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Plant Science
Plants are photosynthetic organisms containing different light absorbing pigments such as chlorophyll . This kingdom includes four
main groups as shown below:
Bryophytes : mosses
Plants are photosynthetic organisms containing different light absorbing pigments such as chlorophyll . This kingdom includes four
main groups as shown below:
These are primitive plants with heights not exceeding 10 millimeters. They have structures similar to roots called rhizoids, stems
and leaves. The rhizoids are mainly used to anchor the plant to the substratum since absorption of nutrients can take place by all
parts of the plants. This is because they are very small with a big surface area to volume ratio for exchange of substances, and they
are surrounded by a humid environment which facilitates the absorption of water and minerals. The plant lacks a transport system
(no vascular tissue), and the nutrients absorbed from the surroundings can be transported efficiently from cell to cell without the
need for xylem tubes.
Sexual reproduction in these plants involves the fusion of eggs and sperms. Sperms have flagella and so
they swim towards the egg. These plants need water for the swimming of the sperms and they need to live
in moist conditions in order to absorb water and nutrients from their surroundings. Bryophytes can also
reproduce asexually by spores growing in sporangia, which burst open when mature releasing the spores.
These spores fall on the soil and grow again into the mature plant.
Filicinophytes (ferns)
These plants are more advanced than bryophytes since they
have vascular tissue made of xylem and phloem. This allows
the plant to grow to much bigger sizes than bryophytes and in
some forests they can grow to a height of 10 meters. The
absorbed water and minerals are carried upwards in xylem
tubes to all the parts of the plant. They can live in dryer
habitats than those of the bryophytes, but still they need water
for their reproduction since they produce sperms that must
swim to the egg in the female structure. These plants can also
reproduce by spores which grow in clusters of sporangia that
appear as spots on the surface of the leaf.
Coniferophytes
These plants are more advanced than the filicinophytes and more adapted to terrestrial life due to the following structures:
1. More advanced vascular tissue
2. Leaves are adapted to conserve water and so in most species the leaves are needle shaped, with thick waxy cuticle and few
numbers of stomata .
3. The roots grow to deep layers in the soil in order to absorb more water
4. The male gametes are pollen grains and not sperms. Pollen is carried by wind and insects and so there is no dependence on
water for reproduction .
5. The male cone produces huge amounts of pollen which is shaken by the wind and falls on the female cone. The female
cone is woody and thus it protects the growing embryo .
6. The presence of seeds in the life cycle of this group is considered a very successful adaptation to terrestrial life. The seed
can withstand dryness on land and can live dormant for thousands of years. This conserves the species and leads to its
wider spread.
7. The seeds are carried by wind and they have a wing like structure that allows them to be carried a long distance away from
the plant. Seed dispersal is important in the conservation of the species since it provides more chances of survival as plants
spread in varied environments where they might adapt.
8. Due to all the above adaptations these plants are spread in many types of habitats and they form forests in various biomes.
The trees are evergreen and so they grow to huge heights and sizes
Coniferous trees: female (left) and male (right) cones grow on the same tree. However the male cones are short lived they grow
mature, shed their pollen which is shaken by the wind and then they dry and fall off. The female cones however, live for a longer
time and they contain the eggs which are fertilized by the pollen, they grow into seeds which fall off the woody cone. These seeds
are winged and they are carried by the wind to other areas where they germinate, grow into seedlings and then into a mature tree.
These pictures were taken from the same tree at different times of the year. Some of the cones are open and have shed their seeds,
while others are still closed with seeds inside them. When these cones dry they open releasing their mature seeds.
Angiospermophytes
These are the flowering plants and in addition to all the structures possessed by the
coniferophytes this group has developed the following structures: Flowers for sexual
reproduction. The flower has led to more successful and efficient methods of pollination due
to the attraction of insects. Insects are attracted to large, colorful, scented and nectar
containing flowers. Pollen stick to the body of insects and by visiting many flowers the insect
transfers pollen to other flowers. The fruit: this structure contains the seed and it helps in seed
dispersal.
Different methods of seed dispersal have evolved in different plants and this includes dispersal by wind, animals, insects and water.
Juicy fruits are eaten by animals, which excrete the undigested seeds at a distance away from the mother plant.
Angiosperms are the most adapted to desert life and so they are the main representatives of desert flora. They are called xerophytes
and their adaptations are described in the next section.
The body of an angiosperm is divided into different parts each with certain functions
and structure. The roots are for absorption and anchoring the plant to the soil. The
stem carries the upper parts of the plant including the leaves, flowers and fruits.
The bud contains a young developing flower or a cluster of growing leaves.
The leaves are the main photosynthetic organs of flowering plants, since they have
chlorophyll and many other adaptations to make them specialized for this process.
Transverse sections in the different parts of a dicotyledonous (dicot) plant are shown
on the next page.
Cross section in the leaf of a dicotyledonous plant
Cross section in a dicot stem
cross section in a dicot root
Transport in angiospermophytes
Absorption of water
Plants absorb water and minerals from the soil. Water enters the plant by osmosis and it always moves from dilute to concentrated
solutions. In other terms it moves from hypotonic to hypertonic solutions. This can also be described as movement from areas of
higher water potential to areas of lower water potential. Water enters the roots through root hair cells, which are adapted for
absorption of water and solutes.
Plants need to grow in hypotonic soils so that water enters the root hair cells by osmosis. If the situation is reversed and plants are
grown in hypertonic soils, water will move from the roots to the soil by osmosis . This results in shrinking of the vacuole, the
cytoplasm and the cell membrane. The plants wilt and if this persists for some time the plant would die. In hypotonic solutions
water enters plant cells and makes them turgid, this contributes to support and turgidity of the plant. In hypertonic solutions, water
leaves the plant cell and so it shrinks and loses support, this is called plasmolysis. A plasmolysed cell is characterized by shrinking
of the cell membrane and its detachment from the cell wall. Herbaceous plants such as grasses and weeds depend mostly on cell
turgidity for support. The leaves of plants also depend mostly on turgidity as a means of support that makes the leaves spread wide
and not droop and fold. This helps them absorb maximum amounts of light and carbon dioxide from the atmosphere.
Transport of water inside the plant
As water enters a root hair cell, it makes the cell hypotonic in comparison to an adjacent cortex cell, and so water moves from it to
the cortex cell by osmosis. This makes the cortex cell hypotonic compared to another cortex cell adjacent to it and so water moves
from it to the next cell. The cells of the cortex are connected together by channels called plasmodesmata. These channels connect
the cytoplasm of all the cortex cells into one continuous fluid called the symplast. The fluid outside the cells also forms one
continuous fluid called the apoplast. Hence water can move in the root from cell to cell through the plasmodesmata (the symplastic
path) or through spaces between the cells and this is the apoplastic path. This movement continues until water reaches the xylem
tubes, then it starts ascending in these tubes due to the following properties:
1. Capillary action : xylem tubes are very thin, they are microscopic and so a microscopic amount of water entering the tube
would move a long distance in it.
2. Adhesion forces between water and the walls of xylem, and cohesion forces between the water molecules, help water to
3.
climb in one continuous column without interruptions and without slipping, since it sticks to the walls of xylem as it is
moving up. Imagine climbing a wall; you need to be in one piece (cohesion forces), and you need to hold on to something
on the wall (adhesion forces).
Transpiration is the loss of water from the stomata of the leaves. This makes epidermal cells in the leaf hypertonic and so
they start pulling water from adjacent cells by osmosis . This process continues until it reaches xylem tubes in the leaves.
Xylem tubes in the leaves are continuous with xylem tubes in the roots and stem, and so water is pulled by transpiration
like pulling water from a straw by suction. Water is also pushed into the roots by osmosis in the root hair cells. Hence
water is under two forces in the xylem tubes; a pulling force exerted by transpiration in the leaves, and a pushing force
exerted by osmosis and entry of water into the root hair cells (root pressure). This allows water to travel in the xylem tubes
from the roots to the leaves like a stream, called the transpiration stream
Diagram showing the direction of water from the soil into the root hair cell, and its transport into the cortex, xylem tubes and then
to the mesophyll layer of the leaf. Water evaporates into the air spaces and then out of the stomata to the atmosphere. The release
of water vapor from the stomata into the atmosphere is called transpiration. Transpiration is one of the forces that pull water up in
the xylem in a continuous column called the transpiration stream. As more water is lost by transpiration it is replaced by
absorption from the soil.
Absorption of mineral ions
Mineral ions are absorbed by root hair cells by active transport , which is a process that needs energy. This is because mineral ions
move against their concentration gradient from lower solute concentration in the soil to higher solute concentration in the root hair
cells. The root hair cell is equipped with a high number of mitochondria , which are needed for aerobic respiration that produces
energy for active transport. The cell membrane of these root hair cells has protein channels for the process of active transport . One
of the requirements in soil management is plowing which helps in aerating the soil and making oxygen available for absorption by
the roots for aerobic respiration . Water logged soils result in poor plant growth due to the lack of oxygen needed for aerobic
respiration in the root hair cells.
The roots are highly adapted to efficient absorption of water and minerals from the soil. These adaptations include highly branching
roots, and cortex cells that have structures specialized for passage of water.
Factors affecting rate of transpiration :
The rate of transpiration in a mesophytic terrestrial plant is affected by many
environmental factors as shown below.
• Light : the stronger the light intensity the faster is the rate of transpiration .
Increased light intensity increases the rate of photosynthesis and so carbon
consumption increases. This lowers carbon dioxide concentration within the leaf
and results in increased stomatal opening.
• Temperature . High temperature causes faster evaporation of water from the
leaves due to increased movement of molecules, and so the rate of transpiration
increases. Desert plants are adapted to close their stomata in the day time in order
to minimize transpiration and the loss of water from the plant. They open their
stomata at night when the temperature is lower, and so they take their carbon
dioxide at night and store it in certain compounds for the day time.
• Wind increases the rate of transpiration . Wind blows away the water vapor that
comes out of the leaves by transpiration, thus making more space for more water
to come out through the stomata into the surroundings.
• Humidity: the higher the humidity of the atmosphere the lower the rate of
transpiration . This is because at high humidity the surroundings are saturated
with water and so the atmosphere cannot take any more water vapor molecules,
and so less water evaporates from the leaves.
Water availability: shortage of water in the soil results in closing of the stomata
and so it lowers the rate of transpiration. This conserves water in the plant and
controls the rate of transpiration. In desert plants the rate of transpiration is greatly
reduced by a multitude of adaptations and measures as will be discussed in the
next section. The rate of transpiration can be measured by a potometer as shown
in the diagram on the right.
Support in terrestrial plants
Support in plants is manifested by two main properties:
Cell turgor, which results from the entry of water into the cells by osmosis . The
cells become swollen but they are prevented from bursting due to the presence of
cellulose in the cell wall . Cellulose is strong and elastic, and so it resists water
pressure and prevents bursting
Adaptations in plants
Concerning water availability plants can be divided into three main groups:
Xerophytes live in an environment characterized by water scarcity and shortage. Desert plants are the most highly adapted plants
to such environments. Water is needed in plants for different functions, including support, photosynthesis , transport , cooling,
enzyme action and many other functions. Hence desert plants have to possess structures and processes that help them in conserving
and obtaining water efficiently. Some of these adaptations are listed below:
 Well established root system that grows in all directions in order to
obtain water from the soil efficiently. The root of a small plant can
sometimes be ten times longer than the plant itself. These long roots also
help to anchor the plant in the sandy deserts and protect them from being
rooted out by the wind. During desert studies, in field work, my students
were always surprised and impressed at the length of the roots and how
deep they have to dig to reach the root apex.
 They have a short life cycle, and so they grow from seeds to mature plants, produce flowers, fruits and seeds again in few days
that coincide with the few days of rain.
 Desert plants show regular distribution and they are well spaced away from each other to avoid competition and to be able to get
enough water for their living.
 The leaves have adapted in different ways to avoid transpiration and loss of water. Many desert plants have succulent leaves,
which are filled with water and this helps them store water, which they can get during the short rainy seasons. Other adaptations
include thick cuticle, few number of stomata , reduced leaves, modification of leaves to spikes, stomata sunken in pits, stomata
surrounded by hairs for minimizing water loss by transpiration .
Left: Succulent leaves; these plants (which do not need watering) are often grown in dry countries on road sides for their aesthetic
value. Right: a variety of cacti demonstrating the different types of adaptations in desert plants.
Hydrophytes are plants that live in watery habitats such as ponds, rivers
and swamps. Elodea is a good example on hydrophytes and the plants here
are adapted by having lots of air spaces in their tissues to help them float,
their leaves stems and other structures are flexible (pliable) in order not to
break due to wave actions and currents. The leaves are divided to small
parts to provide a big surface area for absorption of substances. The root
system is simple and in many cases it functions as an anchoring device,
since absorption can be carried out by all parts of the plant.
Mesophytes are plants that live in moderate amounts of water and so they
do not have special adaptations for water shortage or excess water
availability. They have abundant amounts of rain or irrigation water and so
loss of water by transpiration is balanced by enough water entering the
roots from the soil.
Food storage in plants
Plants make glucose in the process of photosynthesis . Glucose is used for respiration to produce energy which powers different
processes in the plant such as active transport . Glucose is also converted to other substances such as proteins , enzymes ,
nucleotides for building and growth. Excess sugars made in photosynthesis are converted to storage materials such as starch, lipids
and proteins. Some examples on storage materials in plants are shown below:
Starch
Proteins
Lipids
Corn seeds, potato tubers, rice and wheat grains
Seeds of leguminous plants such as peas, beans and lentils.
Olive fruits and seeds, sunflower seeds
Sexual Reproduction in Angiosperms
Flowers are the sexual structures of angiosperms (flowering plants). A flower consists of the following parts:
Sepals are small green leaves surrounding petals and they function in protecting the flower when it is still in its bud stage.
Petals are colorful and so they function in attracting insects for the process of pollination.
The carpel is the female organ and it is divided into three main parts; the ovary where meiosis takes place to produce the egg, the
style which carries the stigma on its tip. The stigma functions in receiving pollen and it secretes a sugar solution that causes the
pollen to stick and to start germinating to produce the germ tube. The germ tube grows down the style to carry the male nucleus to
the female nucleus.
The stamen is the male organ and it is made of a long thin structure called the filament carrying on its tip the anther. The anther is
where meiosis occurs to produce the pollen. It is made of four chambers containing pollen grains.
As the pollen matures, the anther bursts open releasing all the pollen; this is carried to the stigma of the same or other flowers. This
process is called pollination and it can be of two types:
Self pollination is when the pollen of one flower falls on the stigma of the same flower or another flower of the same plant.
Cross pollination is the transfer of pollen from the stigma of one flower to the anther of a flower of another plant of the same
species. Cross pollination can be by wind or animals, mostly insects.
Insect or animal pollination
Flowering plants have evolved various methods of attracting insects for the process of pollination. The insect visits one flower to
get some nectar and by doing this the pollen from the anther sticks to its body, then it flies to another flower of the same species for
the same reason, and by this it deposits some pollen on the stigma of that flower. For insect pollination, plants have evolved the
following adaptations:
Some flowers have a landing or walking platform for insects made from an extension of a petal or some fused petals such as in the
case of orchids. The sunflower attracts insects by being large and brightly colored; it also has nectar as a source of food for insects.
Sweetly scented flowers are common occurrences in all animal and insect pollinated flowers. The number of pollen grains
produced is much smaller than in the case of the wind pollinated flowers, since in most cases the insect as an agent is adapted to
visit flowers of the same species, and so the pollen is specifically carried to its correct destination.
As for wind pollination , the flowers are small and dull in color, the filament hangs out of the flower cup for easy shaking by the
wind, the stigma is feathery and it also hangs out of the flower cup in order to catch the pollen. The pollen is very large in number
since a high percentage of it is lost in the wind and does not reach the flower. Examples on this include wheat, rice and corn
flowers. Pollination is then followed by pollen germination which carries the male nucleus to the egg into the ovule .
Fertilization
During fertilization the pollen nucleus fuses with the female nucleus to produce the zygote. The zygote grows into the seed and the
ovary into the fruit. The fruit has different adaptations for seed dispersal. The seed germinates when provided with all the required
factors including a suitable temperature, water and oxygen. The germinating seed grows into a seedling and the latter into the
mature plant which starts producing flowers and the cycle continues.
The zygote grows inside the ovule and together they form the seed. The ovary grows and concentrates some nutrients to become
the fruit. Fruits can be dry such as in roses, dandelion and peas or juicy such as in apples, cherries and plums.
Seed and fruit dispersal results in the spreading of fruits and seeds to other environments and ecosystems. Seed dispersal is
important for the following reasons:
1. The seeds will not overcrowd in an area and this minimizes competition for resources.
2. It spreads the seeds in other environments which might offer better chances of survival for these plants.
By animals: the hooks in
Self dispersal: in lupins the dry
Wind dispersal by wings such as Wind dispersal by parachute as
burdock fruits stick to the fur of pod splits open and the seeds
in sycamore tree
in dandelion
animals
shoot out at a good distance
Diagrams showing different methods of seed dispersal
Seed Germination
Seeds are dormant or inactive plant structures that help in the survival and conservation of plant species. Seeds are resistant to
various biotic and biotic factors and they can stay dormant for many years, until all the factors around them are suitable. When they
are provided with the right conditions their dormancy breaks and they start germinating and growing into the plant again. Scientists
have found seeds aged about 5000 years in some parts of the world. When these seeds were provided with their requirements they
grew again into a mature plant. This adaptation helps in the conservation of the species and it is one of the factors that lead to the
success of terrestrial plants on land. Land is characterized by different degrees of water shortage, and terrestrial plants have evolved
the seed which can stay dormant for a long time until water and other needed factors are available. Hence the seed is one of the
most successful adaptations to terrestrial life. Seed plants include gymnosperms (non flowering plants such as coniferophytes), and
angiosperms, the flowering plants.
When the seed is provides with suitable factors and materials it starts germinating. In germination dormancy is broken and the seed
is activated. The following sequence of events occur in the activation and the consequent germination of the seed:
Metabolic events in germination
1.
2.
3.
4.
9.
10.
11.
Water enters the seed through a hole in the seed coat called the micropyle.
Water moves into the tissues and cells by imbibition and osmosis .
The seed swells and the seed coat bursts.
Water activates gibberelline, the hormone needed for breaking the
dormancy of the seed.
5. Gibberelline activates amylase which hydrolyses starch to maltose, and
maltose is then hydrolyzed by maltase into glucose.
6. Glucose is mobilized (transported) to the embryo .
7. The embryo absorbs glucose and uses it for respiration (oxygen is needed
in this process).
8. Cell division, growth and elongation occur in the embryo . The radicle
starts growing downwards into a root and the plumule starts growing
upwards into a shoot.
The nutrients needed for growth are all supplied by the food stored in the cotyledons.
As the nutrients in the cotyledons are consumed and exhausted, the first leaves start to appear and the plant starts to
photosynthesize and make its own food.
When photosynthesis starts, the seedling absorbs water and minerals from the soil, carbon dioxide from the
atmosphere and sunlight.
The three main factors needed for the germination of all plant seeds are water, oxygen and a suitable temperature.
 Water : this is needed for the following processes:
1.
2.
3.
4.
5.
Activation of hormones and enzymes .
Swelling of the seeds and bursting of seed coat.
Hydrolysis of storage compounds such as starch into their simple monomers such as glucose.
Transport (mobilization) of the simple materials to the embryo to be used for respiration and growth.
Metabolic reactions and enzyme actions occur in solution, and so water is needed for many reactions.
 Oxygen : all seeds need oxygen for germination. Oxygen is needed for aerobic respiration , which is involved in germination.
Without a supply of oxygen, seeds fail to germinate because of the lack of energy, in the form of ATP . Respiration produces
energy that powers all the reactions involved in this process
C 6 H 12 O 6 + 6 O2 → 6 CO 2 + 6 H 2 O + energy
.
 Suitable temperature : all the reactions occurring in germination are controlled by enzymes . Hence an optimum temperature
for enzyme activity leads to a faster rate of germination. Low temperatures such as freezing inactivates enzymes and stops the
process of germination. Very high temperatures denature enzymes.
Diagrams showing the stages in germination and growth into a seedling in bean, a dicotyledonous plant. As the embryo grows, the
seed shrinks due to consumption of stored food materials by the embryo. Later the seedling starts photosynthesizing and making its
own food.
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