acetyl coenzyme A in the transition stage. The pyruvate and acetyl coenzyme A are processed through the remaining steps of cellular respiration to yield ATP. The ability to use alternate molecules enables cells to keep ATP production going even if they run out of glucose. Marathon runners, for example, first use up their glucose reserves to make ATP, and then draw on their fat reserves to generate the ATP needed to get to the finish line. 34 Anaerobic Pathways to ATP Although most organisms on Earth carry out cellular respiration to generate ATP, a few rely on alternative pathways to make this vital molecule. These pathways are anaerobic—that is, they do not require oxygen. Fermentation is a type of anaerobic pathway used by certain species of bacteria that live in anaerobic environments, such as stagnant ponds or decaying vegetation. Some cells produce ATP using both anaerobic and aerobic pathways. For example, muscle cells typically carry out cellular respiration, but if they do not receive enough oxygen, as can occur during strenuous exercise, muscles switch to fermentation. Yeast cells also carry out both pathways, depending on whether they are in an aerobic environment, such as soil, or an anaerobic one, such as inside a wet lump of dough. Fermentation is much less efficient than cellular respiration, however, producing only two molecules of ATP for every glucose molecule processed. Fermentation occurs in two stages: glycolysis and the recycling stage. The glycolysis pathway in fermentation is virtually the same as the glycolysis pathway of cellular respiration, relying on enzymes, NAD+, and other molecules to transfer the energy of glucose to two molecules each of pyruvate, NADH, and ATP. This is the only stage of fermentation that yields ATP, and while it produces a relatively small amount, it is ample for simple cells. Molecules of NAD+, like other molecules required in glycolysis, must be in constant supply for glycolysis to continue without interruption. In the recycling stage of fermentation, the NADH produced in glycolysis is recycled back into NAD+ to be used again in glycolysis. While all cells that carry out fermentation use the recycling stage to make more NAD+, slight variations exist, depending on the organism involved. Many variations produce a number of interesting by products that are waste as far as the cell is concerned but are useful to humans. In yeast, for example, the recycling stage produces the carbon dioxide gas that makes dough rise in bread making and the alcohol that makes beer and wine intoxicating. In certain kinds of bacteria, the recycling stage produces the lactic acid that turns milk into yogurt or cheese. Other types of anaerobic pathways are carried out by certain species of archaebacteria that live in extreme environments, such as hydrothermal vents, springs of hot water in the deep ocean. These anaerobic pathways begin with glycolysis and are followed by two subsequent stages that resemble the Krebs cycle and electron transport chain of cellular respiration. However, these anaerobic pathways yield about half the amount of ATP that cellular respiration yields. 6. POLLINATION Pollination is defined as the transfer of mature pollen grains from the anther of one flower to the mature stigma of the same flower or another flower of the same plant closely related species. Pollination is the first step which leads to the eventual coming together of male and female gametes for the sake of fertilization. In spite of a common perception that pollen grains are gamete, like the sperm cells of animals, this is incorrect; pollination is a phase in the alternation of generation: achieve each pollen grain is a male haploid plant, a gametophyte, adapted to being transported to the female gametophyte, where it can fertilization by producing the male gametes (or gametes in the process of double fertilization). Sperm gets transported to the stigma, where its two gametes travel down the tube to where the gametophytes(s) containing the female 35 gametes are held within the carpel. One nucleus fuses with the polar bodies to produce the endosperm tissues, and the other with the ovum to produce the embryo. Hence the term” double fertilization”. In gymnosperms the ovule is not contained in a carpel, but exposed on the surface of dedicated support organs such as the scale of a cone, so that the penetration of carpel tissues is unnecessary. Details of the process vary according to the division of gymnosperms in question. The receptive part of the carpel is called a stigma in the flowers of angiosperms. The receptive part of gymnosperm ovule is called the micropyle. Pollination is the necessary step in the reproduction of flowering plants, resulting in the production of offspring that are genetically diverse. The diagrams below shows the flowering parts of pants that deals with pollination. The diagrams clearly show: The male part of flower comprising the anther and filament (together called the stamen) and The female part of the flower: the stigma and style with the ovary containing the ovule at the base of the flower (the carpel). Pollen grains lands and sticky stigma. A pollen tube grows down the style, followed by the male sperm nuclei. The sperm nuclei fuse with the female ovules. The ovules develop into seed and the ovary develops into fruits A complete flower 36 A Diagrammatic Complete Flower The Diagrammatic Representation of Plant Pollination 37 Importance of Pollination Pollens are essentially plant sperm, so pollination is essential to continue the lifecycle of flowering plants. Flowering plant are the basis of the animal, if plant are not pollinated then there will be no progeny and all animal that depend on them would starve. It helps wide flowers reproduce and produce enough seed for dispersal and propagation. Maintain genetic diversity within a population. Develop adequate fruit to entice seed dispersal. It helps in cleaning air, carbon cycling and sequestration. It helps to purify water and prevent erosion. Classes of Pollination i. Wind pollination ii. Insect pollination iii. Water pollination iv. And others. v. Wind Pollination Process of Wind Pollination. The male flowers ripen before the female flowers on the same plant. This favors cross pollination. The wind carries pollen grains of one maize plant to another plant with ripe female flowers. If the pollen falls on ripe stigma, cross pollination takes place. Examples of wind pollinated flowers are Maize, guinea, rice, millet and wheat. Characteristics of wind pollinated flowers. (Anemophilous flowers) They have small, inconspicuous petals/sepals. Flowers are usually dull coloured. There is absence of scent. There is absent of nectars. Large quantity of pollen grains is produced. Pollen grains are small, smooth, light and not sticky. Stigma is elongated and sticky with large surface area. Anthers are attached to the flowers in such a way that they readily swing in the air and release the pollen grains. Insect Pollination (Entomophilies) This often occurs on plants that have developed colored petals and a strong scent to attract insects such as, bees, wasps and occasionally ants (Hymenoptera), beetles (Coleoptera), moths and butterflies (Lepidoptera), and flies (Diptera). The existence of insect pollination dates back to the dinosaur era. Process of pollination by insects (e.g. Pride of Barbados). The insect that normally pollinate pride of Barbados are swallowing tail Butterfly and bees. When the insect lands on the standard petals it uncoils its proboscis and inserts it through the 38 furrow in the standard petals that leads to the nectar. During this process, the hairy body or wings of the butterfly or bees touches the pollen grains and these stick to the body or wings of the insect. When the insect visit another pride of Barbados flowers, the pollens grains on its body or wings may touch the stigma of this flower thereby bringing about cross pollination. 39 A European honey bee collects nectar, while pollen collects on its body. Characteristics of insect pollinated flowers. (Entomophilous flower). They have large conspicuous petals/sepals. Flowers are usually brightly colored. They posses scent. Nectar is also present. Pollen grains are rough, sticky and relatively few. The stigma is flat with sticky surface to enable it receive pollen grains. Petal are shaped and arranged to enable visiting insect becomes dusted with pollen grains. Examples of insect pollinated flowers include: Hibiscus, Delonix, Cowpea, Crotalaria, Pride of Barbados, e.t.c. Water Pollination Water pollinated plants are aquatic and pollen is released into the water. Water currents therefore, act as a pollen vector in a similar way to wind currents. Their flowers tend to be small and inconspicuous with lots of pollen grains and large feathery stigmas to catch the pollen. However, this is relatively uncommon (only 2%of pollination is hydrophilic). And most aquatic plants are insect- pollinated, with flowers that emerge into the air. zoophily: pollination is performed by vertebrates.( such as birds and bats, particularly, hummingbirds, sunbirds, spider hunters ,honeyeaters, and fruit bats.): Plants adapted to using bats or moths as pollinators typically have white petals and a strong scent, while plants that use birds as pollinators tend to develop red petals and rarely develop a scent (few birds rely on a sense of smell to find plant-based food). Anthropophily, pollination by humans: often artificial pollination used in hybridization techniques. 1 Differences between Insect Pollinated Flower and Wind Pollinated Flowers. Insect pollinated flowers Wind pollinated flowers Flowers are usually large and conspicuous. Flowers are usually small and inconspicuous. 2 Flowers are usually brightly coloured. Dull coloured. 3 There is present of scent. There is absent of scent. 4 Pollen grains are rough, sticky and relatively few. Pollen grains are light, smooth very numerous. 5 Anthers may or may not be enclosed by the petals. Filaments are long so that anthers hang outside the flowers. 6 Flowers may be held above the leaves Flowers are carried above the leaves where they are exposed to the wind. 40 7 Stigma is flat or lobed with sticky surfaces for easy adherences of pollen grains. Stigma is a large and feathery hanging outside the flowers providing large surface area for easy trapping of pollen grains. 8 The shapes and flora pairs are such that they enable insect get dusted with the pollen grains during visiting. There is particularly adaptive shape as flowers are small and exposed. 9 Nectars are present. Nectars are absent. The mechanics involve in pollination. Pollination can be accomplished by cross-pollination or by self-pollination: Cross-pollination: This is also called allogamy, occurs when pollen is delivered to a flower from a different plant. Plants adapted to outcross or cross-pollinate often have taller stamens than carpel or use other mechanisms to better ensure the spread of pollen to other plants' flowers. Conditions or device which aid cross pollination. Some plants may have conditions or device which may aid cross pollination to take place. These are dichogamy, Unisexuality and self-sterility. Dichogamy: Dichogamy refers to the ripening of the anthers and stigmas of a bisexual flower at different times. Dichogamy occurs in two ways. These are Protandry and protogyny. (a) Protandry: This refers to the condition in which the anthers of a flower mature earlier than the stigmas that flowers or other flowers of the same plant so that the mature pollen grains are only useful to flowers of other plant which have mature stigma to receive them. (b) Protogyny: Protogyny refers to the condition in which the stigma of flowers matures earlier than its own pollen grains or those of others flowers of the same plant so that it can only receive pollen grains from flowers of other plant. Examples include blue bells and white dead nettle. Unisexuality: Unisexuality is a situation in which some plant bears only male or female flowers and not both on the same plant, e.g. pawpaw. Such plant are said to be dioecious plant. Cross pollination may occur under this condition. On the other hand, in a monoecious plant the male and female flowers are borne by the same plant, the female flowers are usually situated higher than the male flowers so that pollen grains may not reach the stigma of the female flowers. Hence, they will be received only by stigmas of female flowers of other plants. Self-sterility: This is refers to situation in which some plants make themselves sterile. The presence of pollen on their stigmas is injurious to further development of the plant. For 41 examples, they may wither and die. However, when pollen grains come from other plants, fertilization can take place in such plant. Examples are found in passion flowers and tea. Self incompatibility: This mechanism ensures that the pollen tube does not develop unless it has a different genetic composition from that of the stigma. This prevents self pollination. Advantages of cross pollination. Cross pollination leads to the production of healthier offspring than self pollination. It also produces viable seeds. Offspring or individuals produced are more adapted to the environment conditions. It also leads to the formation of new varieties with good characteristic. Disadvantages of cross pollination. It relies on external agent such as wind and insect whose presence at the right time cannot be guaranteed. It may leads to wastage of pollen grains especially pollination by wind. Self-pollination: This occurs when pollen from one flower pollinates the same flower or other flowers of the same individual. It is thought to have evolved under conditions when pollinators were not reliable vectors for pollen transport, and is most often seen in short-lived annual species and plants that colonize new locations. Self-pollination may include autogamy, where pollen moves to the female part of the same flower; orgeitonogamy, when pollen is transferred to another flower on the same plant. Plants adapted to self-fertilize often have similar stamen and carpel lengths. Plants that can pollinate themselves and produce viable offspring are called selffertile. Plants that cannot fertilize themselves are called self-sterile, a condition which mandates cross pollination for the production of offspring. Conditions or device which aid self pollination. Some plants have conditions or device which aid self pollination to take place. These conditions are cleistogamy and homogamy. Cleistogamy: is self-pollination that occurs before the flower opens. The pollen is released from the anther within the flower or the pollen on the anther grows a tube down the style to the ovules. It is a type of sexual breeding, in contrast to asexual systems such as apomixis. Some cleistogamous flowers never open, in contrast to chasmogamous flowers that open and are then pollinated. Cleistogamous flowers by necessity are self-compatible or self-fertile plants. Many plants are self-incompatible, and these two conditions are end points on a continuum. Homogamy: Homogamy refers to the ripening of the anthers and stigma of a bisexual flower at the same time. Under this condition, self pollination may occur in the following ways: 1. A gentle breeze may blow the mature pollen grains which may be shed on mature stigma on mature stigma that are situated below. 42 2. A visiting insect may transfer the mature pollen grains to the stigma of the same flower. 3.Self pollination may also occur when mature stigma push their way out of the corolla tube during which they are brushed against the anthers and in the process pollen grains are collected. 4. In a situation where the filaments are longer than the stigma, the filament may recoil to touch the mature stigmas. 5. In like manner, self pollination may occur where the style are longer than the filament; the style may also bend or recoil to make the stigmas touch the anthers. Advantages of self pollination. 1. It is a sure way of ensuring pollination especially in bisexual flowers. 2. It may not waste pollen grains. Disadvantages of self pollination. 1. It leads to the production of weak offspring as a result of continuous or repeated self pollination. 2. The offspring or individuals produced are less adapted to the environment. Geranium incanum, like most geraniums and pelargoniums, sheds its anthers, sometimes its stamens as well, as a barrier to self-pollination. This young flower is about to open its anthers, but has not yet fully developed its pistil. 43 These Geranium incanum flowers have opened their anthers, but not yet their stigmas. Note the change of colour that signals to pollinators that it is ready for visits. This Geranium incanum flower has shed its stamens, and deployed the tips of its pistil without accepting pollen from its own anthers. (It might of course still receive pollen from younger flowers on the same plant.) Differences between self pollination and cross pollination. Self pollination Cross pollination 1 Self pollination takes place only in bisexual Cross pollination takes place in both plants. unisexual and bisexual plant. 2 Only one parent is involved. Two parents are involved. 3 Pollination may occur without an external agent. This requires external agent, e.g. insect and wind. 4 It does not ensure new variation. It results in the formation of new varieties. 5. Pollen grains are effectively utilized. Much of the pollen grains are wasted. Pollinator and pollenizer Pollination also requires consideration of pollenizer and pollinator. 44 A pollinator is the agent that pollinates plant i.e. involves in the transfer of pollen grains from anthers to stigma, mostly insects like: flies bats moths or birds. A pollenizer is the plant that serves as the pollen source for the other plants; some plants can pollinate themselves i.e. they act as their own pollenizer. A good example is a plant that provides compatible viable and plentiful pollen and blooms at the same time as the plant that is to be pollinated. The wasp Mischocyttarus rotundicollis transporting pollen grains of Schinus terebinthifolius Pollen vectors They are usually insects but also reptiles birds mammals, and others that transport pollen grains and play a vital role in pollination. Any kind of animal that often visits or encounters flower is likely to be a pollen vector to some extent. Plants that rely on pollen vectors tend to be adapted to their particular type of vector. For example day pollinated species tends to be brightly coloured, but if they are pollinated largely by birds or specialist mammals, they tend to be larger nectar and have larger nectar reward than species that are strictly insect pollinated. As for types of pollinator, reptiles pollinators are known but they form minority in most ecological situations. Most species of lizards in the families that seem to be significant in pollination seem to carry out pollen only accidentally especially the larger area species such as varanidae and Iguanidae, but especially several species of the Gekkonidae are active pollutant. Mammals are not generally thought of as pollinators, but some rodents, bats and marsupials are significant and some even specialize in the activities. Examples of pollen vectors include many species of wasps that transport pollen of many plant species, being potential or even efficient pollinators. After of pollination in plants there occurs another important process called fertilization. Fertilisation Fertilization is the union of the male and female gametes to form a zygote. Since the male and female gametes are haploid (n) when the two unite the zygote is diploid (2n). Fertilization starts when a pollen grain lands on the stigma. The pollen grain then grain germinates forming a pollen tube. The tube nucleus controls the growth of the pollen tube. The pollen tube is an example of chemotropism since it is growing toward chemicals produced from the ovule. The generative nucleus travels down the pollen tube. It undergoes mitosis forming two haploid male gamete 45 nuclei. The pollen tube enters the ovule by way of the micropyle. The two male gamete nuclei are released into the embryo sac. The tube nucleus disintegrates. Double Fertilization Since there are 2 sperm nuclei that have reached the embryo sac both nuclei will fuse with female gametes. One sperm nuclei will fuse with the egg cell to form the zygote (2n) while the other sperm nucleus fuses with the 2 polar nuclei in the embryo sac to form an endosperm nucleus (3n). Problems in Fertilization From the time that the pollen comes in contact with the stigma, there are a number of problems that can arise. If the ambient temperatures are too high the pollen can become denatured and if it’s too low then the pollen will just sit until the temperature rises, although the flower’s receptivity is short lived. If the relative humidity is too low at this stage then the stigma or the 46 pollen can desiccate, preventing the germination of the pollen. Low relative humidity has been found to be the culprit in a poor seed set for these reasons in a number of instances in vegetable seed production in the arid western states. The next step in the process of fertilization, the pollen tube growing down through the style can also be derailed by unfavorable weather conditions. The pollen tube is essentially a free living miniature plant (the gametophyte generation) and requires temperatures similar to the mother plant to grow vigorously. The pollen tube’s life cycle is usually 24 hours or less and it must make the trip from stigma to ovule in this period or not be successful in fertilizing the ovule. If the ambient temperature during this short period of time is colder or hotter than temperatures favorable to normal growth of the species than the pollen tube will stop growing and fail restart when the temperature comes back into a favorable range for growth? This will result in no fertilization for that particular pollen tube. In a cool loving crop like spinach that produces luxuriant growth at 58 – 65F (15 – 18C) and virtually stops growth at 78F (25.5C), this means that when it gets hotter than 78F (25.5C) as spinach seed crops are flowering there can be serious damage done to the yield due to poor fertilization of the ovules. Alternately, a heat loving crop like tomatoes can suffer blossom drop producing fewer fruit with low seed yields when tomatoes are exposed to cold night time temperatures during flow. Any Modern Biology or Advance Biology text book. 7. FERTILIZATION AND SEED FORMATION Fertilization Fertilization is a biological term that describes the union of male and female gametes to produce an offspring. In agriculture, it refers to the process of enriching the soil with extra nutrients needed for healthy plant growth. Fertilization is the union of the male and female gametes to form a zygote. Since the male and female gametes are haploid (n) when the two unite the zygote is diploid (2n). Fertilization starts when a pollen grain lands on the stigma. The pollen grain then grain germinates forming a pollen tube. The tube nucleus controls the growth of the pollen tube. The pollen tube is an example of chemotropism since it is growing toward chemicals produced from the ovule. The generative nucleus travels down the pollen tube. It undergoes mitosis forming two haploid male gamete nuclei. The pollen tube enters the ovule by way of the micropyle. The two male gamete nuclei are released into the embryo sac. 47 Double Fertilization Since there are 2 sperm nuclei that have reached the embryo sac both nuclei will fuse with female gametes. One sperm nuclei will fuse with the egg cell to form the zygote (2n) while the other sperm nucleus fuses with the 2 polar nuclei in the embryo sac to form an endosperm nucleus (3n). Steps involved in fertilization The act of fusion of the sperm and the method of approach, entry and eventual fusion of the male and female nuclei constitutes the mechanism of fertilization. This is believed to take place in the following stages. 48 1. Movement of the sperm towards the egg: This is the first step which brings the sperm in physical contact with the ovum. As the male and female gametes are produced in different individuals, there are various mechanisms to bring them nearer. The initial attraction of the sperms towards the egg is supposed to be chmotactic. The sperms swim towards the egg collides with it. Usually several sperms attach themselves to the egg. In individuals with external fertilization large number of eggs and sperms are released outside so that they have a favourable chance of meeting with each other. In individuals with internal fertilization the sperms are discharged into the genital tract of the females by the male individual. From here the sperms move upward and reach the egg present in the uterus. 2. Capacitation and contact: When a large number of sperms approach the egg for contact, the fertilizin and antifertilizin reaction ensures that only a few spermatozoa are allowed to reach the ovum. The initial attachment of the sperm to the egg is believed to be due to the chemical bonding of fertilizin and antifertilizin. 3. Penetration of sperm into ovum: The acrosome portions of the sperm produces some lytic enzymes called sperm lycines which have the ability to dissolved the egg membrane allowing the entry of the sperm into the egg cytoplasm. Usually only the head and the middle piece of the sperm enter the egg while the tial is left outside. 4. Cortical reaction: The entry of the sperm head into the egg brings about several changes in the egg surface. These are the cortical changes and the development of fertilization membrane. In some echinoderm eggs some fine granules are visible in the ooplasm after the entry of the sperm head. The vitelline membrane starts lifting itself up from the point of sperm entry and a perivitelline space is formed between it and the egg surface. The cortical granules from the egg cortex release their contents into the privitelline space. These contents attach themselves to the inner surface of the vitelline membrane forming what is known as a fertilization membrane. The development of the fertilization membrane prevents the entry of other sperms into the egg. 5. Activation of the ovum: The mature egg will be generally in a hibernating condition with very low rates of metabolism and inactive nucleus. The penetration of the sperm triggers the egg into activity. The metabolic rates increase allowing for entry of water and other particles. Metabolically the rate of protein synthesis goes up as these are needed for further cell divisions. At this stage the nucleus (pronucleus) of the egg which has remained in the metaphse II stage (of the meiotic II division) becomes active completing its second division and releases the second polar body. 6. Fusion of male and female pronuclei (amphimixis): In this process there is fusion of the male and female nuclei. Initially the two nuclei remain close together and at the point of contact the nuclear membranes disappear and the chromosomes come to lie on the equator. Finally the nuclear fusion is completed and it becomes a zygote nucleus. 49 The egg is said to have been fertilized and it becomes a zygote. It is now ready to undergo cleavage to develop into the embryo. Germination The embryo will germinate from the seed if the proper environmental conditions are present. When this occurs the embryo resumes its growth. In order for germination to occur the following conditions must be present: i. Water must be present. This allows the seed to swell and enzymes to function. ii. Oxygen must be present in the soil. iii. The temperature must be suitable for the species of plant. Suitable temperatures are usually between 5-30 degrees Celsius depending on the species. iv. The dormancy period must be complete. v. Some seeds need light and others need darkness. · · · · · · · · · · Events of germination When germination begins the first thing that happens is water is absorbed by the seed through the micropyle and through the testa. Enzymes in the soil now digest the foods stored in the seeds: Oils become fatty acids and glycerol Starch becomes glucose Protein becomes amino acids These foods now are absorbed by the embryo. The glucose and amino acids make new structures such as cell walls and enzymes. The fats and glucose are used in cellular respiration to produce energy. The stored food of the seed is being used up as the embryo grows larger. The radicle grows larger and breaks through the testa. It becomes the roots of the new plant. The plumule grows larger and emerges above the ground. Leaves form. 50 Germination occurs differently in different plants. In some plants the cotyledon remains underground while in other plants the cotyledon emerges above ground. The diagrams below show these 2 methods of germination. 51 Problems in Fertilization From the time that the pollen comes in contact with the stigma, there are a number of problems that can arise. If the ambient temperatures are too high the pollen can become denatured and if it’s too low then the pollen will just sit until the temperature rises, although the flower’s receptivity is short lived. If the relative humidity is too low at this stage then the stigma or the pollen can desiccate, preventing the germination of the pollen. Low relative humidity has been found to be the culprit in a poor seed set for these reasons in a number of instances in vegetable seed production in the arid western states. The next step in the process of fertilization, the pollen tube growing down through the style can also be derailed by unfavorable weather conditions. The pollen tube is essentially a free living miniature plant (the gametophyte generation) and requires temperatures similar to the mother plant to grow vigorously. The pollen tube’s life cycle is usually 24 hours or less and it must make the trip from stigma to ovule in this period or not be successful in fertilizing the ovule. If the ambient temperature during this short period of time is colder or hotter than temperatures favorable to normal growth of the species than the pollen tube will stop growing and fail restart when the temperature comes back into a favorable range for growth. This will result in no fertilization for that particular pollen tube. In a cool loving crop like spinach that produces luxuriant growth at 58 – 65F (15 – 18C) and virtually stops growth at 78F (25.5C), this means that when it gets hotter than 78F (25.5C) as spinach seed crops are flowering there can be serious damage done to the yield due to poor fertilization of the ovules. Alternately, a heat loving crop like tomatoes can suffer blossom drop producing fewer fruit with low seed yields when tomatoes are exposed to cold night time temperatures during flowering. Seed Formation Seed formation can be described as a process whereby a pollen grain sends grain to the stigma and a flower or seed is formed. 52 The fertilized becomes the seed. The integuments become the wall of the seed called the testa. The micropyle closes. The endosperm nucleus leads to the formation of triploid endosperm, a food tissue. The diploid zygote, by mitosis, develops into a plant embryo. The developing embryo draws nourishment from the endosperm. The embryo ceases development and goes dormant. The ovule becomes a seed, which contains a dormant plant embryo, food reserve, and the protective coat called the testa. The Embryo The embryo is made up of the radicle or future root and the plumule or future shoot. The endosperm cells divide many times and absorb the nucleus. This is the nutrition (mainly fats, oils and starch) for the embryo. There are 2 types of seeds. Some are endospermic while others are non-endospermic. In endospermic seeds the food reserve is the endosperm, which is outside the plant embryo. Examples of this type of seed are maize and wheat. Non-endospermic seeds have food reserve within the cotyledon(s) of the plant embryo. This occurs in broad beans. 53 Monocots and Dicots Monocots have one cotyledon in the seed while dicots have two cotyledons. The cotyledons are food reserves for the young plant after it germinates from the soil. It uses these food reserves until it is capable of making its own food. In monocots the food is absorbed from the endosperm while in dicots the food is stored in the cotyledons. Monocot Dicot 54 Fruit Development The ovary becomes a fruit. The wall of the ovary becomes the wall of the fruit called the pericarp. The fruit protects the developing seeds and plays an important role in seed dispersal. This process is controlled by auxins produced by the seeds. Once the fruit forms the rest of the flower parts die and fall away. Fruit and Seed Dispersal Seed dispersal is the scattering of offspring away from each other and from the parent plant. As a result of dispersal there is an improved chance of success by reducing competition and overcrowding. Dispersal also enables colonization of new suitable habitats and thus, there is an increased chance of species survival. 55 Methods of Seed Dispersal Wind: The seeds of wind-dispersed plants are lightweight seeds. They have a high air resistance so they can be carried far away from the mother plant. Water Dispersal Fruits which float such as those of the water lily and the coconut palm are carried by water. Coconuts can travel for thousands of kilometres across seas and oceans. The original coconut palms on South Sea Islands grew from fruits, which were carried there from the mainland by ocean currents. Mangroves in the swamp regions of countries such as Thailand are another example. 56 Animal Dispersal Some plants have juicy fruit that animals like to eat. The animal eats the fruit but only the juicy part is digested. The stones and pips pass through the animal's digestive system and are excreted to form new plants. This can be far away from the parent plant. Blackberry, cherry and apple seeds are dispersed in this way. Birds also like to eat fruit and they help to disperse seeds to other areas through their droppings. 57 Mistletoe has sticky fruits that are attractive to birds. The sticky seeds stick to the bird's beak. They then rub their beaks clean on the bark of trees. The sticky seeds are left on the bark to grow into new mistletoe plants - mistletoe is a parasitic plant. Squirrels collect nuts like acorns and bury them for winter food, but they often forget where they have buried them and these grow into new trees. Some fruits like that of the burdock plant have seeds with hooks. These catch on the fur of animals and are carried away. Self-Dispersal: Some plants have pods that explode when ripe and shoot out the seeds. Lupins, gorse and broom scatter their seeds in this way. Pea and bean plants also keep their seeds in a pod. When the seeds are ripe and the pod has dried, the pod bursts open and the peas and beans are scattered. Dormancy 58 Dormancy is a period of inactivity. There is very little cellular activity and no growth. One or many of the following reasons bring about dormancy: Auxins that inhibit growth- Growth Inhibitors Process of seed formation Pollination is the first step in reproduction. Pollen grains are shed from the anthers and fall onto the feathery stigmas. Fertilization is the second step in seed formation. The pollen that reaches the stigma germinates and forms a pollen tube that carries the male nuclei inside the ovary for fusion with the egg nuclei. The complete process from pollination to fertilization takes from 18 to 24 hours. 9. ECOLOGICAL ADAPTATION OF PLANT ON AQUATIC HABITAT Ecology The word ecology can be defined as the study of plant and animal in relation to their environment. Ecology is derived from a Greek word Oikos which means or dwelling place. In other word ecology can be define as a field of study which deals with the relationship of living organism with one another and with the environment in which they live. Ecology is often described as environmental biology. Ecology is divided into two main branches: (a). Autecology: Autecology is concerned with the study of an individual organism or a single species of organism and its environment. For example the study of a single rat and its environment. (b). Synecology: synecology is concerned with the study of inter-relationships between groups of organisms or species living together in an area. For example the study of different organism in a river in relation to their aquatic environment. Ecological Concept There are some important concepts commonly used in the study of ecology which enable one to understand the subject matter. Some of these ecological concepts include: Environment: The environment includes all the factors external and internal, living and nonliving which affect an organism. Biosphere or Ecosphere: The biosphere or ecosphere is the zone of earth occupied by living organisms.it is a layer of life which exists on the earth surface. The biosphere is a narrow zone where complex biological and chemical activities occurred.it can be found on land, soil, water and air.it provide habitat for plant animal and micro-organisms. Lithosphere: The lithosphere is the solid portion of the earth.it is the outermost layer of the earth crust.it is made up of rock and mineral materials, and it also represent 30% 0f the earth surface. Lithosphere forms the basis of all human settlement. 59 Hydrosphere: hydrosphere is the liquid or aquatic part of the earth or living world. It covers about 70% of the earth’s crust. It holds water in various forms- solid (ice), liquid (water) and as gases (water vapour).Examples of hydrosphere are lake, pools spring, ocean or sea, ponds, oasis, rivers and stream. Atmosphere: The atmosphere is the gaseous portion of the earth. It is a layer of gases surrounding the earth. Over 99% of the atmosphere lie within 30km of the earth surface.it contains 78%nitrogen, 21% oxygen, 0.03% carbon dioxide and 0.97 rare or inert gases. Habitat: Habitat is defined as an area occupied by a biotic community. In other words, habitat is any environment in which an organisms live naturaly.it is the natural home of organisms. For example, the habitat of the fish is water. The various types of habitat are aquatic habitat (i.e. live in water) such as rivers, lakes, ponds, stream, lagoons, sea, ocean and terrestrial habitats (live on land) e.g. savannah, forest, desert etc. Biotic Community or Biome: A biotic community is any natural occurring group of different organisms living together and interacting in the same environment. A biome is the largest community of organisms, e.g. rain forest, guinea savannah etc. Ecological Niche: Ecological niche refers to the specific portion of a habitat which is occupied by a particular species or organisms. For example, a caterpillar and an Aphid which live on the same plant occupy different position or ecological niches on the plant. The caterpillar lives mainly on the leaves and feed on them while the Aphid lives on the young shoot and sucks sap from it. Although both organisms live on the same habitat, each has it living space and source of food. Population: Population is defined as the total number of organisms of the species living together in a given area. For example, the total number of Tilapia fish in a pond constitutes the population of Tilapia fish in that habitat. Ecosysyem: An ecosystem refers to a community of plants and animals functioning together with their non-living environment. In other words, ecosystem consist of the living factors (plant and animals) interacting with the non-living factors in an environment. Ecological Factors Ecological factors are those factors in the environment which can influence living organisms or cause changes in any habitat, be it aquatic or terrestrial habitat. Ecological factors are grouped into two categories-Biotic factors and Abiotic factors. Ecological factors common to all habitats Factors affecting or common to all habitats include: Temperature, Rainfall, Light, Wind, Pressure and Hydrogen ion concentration (pH). Of these factors, temperature and rainfall determine the major biomes of the world. Temperature 60 Temperature determines the vegetation of an area. It is necessary for the germination of seeds It regulates the activities of majority of the living things High temperature affects evapotranspiration and reduces the performance of animals. It affects the wilting of field crops, ripening and maturity of crops. It leads to loss of soil nutrients through volatilization. vii) Unfavourable temperature may result in seed dormancy. Rainfall i) Rainfall determines seasons in some places, e.g Nigeria where we have rainy and dry season. ii) It determines the type of vegetation in an area. iii) It determines the distribution of plants and animals. iv) Rainfall is necessary for seed germination. V) Rainfall provides a dwelling place or habitat for some organisms, e.g. fish, crab, shrimps, sea weeds etc vi) it helps to dissolve nutrients in the soil thereby making them available to plants. Plants use water for photosynthesis. viii) It is the main source of water in rivers, ponds, lakes, oceans etc. Wind Wind determines seasons in Nigeria, for instance, the south-West wind is responsible for rainy season while the North-East wind brings harmattan or dry season. It helps in the distribution of rainfall. It can aid the spread of diseases. It aids the pollination of flowers. It also aid the dispersal of seeds and fruits. High velocity wind may cause wind erosion. Wind is responsible for water currents and waves. Light Sunlight is necessary for photosynthesisto take place in green plants. It affects evapo-transpiration. It affects the productivity of crops due to length of day,i.e.,photoperiodism. Light affect flowering and fruiting in plants. Light is the ultimate source of energy for all organisms. It affects the activities of animals,e.g.some animals are active during the day while others are active at night. Pressure Atmospheric pressure decreases as one goes up from the sea level. In aquatic environment, pressure increase as one moves down the water. Plants and animals have special adaptions to a particular level of pressure to enable them survive Too high or too low pressure will affect the lives and activities of plants and animals. Pressure is responsible for the movement of winds 61 Hydrogen ion concentration (pH) PH values range from 1 to 14, with pH 1 being very acidic, PH 7 neutral and PH 14 very alkaline. Living organisms are highly sensitive to pH changes. Too high or too low pH will affect the lives and activities of plant and animals. Plants and animals are adapted to special pH values; e.g pH of fresh water is low while marine PH is high. Most plants thrive well in neutral or slightly alkaline soil while acidic soils support very little vegetation. Aquatic Habitat Aquatic habitat is a body of water in which certain organisms live naturally. In other words, acquatic habitats are habitats or places that relates to lives in water. Organisms that live in water are called acquatic organisms. Examples of aquatic organisms are fish, crabs, toads, plants etc. Other Ecological Factors Common to Aquatic Habitat Salinity: Salinity is defined as the degree of saltiness or concentration of salt solution in water. Salinity is low in fresh water, high in sea water and moderate in brackish water. Aquatic organisms need to maintain the osmotic balance between their body fluids and the aquatic surroundings to survive. For example, organisms living in fresh water will require some adaptive features to enable them get rid of excess water that enters their bodies while those in sea water equally have adaptations to enable them cope with excess water in their bodies. Turbidity/Transparency: Turbidity is caused as a result of suspended materials in water. Clear water has low turbidity. Turbidity is also influenced by season. It is higher during the raining season than in dry season. Turbidity reduces light penetration into the water, resulting in the in ability of aquatic green plant to carry out photosynthesis, and it causes pollination. Dissolved Gases: Dissolved gases in the case refer to oxygen and carbon dioxide. The oxygen concentration of water decreases with depth. Oxygen is required by most aquatic organisms for respiration. It is also required for the decaying of organic substances. Carbon dioxide is required as raw material for photosynthesis. Density: Density of water varies with the type of aquatic habitats. While the density of pier fresh water is 1.00, that the sea water is 1.028 at atmospheric pressure and 0c. Organisms like fish have streamlined bodies which enable them to move them easily through water while other organisms which float on the water surface are sensitive to changes in density. Currents: Water currents increase aeration and the turbidity of the water. It also affects the distribution of aquatic organisms. The type of organisms found in an aquatic habitat is affected by the speed of water current. For example, animal living in fast moving water usually have structures of attaching themselves in rock surfaces so that they cannot be swept away. 62 Tidal Movement And Waves: Tidal movements and waves affect the organisms in aquatic environment. Most organisms in certain level of the water attach themselves to substances or may even live in burrows. Some may possess hard body cover to prevent evaporation of water their bodies. In the open sea, waves cause the aeration of the surface waters, enabling aquatic organisms to have sufficient supply of dissolved gases for their respiration. Types of Aquatic Habitats There are three types of aquatic habitats. These are marine or salt water habitats, estuarine or brackish water habitats and fresh water habitats. Marine Habitats Marine habitats refers to aquatic habitats which contain salt water. Marine habitats include the ocean, lakes, shore and the open seas. Characteristics of Marine Habitat The marine or salt water habitat has the following characteristics: Salinity: Salinity is defined as the degree of saltiness or concentration of salt solution in oceans. The marine habitats have a high salinity and its average salinity is put at 35.2 per 1000. In other words, the average salinity of the ocean is 35.2 parts of salts by weight per 1000 parts of water. Density: The destiny of marine water is high, hence many organisms can float in it. While the destiny of ocean water is about 1.028, that of fresh water is higher than that of fresh water. Pressure: Water pressure increases in depth at the rate of one atmosphere for every ten meters. In other words, pressure varies from one atmosphere at the surface level to about 1000 atmosphere at the greatest depth. This is why animals in marine habitats have features which enables them to adapt especially at the deep level of the sea. Size: Marine habitats represent the largest of all the habitats. The ocean alone occupies over 70% or 360 million square kilometers of the earth’s total area of 510 million square kilometeres. Examples of oceans are atlantic ocean, Indian ocean, Pacific ocean ( the largest) etc. Currents: Currents are always produced down the ocean as a result of certain variations such as salinity and changes in temperature. Tides:Tides are the alternate rise and fall of the ocean approximately twice a day. This alternate rise and fall in water level is due to the gravitational effects of the moon and sun. Oxygen Concentration:The concentration of oxygen in the ocean is highest at the surface while it decreases with depth, and in the very deep parts of the oceans there is practically no oxygen. Hydogen Ion Concentration:Salt water is known to be alkaline in nature with pH of about 8.09.0 near the surface. 63 Waves:Waves are movement of surface waters of the oceans and it can take any direction and are caused by winds. Waves bring about the mixing of sea water especially on the surface of the ocean. Light Penetration:Light penetrates the ocean water only to a maximum depth of 200metres. Therefore, plant life is limited to the upper layers of the ocean where light can penetrate. Penetration of light depends on the water turbidity. Major zones of the marine habitats. The major ecological zones of the marine habitats include: Supratidal or Splash Zone:This is the exposed zone of the marine habitat. It has occasional moisture since it is the area where water splashes when the waves break at the shore. Intertidal or Neritic Zone:This zone which is also called planktonic or euphotic zone is only exposed at low tide or covered by water at high tide. It has high photosynthetic activities because of abundant sunlight. There is also fluctuation of the water temperature. Litoral or Subtidal Zone:This zone is about 200m deep. It is constantly under water, it has abundant sunlight and therefore abundant nutrients. Benthic Zone:Benthic zone is also under water and is about 500m deep. It has low light penetration and low nutrients. Pelagic or Abyssal Zone:This zone is about 7000m deep. It has low temperature, low light penetration, high pressure, low photosynthetic activities and the primary production of food is by chemosynthesis. Hadal or Aphotic Zone:This is the deepest zone of the marine habitat. It is over 7000m deep. It forms the floor or bed of the ocean. There is no light penetration and no photosynthetic activities. On the basis of depth or light penetration or vertical zoning of marine habitat, there exist three major zones. These are euphotic, disphotic and aphotic zones: (1)Euphotic Zone:This is an area which is directly connected with sunshine. Producers, consumers and decomposers are present here. There is enough light penetration for photosynthesis to take place. (2)Disphotic Zone:Disphotic zone is a region of dim light. Consumers and decomposers are also found here. Light penetrates the water but the intensity is too low for photosynthesis to take place. (3)Aphotic Zone:This represents the bottom or bed of the seas and oceans. It is characterised by cold dark water without light penetration and very few living organisms are found in this zone. Distribution of organisms in marine habitats and their adaptive features. The organisms in marine habitats include plants and animals. Plant in marine habitats. Sea Weeds:They possess hold-fast for attachment. They have divided leaves, floating devices or air bladder for bouyancy. 64 Algae, eg. Sargassum: The Algae possess chlorophyll for photosynthetic activities, small size or large surface area for drifting or floating. Sesuvium:Sesuvium possesses thick leaves or reduced leaves for water conservation. Planktons, e.g. Diatoms:They possess air spaces in their tissues, rhizoids (fake feet) for attachment to rocks and air bladder for buoyancy. Food chain in marine habitat. A typical food chain in marine habitat could be up to three or four trophic levels. The phytoplanktons, e.g. diatoms serve as the major producers which support the food chain. Some examples of food chain include: (1)Diatoms - Zooplanktons - Tilapia - Shark. (2)Diatoms - Crabs - Tilapia. Factors Affecting Marine Habitats. Some of the major factors affecting marine habitats are temperature, sunlight, wind, density, pH and salinity. These factors have been explained under the characteristics of marine habitat. Estuarine Habitats. 65 Estuarine habitat is a body of water formed at the coast as a result of the action of tides which mix salt water from the sea with fresh water from the land. The mixing of salt water and fresh water results in the formation of a brackish water. This brackish water is what is called estuarine. Types of estuaries. Estuary is found in the following bodies of water: (1)Delta: A delta is where a river divides into many channels before entry into the ocean or sea. Brackish water or estuary (delta) is formed at the mouth of a river as it enters the sea. (2)Lagoon: Lagoon is a body of ocean water that enters into the land through a canal and therefore has the opportunity of mixing with fresh water from rivers and streams. (3)Bay: Bay is a little or small portion of the sea water which enters into the land and mixes up with fresh water from rivers and streams. It should be noted that a lagoon is bigger than a bay and it may be long enough to join the sea at another end while bay is very small and not long enough to rejoin the sea in another end. Characteristics of Estuarine Habitats. The followings are the characteristics of the estuarine habitats: (1)Fluctuation in Salinity:Salinity fluctuates in this habitat.Salinity is lower at the mouth of a river and gets higher towards the sea. Salinity is also affected by season. While rainy season reduces salinity due to addition of fresh water, dry season increases it. (2)Turbidity:Turbidity of estuarine habitat increases especially during the rainy season when lotsof debris are brought down by rivers to the habitat. This high turbidity also reduces the rate of photosynthesis and respiration by organisms. (3)Shallowness of Water:Unlike the sea water which is deep, the water in estuarine habitat is very shallow. (4)Low Species Diversity:The estuarine habitat has low diversity of species compared to marine habitat. Common plant species are phytoplanktons algae, marsh vegetation etc while animal species are crabs, oysters, lobsters, fishes etc. (5)Water is affected by tides:Sea water usually flows rapidly into estuaries at high tides and rushes back into the ocean at low tides. (6)High Level of Nutrients:The estuarine habitat contains abundant nutrients especially the organic detritus which form the bulk of producers in the habitat. (7)Low Oxygen Content:Oxygen content of estuarine habitat is generally 5very low and as a result, much of the microbiological activities are anaerobic. Distribution of plants in estuarine habitats. Plant species and their adaptive features (1) Planktons, e.g. Diatoms:They possess air spaces in their tissues, rhizoids or false feet for attachment to rock shores and air bladder for buoyancy. (2)Algae:Algae possess chlorophyll for photosynthetic activities and small size or large surface area for floating. (3)Red Mangrove Rhizophora racemosa): It has silt roots which grow down from the stem into the soft mud and develop numerous rootlets which have air spaces for conducting air to the 66 tissues of the roots. The roots also provide support and prevent plants from being washed away by the tides. Again the seeds of red mangrove germinate while they are still on the parent plant thereby preventing the seedlings from being carried away by water current. Red Mangrove (4)White Mangrove(Avicennia Nitida):It has pneumatophores or breathing roots for exchange of gases. 67 White Mangrove Food Chain in Estuarine Habitat. 68 A t ypical food chain in an estuarine habitat may have up to three, four or five trophic levels. The phytoplanktons such as diatoms and detritus form the basic producers which support the food chain. Some examples of food chains in the estuarine habitat include: (1)Detritus – Worms – Snails – Birds. (2)Diatoms – Shrimps – Fishes. Factors affecting estuarine habitat. The factors which affect estuarine habitats are common to aquatic habitats and these include temperature, wind, relative humidity, light, pH etc. Fresh Water Habitats. Fresh water habitat is a body of water formed mainly from inland waters and contain very low level of salinity. Examples of fresh water habitats are rivers, ponds, streams, springs and lakes. Types of Fresh Waters. Fresh waters are classified on the basis of their mobility. Based on this, two types are identified. These are: (1)Lotic Fresh Waters: These include all running waters which can flow continuously in a specific direction. In other words, these are flowing or running waters, e.g. rivers, springs and streams. (2)Lentic Fresh Waters: These include standing or stagnant waters. These do not flow nor move. Example of lentic fresh waters are lakes, ponds, swamps, and dams. Characteristics of Fresh Water Habitats. The following characteristics are associated with fresh water habitats: Low Salinity: Fresh water habitats normally contain very low level of salts.It has about 0.5% of salt compared to about 3.5%for sea water. Small in Size: Fresh water habitat is usually very small compared to the ocean water which is about 75%of the earth surface. Variation in Temperature: The temperation of fresh water habitat usually varies with season and depth. Temperature at the surface of the water varies slightly with that of at the bottom of the water. High Concentration of Oxygen Content: Oxygen is usually available in all parts of the fresh water especially in the surface of the water. Shallowness of Water:Most fresh water habitats are very shallow hence sunlight can easily penetrate through the water to the bottom. Seasonal Variation: Some fresh water habitats like streams and rivers normally dry up during the dry season while others have their volume reduced. The volume of water in rivers also increases during the rainyseason. Turbidity and fast flow of rivers are also high during the rainy season than in dry season. Currents: Currents can affect the distribution of gases, salts and small organisms in fresh water habitats such as rivers and streams. 69 Major Ecological Zones of Fresh Water Habitats The zones of a lentic fresh water habitat, e.g. lake are similar to those of the marine habitats but there are no supratidal and intertidal zones. There are two major zones in a lentic fresh water habitat. These are littoral and benthic zones. (1)Littoral Zone: Littoral zone is the shallow part of fresh water habitat. It contains several plants and animals. The littoral zone has rooted vegetation at its base. It has the highest level of primary production because sunlight can easily penetrate the zone, hence photosynthetic activities are common. Plants associated with this zone include spirogyra, water lettuce, chlamydomonas, water fern,duckweed, diatoms and sedges. Animals associated with this zone include water fleas, water snails, flatworms, frogs, toads, water skaters, ducks, snakes, crocodiles, tadpoles, hippopotamus, and Hydrae. (2)Benthic Zone: Benthic zone is the deepest parts of the lentic fresh water habitat. The benthic zone does not have rooted vegetation like the littoral zone although flowering plants may occur at its surface. Plants associated with the benthic zone have well developed root system in the mud. These plants include water lily, water arum, ferns, crinum lily, commelina and grasses. Animals associated with the benthic zone include protozoa, rotifers, Hydrae, Tilapia fish, mud fish, cat fish, leeches, caddish fly larvae, larvae and pupae of mosquito, water snail, water spider, crayfish, water scorpion, water boatman and water bugs. Lotic Fresh Water Habitat In a lotic fresh water habitat e.g. rivers, there exist two zones. These are: (1)Pool Zone: In this zone, water is relatively slow and calm. (2)Rapid Zone:In this zone, is fast.The loctic fresh water is habitat is not as stratified as the lentic fresh water habitat. Adaptive features of some organisms fresh in water habitat (1)Water Lily (Nymphaea):The plant has air bladders,expanded shape and light weight which keeps it afloat.It has a long petioles attached at the centre of leaf blade which prevent them from being drawn under water by the current. 70 Water Lily. (2)Water Hycinth (Ipomea Grassipsis): They have cavities and intercellular airspace which gives them the ability to float or to maintain buoyancy on water. Water Hycinth 71 Spirogyra (3)Spirogyra: The plant has mucilagenous cover which protects them in water. 72 (4)Water Lettuce (Pistia):Water lettuce hairs on their leaves which help them to trap air and enable them to float. Water Lettuce ECOLOGICAL ADAPTATION OF PLANTS TO TERRESTRIAL HABITAT Habitat A habitat is a place where an organism is commonly found. It may be a physical area, some specific part of the earth surface, air, soil or water. Habitat is a place where organism of different species dwells and communicates. Adaptation enables plants to survive, live and grow in different areas. Adaptation enables plants and animals to live in a particular habitat. It may also be difficult for some plants to adapt in places. Adaptation expanciate on why plants are found in different environment. Example the plants that are found in mangrove swamp forest are different from the plants found in Sudan savannah. Terrestrial habitat is refers to the part of the earth land. It is inhabited by plants which lives and grow on land. The vegetation is influenced by rainfall and temperature. The abundant rainfall 73 and humidity throughout the year flourish the presence of forest while the cold temperature and low rainfall causes poor vegetation on land. Terrestrial habitat is divided into four: i. Rainforest habitat , ii. Savannah or grassland habitat, iii. Marsh or swamp habitat and iv. Arid habitat. Rainforest Habitat It is an extensive community of plants dominated by tall trees which are of different species and heights. The tropical rainforest is hot and it rains a lot, about 80 to 180 inches per year. This abundance of water can cause problems such as promoting the growth of bacteria and fungi which could be harmful to plants. Heavy rainfall also increases the risk of flooding, soil erosion, and rapid leaching of nutrients from the soil (leaching occurs when the minerals and organic nutrients of the soil are "washed" out of the soil by rainfall as the water soaks into the ground). Plants grow rapidly and quickly use up any organic material left from decomposing plants and animals. This results is a soil that is poor. The tropical rainforest is very thick, and not much sunlight is able to penetrate to the forest floor. However, the plants at the top of the rainforest in the canopy, must be able to survive 12 hours of intense sunlight every day of the year. There is a great amount of diversity in plant species in the tropical rainforest. Characteristics of Rainforest Habitat Presence of tall trees They have thin bark They have buttress roots They are hosts to epiphytes. Features of plants that makes them adapt in rainforest Plants have buttress root and tap root which enables them to support the weight of the plants. Examples iroko tree, afara tree e.t.c. They have broad leaves which enhances their mood of respiration and feeding[photosynthesis].example walnut They allow epiphytes on them. Example orchid, mistletoe Drip tips and waxy surfaces allow water to run off, to discourage growth of bacteria and fungi Buttresses and prop and stilt roots help hold up plants in the shallow soil Some plants climb on others to reach the sunlight Some plants grow on other plants to reach the sunlight Flowers on the forest floor are designed to lure animal pollinators since there is relatively no wind on the forest floor to aid in pollination Smooth bark and smooth or waxy flowers speed the run off of water Plants have shallow roots to help capture nutrients from the top level of soil. Many bromeliads are epiphytes (plants that live on other plants); instead of collecting water with roots they collect rainwater into a central reservoir from which they absorb the water through hairs on their leaves 74 Epiphytic orchids have aerial roots that cling to the host plant, absorb minerals, and absorb water from the atmosphere. Drip-tips on leaves help shed excess water. Prop roots help support plants in the shallow soil. Some plants rainwater into a reservoir. collect central Savannah or Grassland Habitat This habitat lies between rainforest and arid. It is dominated by grass, the rainfall is not enough to support the growth of the plant. Types of Grassland There are two types of grasslands which are: a. The tropical grassland [savannah] b. The temperate grassland. Tropical Grassland or Savannah The tropical grassland is located around the equator i.e. between 50 and 200North and South of the equator. Areas where this grassland is found includes; Africa where it is called savannah, in Brazil where it is called Campos, and in South America where it is called llanos. In Nigeria the savannah has three major belts. Guinea savannah, Sudan savannah and Sahel savannah. The luxuriant nature of the grassland decreases North wards from guinea to Sudan and finally to Sahel savannah. Temperate grassland or savannah. The temperate grassland is found in the interior continent of Asia, North America, South America and South Africa where the grasses appear to be uniform. Adaptive features of plants in savannah habitat 1. They have underground stem which helps the plants to withstand intense heat fire and dry season spear grass 2. They have broad and succulent trunks for storage of excess water. Example, baobab. 3. They have long root to search for underground water. Example acacia 75 4. They have a heavy and thick bark which help in reduction of transpiration and also protects them from bush fire. Example elephant grass. Marsh or Swamp Habitat Marshes are permanently or periodically covered with nutrient-rich water. Marshes are characterized by emergent vegetation that is adapted to saturated soils and by submerged vegetation that lives at deeper depths. Plants living in marshes are exposed to three environmental stresses: 1. They are frequently covered by water so they must be able to cope with low oxygen content, 2. They are often exposed to the atmosphere so they can be exposed to factors such terrestrial herbivores and fire, and 3. They are sometimes exposed to the effects of wave action or water movement. Thus, these factors have selected for the herbaceous plants with well developed root systems (that provide anchorage and storage). Salt marshes are found in estuarine areas with high (and fluctuating) salt content. Thus, salt marsh plants must have adaptations for dealing with high salt content in the water that surrounds them, a fourth type of stress. These are low land habitat which is usually flooded all the time. There are two major marsh habitats: 76 a. Fresh water marshes [FWM]. b. Salt water marshes. Fresh Water Marshes Fresh water marshes occur mainly on land beyond limit of the salt water marsh and beyond the areas influenced by tides. Fresh water from rivers overflows the river banks to flood the adjoining low lands resulting in the formation of fresh water marshes. Salt Water Marshes Salt water marshes occur along the coastal areas and they are influenced by tides. The action of the tides in the ocean causes the flooding of adjoining lowlands with brackish water resulting in the formation of salt water marshes. 1. 2. 3. 4. 5. 6. 7. Characteristics of marsh habitat. Lowland habitat High flooding Nature of soil Presence of stagnant water Presence of organic mater High rate of organic decomposition High relative humidity. Plants Found in Marshes Examples of plants commonly found in marshes includes Algae, water lettuce [Pistia], sword grasses, duckweed [Lemma], water lilies [Nymphaea], hornwort, sedges, white mangrove, red mangrove and raphia palm. Low soil oxygen content area Wetland soils have been affected by the permanent cover of water. One problem faced by plants living in marshes is the lack of oxygen in the soil. Oxygen is used by plants (and most other 77 organisms) in the process of cellular respiration in which the energy from glucose (produced by photosynthesis) is released so that the organisms can use the energy to do “biological work.” When glucose is broken down in the presence of oxygen, aerobic respiration occurs and the organisms are able to use a great deal of the stored energy in the glucose. In situations where oxygen is lacking, glucose is broken down by the process of anaerobic respiration which does not release as much energy from each molecule of glucose (aerobic respiration releases about 18 times more energy than anaerobic respiration). Not only is anaerobic much less energy efficient than aerobic respiration, but by products of anaerobic respiration, are toxic. Arid Land or Desert Habitat This refers to the area of very low rainfall and high evaporation rate. Arid lands are the driest habitat receiving less than 25cm of annual rainfall. Types of Arid Land (Deserts) There are two major types of arid lands which are hot and cold desert. Hot Desert Hot desert of the world are located on the western coast of the continent within latitude 15 0 to300 north and south of the equator. Example of hot desert is Sahara desert [North Africa, Great Australia desert and Atacama Desert of South America]. Cold Desert They are located or found in the interior of the continent around 450 to 600 North and South of the equator. The desert is found in the interior of Eurasia, north America and in Patagonia [South America]. 1. 2. 3. 4. 5. 6. 7. 8. 1. 2. 3. 4. Characteristics of Arid Land Habitat Scarcity of water Hot temperature Presence of sandy soil High sunshine Poor vegetation Predominance of strong winds Low relative humidity Presence of drought resistance plant. Adaptive Features of Plants Cactus: It is a leafless plant with prickles to reduce transpiration. It has a thick succulent stem and side branches to store water for long drought. Acacia: This is a drought resistance plant. It has deep root which absorb underground water deep down in the soil. Baobab tree: The leaves are waxy, hairy or needle shape to help reduce the rate of transpiration Wiring grass: It has narrow and slender leaves which help to reduce the rate of transpiration in the plant. 78 5. Oleander: This plant has extremely deep root which is able to absorb underground water deep down the soil. Text Books Ecology and environment biology by Puroht Agrawal Botany for degree student by A.C Dutta A textbook on plant ecology by Schond, R.S Shukla, P.S Chadel A functional approach biology [4th edition] by MBV Roberts. 79