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PLANT KINGDOM Kingdom Plantae popularly known as the ‘plant kingdom’. We must know that the plant kingdom has changed over time. Fungi, and members of the Monera and Protista having cell walls have now been excluded from Plantae though earlier classifications put them in the same kingdom. So, the cyanobacteria that are also referred to as blue green algae are not ‘algae’ any more. Let us look to different form of classifications: Artificial System of Classification of Plants: i. The system of classification in which one or very few character are considered as the key feature of classification is called as artificial system. ii. This system of classification never throws light on the affinities or relationship of the plants with one another. iii. This classification is made only on the basis of presence on absence of the particular character that taken into account leaving the others. iv. In artificial system plants are categorized into a particular taxonomical rank but the ranks are not stepwise. v. In this classification very few species are included which were discovered until that period. There are many taxonomist classify their own way of understanding in to different group taking one or few characters. These plants are grouped in to artificial groups or classes or orders. Out of these artificial system, the system which primarily proposed by Linnaeus was taken to consideration. Natural Classification: George Bentham (1800-1884) and Sir Joseph Dalton Hooker (1817-1911) were great plant explorer and geographer associated with royal botanical gardens and adopted a very comprehensive system of classification in their jointly published book genera pantarum. Feature of Classification: i. This includes the names and descriptions of all genera of seed plant known so far and classified accordingly. ii. The plant kingdom comprises about 97205 species of seed plantsunder 202 orders in which orders treated now as families. iii. This orders further grouped under several cohorts, now treated as orders. iv. This orders further grouped under several cohorts, now treated as orders. v. They placed first dicotyledons, then gymnosperms and lastly the monocotyledons. vi. They classify dicotyledonsinto 165 order gymnosperms into 3 orders and mono cotyledons into 34 orders. vii. They devide dicotyledons in to 3 divisions and 14 series which further divided into cohorts and orders. viii. The dicots started with family ranunculadceae, with free sepals and petals and indefinite number of stamens and carpels are free. The dicots e4nds with family labiatae with fused swpals and petals with definite number of carpels and stamens. ix. Monocots divided into seven series. Which are directed grouped into orders without interpretation of cohorts. Orders with epigenous flowers, i.e. orchidaceous and scitaminae were kept first, follower by orders with petaloid hypogynous flowers, i.e. liliaceae. Then kept palmaceae and araceae, finally ended with graminal and cyperaceae. Phylogenetic Classification: Phylogeny The evolutionary history of a group of genetically related organisms is called a phylogeny. It includes ancestor species and descendant species. A phylogeny is usually represented by a tree diagram called a phylogenetic tree. An early example of a phylogenetic tree is Darwin’s “Tree of Life” (see Figure below). In this diagram, Darwin was trying to show how he thought evolution had occurred. The tree shows how species evolved through time, from the bottom of the tree to the top. As species evolved, they formed new branches on the tree of life. Some of these species eventually branched into additional descendant species. Others died out, or went extinct, without leaving any descendants. . This branching diagram represents the evolutionary histories of different species. It is the only diagram that originally appeared in Darwin Modern biologists still use phylogenetic trees to represent evolutionary histories. A simple phylogenetic tree is shown in Figure below. The tips of the branches represent genetically related species. The branching points represent common ancestors. A common ancestor is the last ancestor species that two descendant species shared before they took different evolutionary paths. In the tree in Figure below, species 1 and 2 shared a more recent common ancestor with each other than with species 3. Therefore, species 1 and 2 are more closely related to one another than to species 3. . This phylogenetic tree shows how hypothetical species 1, 2, and 3 are related to one another through common ancestors. Ancestor species are like your own ancestors. Your most recent common ancestor with any siblings you may have is a shared parent. Your most recent common ancestor with a first cousin is a shared grandparent. Your most recent common ancestor with a second cousin is a shared great-grandparent. In general, the more distant the relationship between you and relatives in your own generation, the farther in the past you shared a common ancestor. The same holds true for related species. The more distant the relationship between two related species, the farther back in time they shared a common ancestor. Numerical Taxonomy which is now easily carried out using computers is based on all observable characteristics. Number and codes are assigned to all the characters and the data are then processed. In this way each character is given equal importance and at the same time hundreds of characters can be considered. Cytotaxonomy that is based on cytological information like chromosome number, structure, behaviour and chemotaxonomy that uses the chemical constituents of the plant to resolve confusions, are also used by taxonomists these days. .Gross Morphology: done on the basis of functions. .Vegetative Characterstics: Plant morphology treats both the vegetative structures of plants, as well as the reproductive structures. The vegetative (somatic) structures of vascular plants include two major organ systems: (1) a shoot system, composed of stems and leaves, and (2) a root system. .Androecium: Flowers are made up of both reproductive and non-reproductive structures arranged in four whorls. These include the calyx, the corolla, the gynoecium, and the androecium. 1 Calyx: outermost whorl made up of usually green, leaf-like structures called sepals 2 Corolla: whorl that contains often brightly colored petals 3 Androecium: third whorl that contains male reproductive structures called stamens 4 Gynoecium: innermost whorl made up of female reproductive structures called carpels Both the calyx and corolla are the non-reproductive structures of a flower, while the androecium and gynoecium are the reproductive structures. The gynoecium produces egg cells, and the androecium produces sperm cells. In this lesson, we will focus on the structure of the androecium. .Cytological: study of cell ALGAE Algae: .chlorophyll .simple .thalloid (An organism or structure resembling a thallus) Thallus .autotrophic .largely aquatic (fresh water and marine) They occur in variety of other habitats like moist stones, soils and wood. Some of them also occur in association with fungi (lichen) and animals (sloth bear). Sloth bear The size ranges from the microscopic unicellular forms like Chlamydomonas, to colonial forms like Volvox and to the filamentous forms like Ulothrix and Spirogyra. A few of the marine forms such as kelps, form massive plant bodies. VOLVOX Ulothrix REPRODUCTION: ALGAE The algae reproduce by vegetative, asexual and sexual methods. Vegetative reproduction is by fragmentation. Each fragment develops into a thallus. Asexual reproduction is by the production of different types of spores, the most common being the zoospores. They are flagellated (motile) and on germination gives rise to new plants. Sexual reproduction takes place through fusion of two gametes. These gametes can be flagellated and similar in size (as in Chlamydomonas) or non-flagellated (non-motile) but similar in size (as in Spirogyra). Such reproduction is called isogamous (form of sexual reproduction that involves gametes of similar morphology). Fusion of two gametes dissimilar in size, as in some species of Chlamydomonas is termed as anisogamous. Fusion between one large, non-motile (static) female gamete and a smaller, motile male gamete is termed oogamous, e.g., Volvox, Fucus. FUCUS USES: ALGAE Algae are useful to man in a variety of ways. At least a half of the total carbon dioxide fixation on earth is carried out by algae through photosynthesis. Being photosynthetic they increase the level of dissolved oxygen in their immediate environment. They are of importance as primary producers of energy-rich compounds which form the basis of the food cycles of all aquatic animals. Many species of Porphyra, Laminaria and Sargassum are among the 70 species of marine algae used as food. Certain marine brown and red algae produce large amounts of hydrocolloids (water holding substances), e.g., algin (brown algae) and carrageen (red algae) are used commercially. Agar, one of the commercial products obtained from Gelidium and Gracilaria are used to grow microbes and in preparations of ice-creams and jellies. Chlorella and Spirullina are unicellular algae, rich in proteins and are used as food supplements even by space travellers. The algae are divided into three main classes: Chlorophyceae, Phaeophyceae and Rhodophyceae TYPES: ALGAE Chlorophyceae The members of chlorophyceae are commonly called green algae. The plant body may be unicellular, colonial or filamentous. They are usually grass green due to the dominance of pigments chlorophyll a and b. The pigments are localised in definite chloroplasts. The chloroplasts may be discoid (disc like structure), plate-like, reticulate ,cup-shaped, spiral or ribbon-shaped in different species. Most of the members have one or more storage bodies called pyrenoids located in the chloroplasts. Pyrenoids contain protein besides starch. Some algae may store food in the form of oil droplets. Green algae usually have a rigid cell wall made of an inner layer of cellulose and an outer layer of pectose (something insoluble to water). (reticulate) Vegetative reproduction usually takes place by fragmentation or by formation of different types of spores. Asexual reproduction is by flagellated zoospores produced in zoosporangia. The sexual reproduction shows considerable variation in the type and formation of sex cells and it may be isogamous, anisogamous or oogamous. Some commonly found green algae are: Chlamydomonas, Volvox, Ulothrix, Spirogyra and Chara CHLOROPHYLL Chlorophyll a Chlorophyll a is a specific form of chlorophyll used in oxygenic photosynthesis. It absorbs most energy from wavelengths of violet-blue and orange-red light.[3] It also reflects green/yellow light, and as such contributes to the observed green color of most plants. This photosynthetic pigment is essential for photosynthesis in eukaryotes, cyanobacteria and prochlorophytes because of its role as primary electron donor in the electron transport chain. Chlorophyll b Chlorophyll b is a form of chlorophyll. Chlorophyll b helps in photosynthesis by absorbing light energy. It is more soluble than chlorophyll a in polar solvents because of its carbonyl group. Its color is yellow, and it primarily absorbs blue light. Chlorophyll c Chlorophyll c is a form of chlorophyll found in certain marine algae, including the photosynthetic Chromista(separate kingdom to protista) (e.g. diatoms, brown algae) and dinoflagellates. Chlorophyll d Chlorophyll d is a form of chlorophyll, identified by Harold Strain and Winston Manning in 1943.[1][2] It is present in marine red algae and cyanobacteria which use energy captured from sunlight for photosynthesis.[3] Chlorophyll d absorbs far-red light, at 710 nm wavelength, just outside the optical range.[4] An organism that contains chlorophyll d is adapted to an environment such as moderately deep water, where it can use far red light for photosynthesis,[5] although there is not a lot of visible light Chlorophyll c Phaeophyceae The members of phaeophyceae or brown algae are found primarily in marine habitats. They show great variation in size and form. They range from simple branched, filamentous forms (Ectocarpus) to branched forms as represented by kelps (large seaweeds), which may reach a height of 100 metres. They possess chlorophyll a, c, carotenoids (any of a class of mainly yellow, orange, or red fatsoluble pigments, including carotene, which give colour to plant parts such as ripe tomatoes and autumn leaves) and xanthophylls (yellow pigments that occur widely in nature and form one of two major divisions of the carotenoid group). They vary in colour from olive green to various shades of brown depending upon the amount of the xanthophyll pigment, fucoxanthin (accessory pigment in the chloroplasts of brown algae)present in them. Food is stored as complex carbohydrates, which may be in the form of laminarin (The molecule laminarin (also known as laminaran) is a storage glucan (a polysaccharide (polymeric carbohydrate molecules composed of long chains of monosaccharide units) of glucose) found in brown algae)or mannitol. The vegetative cells have a cellulosic wall usually covered on the outside by a gelatinous coating of algin. The protoplast contains, in addition to plastids, a centrally located vacuole and nucleus. The plant body is usually attached to the substratum by a holdfast, and has a stalk, the stipe and leaf like photosynthetic organ – the frond. Vegetative reproduction takes place by fragmentation. Asexual reproduction in most brown algae is by biflagellate (two flagella) zoospores that are pear-shaped and have two unequal laterally attached flagella. Sexual reproduction may be isogamous, anisogamous or oogamous. Union of gametes may take place in water or within the oogonium (oogamous species). The gametes are pyriform (pear-shaped) and bear two laterally attached flagella. The common forms are Ectocarpus, Dictyota, Laminaria, Sargassum and Fucus . Polymer: large molecule, or macromolecule, composed of many repeated subunits. Because of their broad range of properties, both synthetic and natural polymers play an essential and ubiquitous role in everyday life Ectocarpus Mannitol: a colourless sweet-tasting crystalline alcohol which is found in many plants and is used in various foods and medical products. Rhodophyceae Rhodophyta are commonly called red algae because of the red pigment, rphycoerythrin (red protein-pigment) in their body. Majority of the red algae are marine with greater concentrations found in the warmer areas. They occur in both well-lighted regions close to the surface of water and also at great depths in oceans where relatively little light penetrates. Red algae are multicellular. Some of them have complex body organisation. The food is stored as floridean (type of storage glucan) starch which is very similar to amylopectin (highly branched polymer found in plants)and glycogen (form of storage in animals and fungi) in structure. The red algae usually reproduce vegetatively by fragmentation. They reproduce asexually by non-motile spores and sexually by non-motile gametes. Sexual reproduction is oogamous and accompanied by complex post fertilisation developments (After fertilization, a series of event occurs in the zygote to develop into a seed). The common members are: Polysiphonia, Porphyra , Gracilaria and Gelidium. Gracilaria Gelidium BRYOPHYTES Bryophytes include the various mosses and liverworts that are found commonly growing in moist shaded areas in the hills . Bryophytes are also called amphibians of the plant kingdom because these plants can live in soil but are dependent on water for sexual reproduction. They usually occur in damp, humid and shaded localities. They play an important role in plant succession on bare rocks/soil. PLANT SUCCESSION Many plant communities are not self-sustaining. A field in the temperate deciduous forest biome will remain a field only as long as it is grazed by animals or mowed regularly. If these factors are eliminated, the balance tips in favor of other species. The newcomers will, in turn, establish conditions that no longer favor them but promote the growth of still other species. The revered naturalist and writer, Henry David Thoreau, called this process succession. Primary Plant Succession The process of plant succession begins just as soon as a land area capable of supporting plant life is formed. Some examples: 5 accumulation of sand dunes at the edge of the ocean or a lake 6 cooling of a lava flow 7 exposure of rock by a retreating glacier Bare rock succession in a temperate deciduous forest biome • The first colonizers are lichens and certain mosses. Acids secreted by the lichens attack the rock and provide bits of soil. Additional soil particles may be formed by weathering or be blown in from elsewhere. Damage and decay to the lichens supplies some humus, and eventually enough soil is formed to support other mosses. • The growth, death, and decay of mosses produces more humus, and soon there is enough to support the growth of • grasses and • shrubby growth such as low-bush blueberries and huckleberries. These, in turn, provide the conditions for such sun-loving, fast-growing species as • gray birch trees and poplars (quaking aspens). • In time, white pines replace these. In the dense shade of mature white pines, only shade-tolerant maples and beech seedlings can gain a foothold. If the shallow-rooted white pines are removed by a hurricane or by lumbering, the maples and beeches can take over. Bog succession Another example of plant succession occurs as shallow ponds gradually fill in with soil washed in from the surrounding terrain and organic matter produced by underwater plants. As we walk from the edge of a poorly-drained, boggy pond back into a temperate deciduous forest, we pass through a series of zones that recreate in space the plant succession that has been occurring in time. • From the swamp loosestrife at the waters edge past • sphagnum moss, leatherleaf, sheep laurel, and pitcher plants, then • swamp azaleas, high-bush blueberries and poison sumac, followed by • black spruce and American larch and, finally, • swamp maples and alders one passes concentric zones, each representing a later stage of plant succession as the soil has become firmer and the shade denser. Secondary Plant Succession Lumbering, farming, fires, and hurricanes interrupt the process of succession by removing the dominant plants in the community. Their elimination sets the stage for a new succession to begin. The many abandoned farms in New England (I live on one) illustrate this. People often wonder why our pioneers built stone walls through the woods. The answer is that they did not. The walls in the woods today once marked the boundaries of fields and pastures, but when cultivation and grazing ceased, a secondary succession began. Where I live, • • • • • the grass of abandoned fields soon became invaded by low-growing, herbaceous species like plantains, and golden rods. These were quickly followed by woody shrubs like the common juniper, blueberries and gray-stemmed dogwood Soon sun-loving gray birch, poplars, and Eastern red cedar ("old-field cedar") became established. White pines or, in sandy well-drained locations, oaks have followed. These will persist until some disturbance such as fire, hurricanes, or lumbering open up the land, and the process of secondary succession begins again. In general, plant succession is a reflection of the increasing efficiency of the community at intercepting the energy of the sun and converting it into chemical energy. As one stage of succession follows another, • the biomass of the community increases. This is the outcome of the increasing amount of • net productivity — calories stored by the plant community. • This, in turn, provides calories for a larger community of consumers. • As succession continues, the diversity of species in the community increases — at least for a time. • When the system approaches its climax, the rate of increase in net productivity of the plants is consumed by its own heterotrophs. • The system comes into equilibrium and reaches peak efficiency at channeling the energy of the sun into the food web of the community. The graph (from Whittaker, R. H., Communities and Ecosystems, Macmillan, 1970) shows the changes in number of species, biomass, and net productivity during secondary succession in a temperate deciduous forest over a period of 160 years. The plant body of bryophytes is more differentiated than that of algae. It is thallus-like and prostrate (growing along the ground)or erect (straight), and attached to the substratum by unicellular or multicellular rhizoids. They lack true roots, stem or leaves. They may possess root-like, leaf-like or stem-like structures. Rhizoid: a filamentous outgrowth or root hair on the underside of the thallus in some lower plants, especially mosses and liverworts, serving both to anchor the plant and (in terrestrial forms) to conduct water. The main plant body of the bryophyte is haploid (having a single set of unpaired chromosomes). It produces gametes, hence is called a gametophyte. The sex organs in bryophytes are multicellular. The male sex organ is called antheridium. They produce biflagellate antherozoids. The female sex organ called archegonium is flask-shaped and produces a single egg. The antherozoids are released into water where they come in contact with archegonium. An antherozoid fuses with the egg to produce the zygote (an ovum (female gamete) and a sperm cell (male gamete)—combine to form a single diploid cell). Zygotes do not undergo reduction division immediately. They produce a multicellular body called a sporophyte. The sporophyte is not free-living but attached to the photosynthetic gametophyte and derives nourishment from it. Some cells of the sporophyte undergo reduction division (meiosis) to produce haploid spores. These spores germinate to produce gametophyte. Bryophytes in general are of little economic importance but some mosses provide food for herbaceous mammals, birds and other animals. Species of Sphagnum, a moss, provide peat that have long been used as fuel, and because of their capacity to hold water as packing material for trans-shipment of living material. Mosses along with lichens are the first organisms to colonise rocks and hence, are of great ecological importance. They decompose rocks making it suitable for the growth of higher plants. Since mosses form dense mats on the soil, they reduce the impact of falling rain and prevent soil erosion. The bryophytes are divided into liverworts and mosses. LIVERWORTS The liverworts grow usually in moist, shady habitats such as banks of streams, marshy ground, damp soil, bark of trees and deep in the woods. The plant body of a liverwort is thalloid, e.g., Marchantia. The thallus is dorsiventral (having dissimilar dorsal and ventral surfaces)and closely appressed (push) to the substrate (layer). The leafy members have tiny leaf-like appendages (a thing that is added or attached to something larger or more important) in two rows on the stem-like structures. Asexual reproduction in liverworts takes place by fragmentation of thalli, or by the formation of specialised structures called gemmae (sing. gemma). Gemmae are green, multicellular, asexual buds, which develop in small receptacles (a hollow object used to contain something) called gemma cups located on the thalli. The gemmae become detached from the parent body and germinate to form new individuals. During sexual reproduction, male and female sex organs are produced either on the same or on different thalli. The sporophyte is differentiated into a foot, seta and capsule. After meiosis, spores are produced within the capsule. These spores germinate to form freeliving gametophytes. A sporophyte is the diploid multicellular stage in the life cycle of a plant or alga. It develops from the zygote produced when a haploid egg cell is fertilized by a haploid sperm and each sporophyte cell therefore has a double set of chromosomes, one set from each parent. Mosses The predominant stage of the life cycle of a moss is the gametophyte which consists of two stages. The first stage is the protonema (A protonema (plural: protonemata) is a thread-like chain of cells that forms the earliest stage (the haploid phase) of a bryophyte or pteridophyte life cycle)stage, which develops directly from a spore. It is a creeping, green, branched and frequently filamentous stage. The second stage is the leafy stage, which develops from the secondary protonema as a lateral bud. They consist of upright, slender axes bearing spirally arranged leaves. They are attached to the soil through multicellular and branched rhizoids. This stage bears the sex organs. Vegetative reproduction in mosses is by fragmentation and budding in the secondary protonema. In sexual reproduction, the sex organs antheridia (male sex organ) and archegonia (female sex organ) are produced at the apex (highest)of the leafy shoots. After fertilisation, the zygote develops into a sporophyte, consisting of a foot, seta (stalk supporting the capsule of a moss or liverwort, and supplying it with nutrients) and capsule. The sporophyte in mosses is more elaborate than that in liverworts. The capsule contains spores. Spores are formed after meiosis. The mosses have an elaborate mechanism of spore dispersal. Common examples of mosses are Funaria, Polytrichum and Sphagnum. PTERIDOPHYTES The Pteridophytes include horsetails (Equisetum or horse tails is the only living genus in Equisetaceae, a family of vascular plants that reproduce by spores rather than seeds) and ferns. Pteridophytes are used for medicinal purposes and as soil-binders (Soil binders consist of applying and maintaining a soil stabilizer to exposed soil). They are also frequently grown as ornamentals. Evolutionarily, they are the first terrestrial plants to possess vascular tissues – xylem and phloem. In pteridophytes, the main plant body is a sporophyte which is differentiated into true root (A true root system consists of a primary root and secondary roots or lateral roots), stem and leaves . These organs possess well-differentiated vascular tissues. The leaves in pteridophyta are small (microphylls: very short leaf) as in Selaginella (Selaginella is the sole genus of vascular plants in the family Selaginellaceae, the spikemosses or lesser clubmosses) or large (macrophylls) as in ferns. The sporophytes bear sporangia that are subtended by leaf-like appendages called sporophylls. In some cases sporophylls may form distinct compact structures called strobili or cones (Selaginella, Equisetum or horse tail). The sporangia produce spores by meiosis in spore mother cells. The spores germinate to give rise to inconspicuous (not clearly visible), small but multicellular, free-living, mostly photosynthetic thalloid gametophytes called prothallus. These gametophytes require cool, damp, shady places to grow. Because of this specific restricted requirement and the need for water for fertilisation, the spread of living pteridophytes is limited and restricted to narrow geographical regions. The gametophytes bear male and female sex organs called antheridia and archegonia, respectively. Water is required for transfer of antherozoids – the male gametes released from the antheridia, to the mouth of archegonium. Fusion of male gamete with the egg present in the archegonium result in the formation of zygote. Zygote thereafter produces a multicellular well-differentiated sporophyte which is the dominant phase of the pteridophytes. In majority of the pteridophytes all the spores are of similar kinds; such plants are called homosporous. Genera like Selaginella and Salvinia (Salvinia, a genus in the family Salviniaceae, is a floating fern named in honor of Anton Maria Salvini, a 17th-century Italian scientist. Watermoss is a common name for Salvinia) which produce two kinds of spores, macro (large) and micro (small) spores, are known as heterosporous. The megaspores and microspores germinate and give rise to female and male gametophytes, respectively. The female gametophytes in these plants are retained on the parent sporophytes for variable periods. The development of the zygotes into young embryos take place within the female gametophytes. This event is a precursor to the seed habit considered an important step in evolution. SEED HABIT A seed consists of an embryo, stored food and a seed coat. The seed habit is the most complex and evolutionary successful method of sexual reproduction. It is found in vascular pIants. Today, seed plants, gymnosperms and angiosperms flowering plants are the most diverse lineage within the vascular plants. Most of this diversity in angiosperms occurred during Cretaceous time. The seed plants have an adaptive advantage. They occur in a wide variety of habitats and dominate today’s flora. This evolutionary success is due to the seed. It is one of the most dramatic innovations during land plant evolution. The origin and evolution of the seed habit was started in late Devonian times about 385 M. The pteridophytes are further classified into four classes: Psilopsida (Psilotum); Lycopsida (Selaginella, Lycopodium), Sphenopsida (Equisetum) and Pteropsida (Dryopteris, Pteris, Adiantum). Horse-tails Clubmosses: Lycopodium is a genus of clubmosses, also known as ground pines or creeping cedar, in the family Lycopodiaceae, a family of fern-allies Selaginella Lycopodium Sphenopsida Pteropsida Dryopteris, Pteris, Adiantum GYMNOSPERMS The gymnosperms (gymnos : naked, sperma : seeds) are plants in which the ovules are not enclosed by any ovary wall and remain exposed, both before and after fertilisation. The seeds that develop post-fertilisation, are not covered, i.e., are naked. Gymnosperms include medium-sized trees or tall trees and shrubs (Figure 3.4). One of the gymnosperms, the giant redwood tree Sequoia is one of the tallest tree species. The roots are generally tap roots. Roots in some genera have fungal association in the form of mycorrhiza (Pinus), while in some others (Cycas) small specialised roots called coralloid roots are associated with N2fixing cyanobacteria. The stems are unbranched (Cycas) or branched (Pinus, Cedrus). The leaves may be simple or compound. In Cycas the pinnate leaves persist for a few years. The leaves in gymnosperms are well-adapted to withstand extremes of temperature, humidity and wind. In conifers, the needlelike leaves reduce the surface area. Their thick cuticle (a protective and waxy or hard layer covering the epidermis of a plant, invertebrate, or shell) and sunken (having sunk or been submerged in water) stomata also help to reduce water loss. The gymnosperms are heterosporous (Heterospory is the production of spores of two different sizes and sexes by the sporophytes of land plants); they produce haploid microspores and megaspores. The two kinds of spores are produced within sporangia that are borne on sporophylls which are arranged spirally along an axis to form lax or compact strobili or cones. The strobili bearing microsporophylls and microsporangia (a sporangium containing microspores)are called microsporangiate or male strobili. The microspores develop into a male gametophytic generation which is highly reduced and is confined to only a limited number of cells. This reduced gametophyte is called a pollen grain. The development of pollen grains take place within the microsporangia. The cones bearing megasporophylls with ovules or megasporangia are called macrosporangiate or female strobili. The male or female cones or strobili may be borne on the same tree (Pinus) or on different trees (Cycas). The megaspore mother cell is differentiated from one of the cells of the nucellus. The nucellus is protected by envelopes and the composite structure is called an ovule. The ovules are borne on megasporophylls which may be clustered to form the female cones. The megaspore mother cell divides meiotically to form four megaspores. One of the megaspores enclosed within the megasporangium (nucellus) develops into a multicellular female gametophyte that bears two or more archegonia or female sex organs. The multicellular female gametophyte is also retained within megasporangium. Unlike bryophytes and pteridophytes, in gymnosperms the male and the female gametophytes do not have an independent free-living existence. They remain within the sporangia(a receptacle in which asexual spores are formed)retained on the sporophytes. The pollen grain is released from the microsporangium. They are carried in air currents and come in contact with the opening of the ovules borne on megasporophylls. The pollen tube carrying the male gametes grows towards archegonia in the ovules and discharge their contents near the mouth of the archegonia. Following fertilisation, zygote develops into an embryo and the ovules into seeds. These seeds are not covered. .EMBRYO: an unborn or unhatched offspring in the process of development Sequoia Cedrus , Conifers ANGIOSPERMS Unlike the gymnosperms where the ovules are naked, in the angiosperms or flowering plants, the pollen grains and ovules are developed in specialised structures called flowers. In angiosperms, the seeds are enclosed by fruits. The angiosperms are an exceptionally large group of plants occurring in wide range of habitats. They range in size from tiny, almost microscopic Wolfia to tall trees of Eucalyptus (over 100 metres). They provide us with food, fodder, fuel, medicines and several other commercially important products. They are divided into two classes : the dicotyledons and the monocotyledons. The dicotyledons are characterised by having two cotyledons in their seeds while the monocolyledons have only one. The male sex organs in a flower is the stamen. Each stamen consists of a slender filament with an anther at the tip. The anthers, following meiosis, produce pollen grains. The female sex organs in a flower is the pistil or the carpel. Pistil consists of an ovary enclosing one to many ovules. Within ovules are present highly reduced female gametophytes termed embryosacs. The embryo-sac formation is preceded by meiosis. Hence, each of the cells of an embryo-sac is haploid. Each embryo-sac has a three-celled egg apparatus – one egg cell and two synergids, three antipodal cells and two polar nuclei. The polar nuclei eventually fuse to produce a diploid secondary nucleus. Pollen grain, after dispersal from the anthers, are carried by wind or various other agencies to the stigma of a pistil. This is termed as pollination. The pollen grains germinate on the stigma and the resulting pollen tubes grow through the tissues of stigma and style and reach the ovule. The pollen tubes enter the embryo-sac where two male gametes are discharged. One of the male gametes fuses with the egg cell to form a zygote (syngamy). The other male gamete fuses with the diploid secondary nucleus to produce the triploid primary endosperm nucleus (PEN). Because of the involvement of two fusions, this event is termed as double fertilisation, an event unique to angiosperms. The zygote develops into an embryo (with one or two cotyledons) and the PEN develops into endosperm which provides nourishment to the developing embryo. The synergids and antipodals degenerate after fertilisation. During these events the ovules develop into seeds and the ovaries develop into fruit. PLANT LIFE CYCLES AND ALTERNATION OF GENERATIONS NCERT In plants, both haploid and diploid cells can divide by mitosis. This ability leads to the formation of different plant bodies - haploid and diploid. The haploid plant body produces gametes by mitosis. This plant body represents a gametophyte. Following fertilisation the zygote also divides by mitosis to produce a diploid sporophytic plant body. Haploid spores are produced by this plant body by meiosis. These in turn, divide by mitosis to form a haploid plant body once again. Thus, during the life cycle of any sexually reproducing plant, there is an alternation of generations between gamete producing haploid gametophyte and spore producing diploid sporophyte. However, different plant groups, as well as individuals representing them, differ in the following patterns: 1. Sporophytic generation is represented only by the one-celled zygote. There are no free-living sporophytes. Meiosis in the zygote results in the formation of haploid spores. The haploid spores divide mitotically and form the gametophyte. The dominant, photosynthetic phase in such plants is the free-living gametophyte. This kind of life cycle is termed as haplontic. Many algae such as Volvox, Spirogyra and some species of Chlamydomomas represent this pattern. 2. On the other extreme, is the type wherein the diploid sporophyte is the dominant, photosynthetic, independent phase of the plant. The gametophytic phase is represented by the single to few-celled haploid gametophyte. This kind of lifecycle is termed as diplontic. All seedbearing plants i.e. gymnosperms and angiosperms, follow this pattern . 3. Bryophytes and pteridophytes, interestingly, exhibit an intermediate condition (Haplo-diplontic); both phases are multicellular and often free-living. However, they differ in their dominant phases. A dominant, independent, photosynthetic, thalloid or erect phase is represented by a haploid gametophyte and it alternates with the short- lived multicelluler sporophyte totally or partially dependent on the gametophyte for its anchorage and nutrition. All bryophytes represent this pattern. The diploid sporophyte is represented by a dominant, independent, photosynthetic, vascular plant body. It alternates with multicellular, saprophytic/autotrophic, independent but short-lived haploid gametophyte. Such a pattern is known as haplo-diplontic life cycle. All pteridophytes exhibit this pattern (Figure 3.7 c). Interestingly, while most algal genera are haplontic, some of them such as Ectocarpus, Polysiphonia, kelps are haplo-diplontic. Fucus, an alga is diplontic. REFERENCE THE LIFE CYCLE OF PLANTS Alternation of Generations All plants undergo a life cycle that takes them through both haploid and diploid generations. The multicellular diploid plant structure is called the sporophyte, which produces spores through meiotic (asexual) division. The multicellular haploid plant structure is called the gametophyte, which is formed from the spore and give rise to the haploid gametes. The fluctuation between these diploid and haploid stages that occurs in plants is called the alternation of generations. The way in which the alternation of generations occurs in plants depends on the type of plant. In bryophytes (mosses and liverworts), the dominant generation is haploid, so that the gametophyte comprises what we think of as the main plant. The opposite is true for tracheophytes (vascular plants), in which the diploid generation is dominant and the sporophyte comprises the main plant. Bryophyte Generations Bryophytes are nonvascularized plants that are still dependent on a moist environment for survival (see Plant Classification, Bryophytes . Like all plants, the bryophyte life cycle goes through both haploid (gametophyte) and diploid (sporophyte) stages. The gametophyte comprises the main plant (the green moss or liverwort), while the diploid sporophyte is much smaller and is attached to the gametophyte. The haploid stage, in which a multicellular haploid gametophyte develops from a spore and produces haploid gametes, is the dominant stage in the bryophyte life cycle. The mature gametophyte produces both male and female gametes, which join to form a diploid zygote. The zygote develops into the diploid sporophyte, which extends from the gametophyte and produces haploid spores through meiosis. Once the spores germinate, they produce new gametophyte plants and the cycle continues. Tracheophyte Generations Tracheophytes are plants that contain vascular tissue; two of the major classes of tracheophytes are gymnosperms (conifers) and angiosperms (flowering plants). Tracheophytes, unlike bryophytes, have developed seeds that encase and protect their embryos. The dominant phase in the tracheophyte life cycle is the diploid (sporophyte) stage. The gametophytes are very small and cannot exist independent of the parent plant. The reproductive structures of the sporophyte (cones in gymnosperms and flowers in angiosperms), produce two different kinds of haploid spores: microspores (male) and megaspores (female). This phenomenon of sexually differentiated spores is called heterospory. These spores give rise to similarly sexually differentiated gametophytes, which in turn produce gametes. Fertilization occurs when a male and female gamete join to form a zygote. The resulting embryo, encased in a seed coating, will eventually become a new sporophyte. Problem : In gymnosperms and angiosperms, spores of two distinct sexes are produced and give rise to sex-specific gametophytes. What is this phenomenon called? Heterospory. Problem : What is the difference between microspores and megaspores? Microspores are spores that are specifically male and give rise to male gametophytes; megaspores, on the other hand, are specifically female and give rise to female gametophytes. Problem : Define alternation of generations. The fluctuation between the diploid (sporophyte) and haploid (gametophyte) life stages that occurs in plants. Problem : Through what process do sporophytes generate spores? Meiosis. Fertilization in plants occurs when haploid gametes meet to create a diploid zygote, which develops into an embryo. In gymnosperms (conifers) and angiosperms (flowering plants), the meeting of the gametes occurs in the following way: male gametes are enclosed in pollen grains and are carried by wind or insects to the female reproductive organs. The final product of fertilization--the embryo--is encased in a seed. For this reason, these two types of tracheophytes are termed seed plants. Gymnosperm Fertilization The female gametophyte contains several archegonia, where the egg cells originate and develop. The gametophyte itself is surrounded by layers of sporangia and integument; all of these elements comprise an ovule, which is found on the surface of a female cone. Fertilization occurs when pollen grains (male gametophytes) are carried by the wind to the open end of an ovule, which contains the eggs, or female gametophyte. There, the pollen grain develops an outgrowth called a pollen tube, which eventually penetrates to the egg cell within one of the archegonia. The sperm cells within the pollen tube then vie to fertilize the egg. Once fertilization has occurred, the embryo develops within the female gametophyte, and the ovule becomes the seed, complete with a food source (the gametophyte tissue) and a seed coat (the integument). This embryo, which will eventually become a new sporophyte, consists of two embryonic leaves, the epicotyl and hypocotyl. Angiosperm Fertilization The female reproductive organ of angiosperms is the pistil, located in the middle of the flower. As in gymnosperms, the male gametophyte is the pollen grain. In order for fertilization to occur in most flowering plants, insects or other animals must transport the pollen to the pistil. A major distinguishing feature of angiosperms is the practice of double fertilization. Figure %: Double Fertilization An angiosperm ovule contains an egg cell and a diploid fusion nucleus, which is created through the joining of two polar nuclei within the ovule. When a pollen grain comes into contact with the stigma, or top of the pistil, it sends a pollen tube down into the ovary at the pistil's base. As the pollen tube penetrates the ovule, it releases two sperm cells. One fuses with the egg to create a diploid zygote, while the other joins with the fusion nucleus to form a triploid nucleus. This triploid nucleus turns into an endosperm, which nourishes the developing embryo (filling the role of gametophyte tissue in the gymnosperm seed). As in gymnosperms, the ovule becomes a seed, encasing the embryo and endosperm in a seed coat. But unlike gymnosperms, in angiosperms the ovary containing the ovules develops into a fruit after fertilization. The fruit gives the embryos the double benefit of added protection against desiccation and increased dispersal, since it is eaten by far-ranging animals who then excrete the seeds. In order for fertilization to occur, angiosperms either self-pollinate, in which a particular plant fertilizes itself, or cross-pollinate, in which one plant is fertilized by another of the same species. Cross-pollination generally produces far more vigorous plants, and is encouraged through differential development of the male and female gametophytes on a flower, or through the positioning of these gametophytes so that self- pollination is difficult. Problem : How does a pollen grain function in fertilization? Pollen grains, or the male gametophytes of flowering plants, carry the sperm cells to the female reproductive organs. They are transported either by the wind (as in conifers) or by insects (as in most flowering plants). Problem : What are the three main components of a seed, and from what are they derived? The three components of the seed are the embryo, the food source (derived from gametophyte tissue in gymnosperms and from endosperm in angiosperms), and the seed coat (derived from the integument of the ovule). Problem : What is the female reproductive organ of a flowering plant, and where must pollen land in order to fertilize the eggs it contains? Pistils are the female reproductive organs of angiosperms, and pollen grains must land on their stigmas, or tips, in order to reach the egg cells. Problem : Are ovaries characteristic of angiosperms or gymnosperms? What are their functions? Ovaries, found in angiosperms, enclose the ovules and develop into fruits after fertilization. Ovaries protect the embryos from drying out and help in their dispersal. Problem : How is cross-pollination encouraged in most angiosperms? Cross-pollination is encouraged through differential development of the male and female gametophytes on a flower, or through the positioning of these gametophytes so that self-pollination is difficult. Through asexual reproduction, many plants can produce genetically identical offshoots (clones) of themselves, which then develop into independent plants. This process is called vegetative propagation, or vegetative reproduction. One way in which vegetative propagation occurs is through fragmentation, a process in which a severed plant part can grow into a whole new plant. Other modes of vegetative propagation include the production of specialized structures such as tubers, runners, and bulbs. The advantages to this kind of asexual reproduction, which can occur either naturally or artificially, stem from the fact that it can occur more rapidly than seed propagation and can allow a genetically superior plant to produce unlimited copies of itself without variation. Tubers Tubers, such as potatoes, are fleshy underground storage structures composed of enlarged parts of the stem. A tuber functions in asexual propagation as a result of the tiny scale leaves equipped with buds that grow on its surface. Each of these buds can form a new plant, genetically identical to the parent. Runners Runners, such as those found on strawberry plants, are slender horizontal stems that spread outward from the main plant . Entirely new plants can develop from nodes located at intervals on the runners; each node can give rise to new roots and shoots. Bulbs Bulbs, such as onions and tulips, are roughly spherical underground buds with fleshy leaves extending from their short stems. Each bulb contains several other buds which can give rise to new plants. Problem : Explain the difference between seed propagation and vegetative propagation. With seed propagation, reproduction occurs through embryos (contained in seeds) that are produced sexually. With vegetative propagation, by contrast, plants reproduce asexually through genetically identical offshoots (clones), which then develop into independent plants. Problem : What are the advantages to vegetative propagation in comparison with seed propagation? Vegetative propagation can occur more rapidly than seed propagation and can allow a genetically superior plant to produce unlimited copies of itself without variation. Problem : What is the name for the process by which a severed plant part grows into a whole new plant? Fragmentation. Problem : List three kinds of specialized structures developed by some plants to allow vegetative propagation. Tubers, runners, and bulbs. Problem : What occurs at the nodes of runners? Each node can give rise to new roots and shoots, so that an entirely new plant can develop. Grafting is an artificial form of vegetative propagation in which parts of two young plants are joined together, first by artificial means and then by tissue regeneration. Typically, a twig or bud is cut from one plant and joined to a rooted plant of a related species or variety. The twig or bud is called the scion, and the plant onto which is it grafted (and that provides the roots) is called the stock. The scion eventually develops into an entire shoot system. Grafting often allows horticulturalists to combine the best features of two different plants into one plant. Sometimes the stock and scion retain independent characteristics, and sometimes the stock alters the characteristics of the scion in some desirable way. Problem : What is grafting? Grafting is an artificial form of vegetative propagation in which parts of two young plants are joined together, first by artificial means and then by tissue regeneration. Problem : From a horticultural standpoint, what is the purpose of grafting? Grafting often makes it possible to combine the best features of two different plants into a single plant. Problem : When parts of two plants are grafted together, what is the rootproviding plant called? The stock. Problem : When parts of two plants are grafted together, what is the rootless twig or bud called? The scion. Problem : Do the characteristics of the stock always affect those of the scion? No; although this is sometimes the case, it is possible for the scion to retain independent characteristics after being grafted onto a stock with different characteristics.