PLANT BIOLOGY
CLASSIFICATION OF PLANTS
Plant Classification
Seed plants show the greatest evolutionary complexity in the plant kingdom and are
dominant in most terrestrial environments. They have real roots, stem and leaves. There
are two groups of seed plants: Gymnosperms (open-seeded) and angiosperms (closed
seeded)
1) Gymnosperms (Open seeded plants/cone bearers) (Coniferophyta)
 There are about 650 species.
 The seeds of gymnosperms are unprotected. Their seeds are formed inside a cone
(kozalak) instead of a fruit.
 They are woody, ever-green plants with needle-like leaves, i.e. conifers.
2) Angiosperms (Angiospermatophyta)



There are at least 235.000 species.
They are flowering plants that produce their seeds within a fruit. Angiosperms are far
more developed than gymnosperms because the reproductive structures are protected
against environmental changes.
The first leaf or the first pair of leaves produced by the embryo of a seed plant is
called a cotyledon. According to the number of cotyledons in a seed, Angiosperms are
classified into two groups as monocotyledons and dicotyledons.
a) Monocotyledons: They are mostly herbaceous plants with leaves that are usually long and
narrow and have parallel veins. Monocot seeds have a single cotyledon
b) Dicotyledons: They may be herbaceous or woody. Dicot's leaves are variable in shape, but
usually broader than monocot leaves and have netted veins (there are branched veins similar
to a net.) Two cotyledons are present in seeds of dicots.
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Comparison of structures of monocots and dicots
PLANT STRUCTURE
The organs of a plant are its roots, stems, leaves and reproductive structures. There are
extensive modifications of roots, stems, and leaves in different types of plants. These
modifications allow plants growing in varied environments to survive.
Roots: There are two major types of root systems: taproot systems and fibrous root systems.
Plants with taproot systems have one main vertical root (the tap root) that develops from the
embryonic root. When there is a fibrous root system, there is generally a group of thin roots
spread out in the soil without a main central root. Some of the roots have specialized cells
within the roots that store large quantities of carbohydrates and water. Carrots and beets are
among these storage roots.
Some structures in dicots
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Stems: Stems are capable of great modification as they carry out the basic functions of
conduction and support of photosynthetic organs, they often take on very different forms as
shown in this table.
Kalanchoe plant and Tendrils
Leaves: Leaves also show great structural modification while maintaining their basic function
of photosynthesis.
PLANT TISSUES
Plants, like animals are made of tissues that form organs. Plants have fewer types of tissues
than animals. Some plant tissues are made of only one type of cell. Others are made of two or
more types of cells that work together. Some tissues are found throughout the plant, while
others are found only in specific structures. Plant tissues can be divided into two groups as
meristematic and permanent tissues.
In most animals, cell division takes place throughout the animal's body during periods of
growth. In plants, however, cell divison takes place in certain regions called meristems.
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Meristematic tissues are made of cells that undergo cell division frequently. Meristematic
tissue cells have the following properties:
- The cells are living
- Thin walled with a lot of cytoplasm
- Their nucleus is large and if vacuoles are present, they are small
- There is no intercellular space (air space) between them.
- They have ability to divide
- They are important in growth, differentiation and repair.
When a meristem cell divides, one of the newly formed cells becomes part of a new structure
and the other remains in the meristem, where it can continue to form more cells. A plant has
two kinds of meristem: apical and cambium.
Primary (Apical) Meristem: Apical meristem is present in the growing tips of stems,
branches
and
roots.
It
cause
roots
and
stems
to
grow
longer.
Longitudinal sections through the vegetative regions of root and stem show the location of
meristematic tissue.
4
Secondary Meristem (Cambium): Cambium occurs as cylinders of meristem within the
stem and root in woody plants. It adds tissues that increase the thickness of stems and roots.
Some of the permanent tissue cells,
by the action of hormones, regain the
ability to divide and differentiate into
secondary
meristem
cells.
There are two types of cambium.
a) The vascular cambium produces
layers of tissues that transport water
and nutrients, that is, xylem and
phloem.
b) The cork cambium, produces a
layer of tissue called cork (periderm).
It differentiates from epidermis.
In trees that have a growing season,
vascular cambium is dormant during
the winter. In the spring, when moisture is plentiful and leaves require much water for growth,
the secondary xylem contains wide vessel elements with thin walls. In this so-called spring
wood, wide vessels transport sufficient water to the growing leaves. Later in the season,
moisture is scarce, and the wood at this time, called summer wood, has a lower proportion of
vessels
When the trunk of a tree has spring wood followed by summer wood, the two together
make up one year's growth, or an annual ring. You can tell the age of a tree by counting the
annual rings. The outer annual rings, where transport occurs, are called sapwood. In older
trees, the inner annual rings, called heartwood, no longer function in water transport.
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Tree trunk. a. A cross section of a 39-year-old larch, Larix decidua.The xylem within the
darker heartwood is inactive; the xylem within the lighter sapwood is active. b. The
relationship of bark, vascular cambium, and wood is retained in a mature stem. The pith has
been buried by the growth of layer after layer of new secondary xylem.
PERMENANT TISSUES:
Permenant tissue cells, which do not have the ability to divide, are formed from meristematic
tissue cells.
A- PROTECTIVE TISSUE:
Protective tissue is also known as dermal tissue. It is the plant's outer covering in contact with
the environment. Functions of the protective tissue can be listed as follows:
- Protects against excess loss of water by evaporation.
- Protects against mechanical damage.
- Protects against infections.
- Specialized cells like stoma and root hair help in exchange of materials and absorption.
There are two kinds of protective tissue; epidermis and periderm.
a) Epidermis:
The epidermis is the protective tissue that forms the outer layer on leaves, green stems
and roots. In a young plant, epidermis covers all parts; in an adult plant, it covers the
leaves and roots but is replaced by a thicker tissue, the periderm, in the stem. The cells
of the epidermis which cover the above ground parts of a plant secrete a waxy
substance called cutin. Cutin forms a layer over the outer surface of the epidermis.
This layer, which is called the cuticle, cuts down on water loss and protects against
infection by microorganisms. In low water environments, thickness of cuticle
increases, and size of the leaf decreases to prevent water loss. Epidermal cells of the
root do not produce cuticle for it would interfere with water uptake.
Properties of epidermis are as follows:
- Usually single cell layer.
- Cells do not contain chloroplast, so light can penetrate and reach the photosynthetic cells
beneath them.
- Cells have thick cell walls.
- No intercellular space (Cells fit tightly together).
Groups of epidermal cells may form elongated structures, called hairs, that reflect light (to
prevent overheating) and restrict water loss. In some plants these hairs are sticky or toxic, so
provide
protection.
Epidermal cells of the root produce root hairs that increase the surface area for absorption of
water. Some epidermis cells on leaves (mainly on lower surface) and green stems form tiny
holes called stomata through which water vapor, carbon dioxide and oxygen gas may enter
and exit a leaf.
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The stomata are not open all the time. They open and they close according to the needs of the
leaf.
Each stomata is surrounded by a pair of specialized epidermal cells called guard cells which
regulate
the
opening
and
closing
of
the
stomatas.
A guard cell has a thick wall on the inner side and thinner walls on the other sides. They have
chloroplasts unlike normal epidermal cells.
Stomata open in response to light. In the presence of light, photosynthesis takes place and the
amount of CO2 decreases (indicating a need for CO2). This stimulates the absorption of
potassium ions (K+) by the guard cell. This causes an increase in solute concentration inside
the guard cells, so water is pulled in by osmosis from nearby epidermal cells. Guard cells
swell and their thin walls balloon out and their thick walls bend. This forms an opening
between the two guard cells.
Stomata close in response to low concentrations of water vapor (to conserve water) and to
high
concentrations
of
CO2
inside
the
leaf.
To close stomata, guard cells pump out potassium ions and water follows by osmosis. The
cells shrink and their thick walls straighten. The hole between them closes.
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The number and distribution of stomata in the leaves of plants are related with the adaptations
of plants to live at different environmental conditions. In environment where water amount is
low, number of stomata decreases and they are present deep in epidermis.
b) Periderm:
Periderm is a protective tissue that covers the surface of woody stems and roots. It
protects the more delicate inner tissues from mechanical injury. It also waterproofs the
outer
surface
and
prevents
infections.
Periderm consists of cork cells produced from cork cambium and epidermis. Cork
cells live for only a short time. Fully grown cork cells are dead. Their cell walls are
impregnated with a waxy substance, suberin.



Outer layer cells are dead
Cell walls are thickened by the addition of suberin
Contains lenticels which are responsible for the exchange of oxygen, carbon dioxide
and water vapor.
Lenticels- small raised openings- are scattered throughout the cork tissue to allow passage of
oxygen
and
carbon
dioxide.
As the stem widens, the outer layer of cork becomes too small and splits. New cork is formed
continuously, by the cork cambium, to replace the cork that splits. Unlike stomata, lenticels
do not have the ability to open and close. They are always open. Also, they do not contain
chloroplast.
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B- PARENCHYMA TISSUE:
A plant is formed chiefly of parenchyma cells. Parenchyma cells retain their ability to
multiply but formation of new cells is not the main function of a parenchyma cell; it does so
only to replace an injured part. Parenchyma cells are alive cells, have thin cell walls.
There are four different types of parenchyma tissues according to their function.
1.
AssimilationParenchyma:
The main function of assimilation parenchyma is photosynthesis. It is found in the mesophyll
layer of leaves. Mesophyll layer consists of two kinds of parenchyma: palisade and spongy
parenchyma.
Palisade parenchyma is found on the upper part of the mesophyll layer. Its cells are
cylindrical in shape and abundant chloroplasts. There is little intercellular space between the
cells.
Spongy parenchyma is found on the lower part of the mesophyll layer. Its cells are irregularly
shaped and contain less chlorophyll compared to palisade parenchyma. There are large air
spaces between cells.
Diagram of leaf structure. Note the arrangement of tissue layers within the leaf. Image from
Purves et al., Life: The Science of Biology, 4th Edition, by Sinauer Associates
(www.sinauer.com) and WH Freeman (www.whfreeman.com), used with permission
2. Storage Parenchyma:
- Found in the storage organs such as roots, stems, fruits and seeds.
- Store starch, proteins, lipids (in leucoplasts) and water (in vacuoles).
- Have few or no chloroplasts.
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3. Transport Parenchyma:
- Found between assimilation parenchyma and vascular bundles.
- Function in the transport of food materials and water between these tissues.
4. Aeration Parenchyma:
-Present
in
the
roots
and
stems
of
water
plants.
- The cells have large airspaces in between. This increases the rate of diffusion and also
makes plant lighter so it can float on water.
C- SUPPORT TISSUE:
In young plants, support is provided by turgor pressure. In others, support tissues are present.
The two types of support tissue are collenchyma and sclerenchyma.
a) Collenchyma:
Collenchyma cells provide flexible support to the
leaves, flowers, and growing parts of a plant. They
form long strands within leaves, helping them to lie
flat, and thin sheets just beneath the surface of growing
stems, helping them to remain erect yet bend with the
wind.
A collenchyma cell has a thick cell wall, reinforced at
certain parts with pectin and additional cellulose. The
cell wall is uneven in thickness. According to the
thickening pattern, there are two types of collenchyma
cells.
Angular: The cell wall is thickened on the corners.
Lamellar: The cell walls are thickened at the sides.
Diagram of some collenchyma cells. The above image is from
http://www.biosci.uga.edu/almanac/bio_104/notes/apr_9.html.
b) Sclerenchyma:
Sclerenchyma cells give rigid support to non-growing pats. They have greately
thickened cell walls that are stiffened with a substance called lignin. A sclerenchyma
cell does not begin to function until it dies. Its cell walls, which remain after death,
give
support
and
shape
to
the
plant.
When fully grown, the cells usually do not have cytoplasm. In fact the cell walls are so
thick that the space inside the cell is nearly
eliminated. There are two kinds of sclerenchyma
cells;
sclereid
cells
and
fiber
cells.
Sclereid cells have variety of shapes. They may be
scattered throughout a structure, as in pear and
quince (making this fruit gritty), or arranged in
sheets, as in seed coats or walnut shell.
Fiber cells are long, slender, and often arranged in
bundles to form very strong materials. They are
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thickened as fibers. They are most abundant in the wood and bark of trees and in
plants like hemp and linen.
The above image is from http://www.biosci.uga.edu/almanac/bio_104/notes/apr_9.html.
.
D- VASCULAR TISSUE:
Vascular tissues form tubes that transport fluids and support
the plant in an upright position. These tissues are built of
tube-forming cells as well as paranchyma cells (For storage
of materials) and sclerenchyma cells (mostly fiber cells, for
added strength). There are two kinds of vascular tissue:
xylem and phloem. The xylem and phloem lie next to each
other, forming a vascular bundle.
gopher://wiscinfo.wisc.edu:2070/I9/.image/.bot/.130/Stem/Z
ea_cross_section/Vascular_Bundle_labelled
a) Xylem: Most of the cells that form
mature xylem are dead. They do not have
cytoplasm. They form tubes that go from
the roots up through the stems and leaves.
Xylem is mainly made of two types of
cells- tracheids and vessel elements. In
addition, xylem contains paranchyma
cells and fiber cells (sclerenchyma). The
parenchyma cells store starch and ions
and the fiber cells provide support.
http://www.biosci.uga.edu/almanac/bio_104/notes/apr_9.html.
A tracheid is a tube formed of tracheid cells. These long, narrow cells have small pits in the
sides of their end walls, where they contact neigbouring cells. The pits are thinner regions, but
not actual holes, in the cell walls. A tracheid cell dies before it becomes functional, leaving
only its walls to form part of a tube: fluids flow through the tube, passing through the pits
from one dead tracheid cell to the next.
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Vessel elements are cells that form conducting tubes. These cells, which do not have end
walls, are placed end to end, forming long, thin, hollow tubes called vessels. Vessel members,
like
tracheid
cells,
are
dead
when
functional.
Since the cells are dead, transport in the xylem occurs passively.
c)
Phloem: Unlike the tracheids and vessel elements of xylem, most of the cells that
make up phloem are alive and contain cytoplasm. The substances transported by phloem are
mainly organic compounds dissolved in water. Food made in the leaves moves through the
phloem to other parts of the plant. Unneeded food is often carried to the roots where it is
stored. In the spring, sap, containing dissolved materials, is moved upward through the
phloem. Phloem is made of two types of cells, sieve cells and companion cells. Like xylem,
phloem also contains parenchyma cells for storing materials and fiber cells for support.
Sieve cells form tubes by loosing their nuclei and most organelles rather than by dying. The
remaining cytoplasm lies next to the cell walls, leaving room for fluids to pass through the
center. Fluids flow from one cell to the next through holes in the end walls.
Companion cells are connected to neighboring sieve tubes by thin strands of cytoplasm.
These cells, which have nuclei and cytoplasm, are thought to control the transport activities of
sieve cells.
Structure
Function
XYLEM
- Mature cells are dead
- Made of tracheids,
Vessel members,
parenchyma and
sclerenchyma
- Transport of water and
minerals from roots to
upper parts
- Transport is
unidirectional
- Transport is passive
PHLOEM
- Cells are alive
- Made of sieve tubes,
companion cells, paranchyma
and sclerenchyma
- Transport of organic molecules
such as sucrose, amino acids,
fatty acids, etc.
- Transport is bidirectional
- Transport is passive and active
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Tissues in the stem of a dicotyledon
dicotyledon
Tissue distribution in the root of a
E- SECRETORY TISSUE:
- Cells have large nuclei and cytoplasm.
- Produce and secrete resins, nectar, antibiotic like substances and hormones.
Functions of secretory tissue are:
- Protection
- Some help pollination
- Have role in metabolic activities
MOVEMENT AND HORMONAL REGULATIONS IN PLANTS
Plants give response to external stimuli like light, gravity or touch. These responses can be
tropism or nastic movement.
Generalized plant responses to a stimulus are called nastic movements. These include the
opening of bud scales and of flower petals, growth movements that occur in response to
stimuli such as light and heat without regard for the direction of the stimulus. Some spring
flowers exhibit thermonasties, i.e., their flowers open in response to warmth rather than the
amount of light. Nastic movements are achieved by changes in turgor pressure. Turgor
movement is affected by changes in the water content of cells and is often quite rapid.
Examples are the “sleep movements” of clover, the sudden droping of the leaves of the
sensitive plant (mimosa) when touched (thigmotropism), and the reactions of insectivorous
plants to the presence of their prey.
Tropism is the involuntary response of an organism or part of organism, involving orientation
toward (positive tropism) or away from (negative tropism) one or more external stimuli.
Tropistic growth in plants is believed to be triggered by the presence of plant hormones that
promote cell growth.
Tropisms are plant responses in which the direction of movement is strongly related to the
direction of some environmental factors. Gravitropism and phototropism are the two principle
responses and thus the majority of work done has been on these. Gravitropism (also known as
geotropism) is the response in which the direction of growth is determined by the effects of
gravity on the plant. Phototropism is a response to a gradient of light intensity.
Plants don't have nervous system. They have hormones which regulate certain processes of
the plants such as growth, germination, flowering, fruit formation, leaf or fruit fall
(abscission). Their tropic movements are also regulated by hormones.
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Main Plant Hormones:
1. Auxins
2. Gibberellins
3. Cytokinins
4. Ethylene
5. Abscisic acid
The auxin, gibberelline and cytokinin are promotors
whereas ethylene and absicic acid are inhibitors
1. AUXINS: They are produced in the embryo of seed and
apical meristems.
Main functions :
a) Responsible to promote growth through cell elongation. High concentrations have
INHIBITING effect
b) Related with tropic movements. Unequal distribution of auxin results in asymmetric
growth.
c) Delay leaf - fall and responsible for apical dominance
d) May be used to produce fruits without fertilization (seedless fruit)
e) Responsible for differentiation (in combination with cytokinins)
f) Used to prevent premature fruit drop
Experiments on Oat Seedlings (yulaf filizi) to show the effect of auxins on growth.
Experiments showing the effect of auxins on tropism
2. GIBBERELLINS: Found in apical meristems and germinating seeds
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Main functions:
a) Stimulate and promote germination (seed and bud)
b) Promote growth through cell elongation (with auxins), (stem growth)
c) Stimulate flowering and fruit development
d) May be used to produce seedless fruits
e) stimulate breakdown of starch
3. CYTOKININS
Found in apical meristems and germinating seeds.
Main functions:
a) Stimulate cell division and differentiation (in combination with auxins)
b) Stimulate development of chloroplasts and chlorophyll synthesis
c) Delay aging and provide prolong shelf life of cut flowers
4. ETHYLENE
Found mainly in mature leaves, fruits
Main functions:
a) Promotes ripening of fruits
b) Responsible for aging of tissues and leaf-fall.
5. ABSCISIC ACID
Found in aging tissues
Main functions:
a) Inhibits growth
b) Responsible for dormancy of buds and seeds
c) Responsible for leaf and fruit abscission (=fall)
d) Prevents excess loss of water by causing the stomata to close. Therefore, this hormone is
also known as STRESS hormone.
PHOTOPERIODISM
Many angiosperms flower at about the same time every year. This occurs even though they
may have started growing at different times. Their flowering is a response to the changing
length of day and night as the season progresses. The response by an organism to the duration
and timing of light and darkness is called as photoperiodism. It helps promote cross
pollination.
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Plant type
Long-day
plants
Flowering and light
Bloom when days are longest and
nights are shortest (midsummer)
Short-day
plants
Bloom in spring, late summer, and
autumn when days are shorter
Day-neutral
plants
Flower without regard to day length
Examples
 spinach
 Arabidopsis
 sugar beet
 radish
 clover
 chrysanthemums (bloom in
the fall)
 rice (Oryza sativa)
 poinsettias
 morning glory (Pharbitis nil)
 the cocklebur (Xanthium)





tomatoes
roses
dandelions
sunflower
corn
In 1920 two employees of the U. S. Department of Agriculture, W. W. Garner and H. A.
Allard, discovered a mutation in tobacco — a variety called Maryland Mammoth — that
prevented the plant from flowering in the summer as normal tobacco plants do. Maryland
Mammoth would not bloom until late December.
Experimenting with artificial lighting in winter and artificial darkening in summer, they found
that Maryland Mammoth was affected by photoperiod. Because it would flower only when
exposed to short periods of light, they called it a short-day plant. Short day plants flower
when the day length is below some critical value, usually in spring or fall.
Long-day plants bloom when the day length exceeds some critical value, usually in summer.
Day-neutral plants are independent of day length and can bloom whether days are long or
short.
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If the critical element in the photoperiodism of flowering is day length, as the terms ‘longday’ and ‘short-day’ imply, then we should be able to prevent a long-day plant from flowering
at he proper season by shielding it from light for an hour or so during the middle of the day.
But if this done, nothing happens; the plant flowers normally. If, however, a short-day plant is
illuminated by a bright light for a few minutes, or even seconds, in the middle of the night
during the normal flowering season, it will not bloom. The same sort of experiment will
induce flowering at the wrong season by a long-day plant. Clearly, then, the critical element
of the photoperiod is actually the length of the night, not the length of the day. Instead of
speaking long-day and short-day plants, we would be more accurate to speak of short-night
and long-night plants.
The difference between long-day and short-day plants does not depend upon the precise
duration of darkness at the time of flowering. Rather, the basic distinction is that long-day
(short-night) plants will flower only when the night is shorter than a critical value, whereas
short-day (long-night) plants will flower only when the night is longer than a critical value.
The critical night length is thus a maximum value for flowering by long-day plants and a
minimum value for flowering by short-day plants.
Photoperiodism also explains why some plant species can be grown only in certain latitude.


Spinach, a long-day plant, cannot flower in the tropics because the days never get long
enough (14 hours).
Ragweed, a short-day plant, fails to thrive in northern Maine because by the time the
days become short enough to initiate flowering, a killing frost in apt to occur before
reproduction and the formation of seeds is completed.
It was first found that red light (wavelength about
660nm) is the most effective in inhibiting
flowering in this short-day plants; the same red
light is very effective in inducing flowering in
long-day plants.
Later, it was found that far-red light(wavelength
about 730nm) , which is invisible to the human
eye, has effects exactly contrary to those of the
red light; it induces flowering in short-day plants
and inhibits flowering in long day plants. Not
only do red and far-red light have opposite
effects, but each reverses the effect of prior
exposure to the other. A short-day (long-night)
plant will not flower if its long night is interrupted
by a bright flash of red light; if, however, the red
flash is followed immediately by a far red flash, the plant flowers normally. Almost any
number of successive flashes can be used, the final effect depending solely on whether the last
flash was red or far-red.
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Recent work — mostly in the long-day plant, Arabidopsis — supports a different model of
photoperiodism. This work suggests that the photoperiodic response is governed by the
interaction of


daylight with
innate circadian rhythms of the plant.
The mechanism represents a biological clock that is present in most plants. Such biological
clocks are important in controlling a number of stages in the life of a plant. Once the red/far
red light mechanism and the internal clock mechanism have together indicated to the plant
that the photoperiod is appropriate for flowering, the leaves must initiate the next step altering
hormone production or sensitivity to hormone ratios as necessary to bring about actual
flowering.
TRANSPORT IN PLANTS
The movement of substances through the conducting, or vascular tissues of plants is called
TRANSLOCATION. There are two types of conducting or vascular plant tissues. These are
xylem and phloem. Xylem translocates mainly water and mineral salts from roots to the aerial
parts of the plant. Phloem translocates mainly organic materials. The xylem generally
transport upwards, from soil to upper plant parts, while substances can move both up and
down through phloem tissue.
Absorption of Water and Minerals by Roots
Water and minerals enter the plant through the epidermis of roots, cross the root cortex, pass
into the xylem vessels and then flow up to the shoot system.
Minerals are needed for the growth of plants. Growth is limited to the mineral that is found in
the least amount in the soil. Farmers overcome this issue by adding fertilizers to the soil.
Fertilizers contain minerals that are essential for the growth of plants.
Root hairs are special projections of the epidermis cells of roots. The main function of root
hairs is to absorb water and minerals.
Water in the cytoplasm of root hairs contains large amounts of dissolved minerals, sugars,
amino acids, and other substances. In general, the soil has a greater concentration of water
than it has dissolved minerals. Water, therefore, flows by osmosis from the soil into the root
hairs.
An essential mineral present in the soil solution in high concentration may enter the root hairs
by diffusion, but plants do not rely on diffusion alone to absorb most minerals.
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The soil solution is usually very dilute, and roots can accumulate essential minerals to
concentrations that are hundreds of times greater than the concentration of these minerals in
soil. In this case mineral uptake is accomplished by active transport.
Minerals that need to be taken up from the soil are:
 N as NO3- or NH4+
 K as K+
 P as PO4-3
 Ca as Ca+2
There are three main methods for the absorption of minerals from the soil into roots:
 Diffusion
 Mass flow of water in the soil carrying these ions
 Aid provided by fungal hyphae
 Active transport
Diffusion: Some minerals are more concentrated in the soil than in the roots, and when
dissolved in water, will diffuse into the root.
Mass Flow of Water: The plant takes up large volumes of water which contains some
dissolved minerals. This is called mass flow.
Mutualistic relationship for absorption of minerals: Many plant species live together with
a fungus to help absorb minerals. There is a symbiotic relationship between roots and fungi.
Large numbers of fungal filaments called hyphae form a cover over the surface of young
roots. They also grow into the plant roots and transport these minerals to the roots. This helps
the plant obtain the minerals they would not be able to absorb without fungus because hyphae
create larger surface area available for water and mineral ion absorption. This mutualistic
relationship is referred to as a mycorrhiza.
Some plants, such as members of the legume family (e.g., beans, clover, and alfalfa), have
roots colonized by Rhizobium bacteria, which can fix atmospheric nitrogen (N2). They break
the N ≡ N bond and reduce nitrogen to NH4+ for incorporation into organic compounds. The
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bacteria live in root nodules and are supplied with carbohydrates by the host plant. The
bacteria, in turn, furnish their host with nitrogen compounds.
Active Transport: Since it has been found that the concentration of some minerals in side the
root may exceed that of a specific mineral in the soil, these minerals are taken up by active
transport. The process requires energy.
Translocation in the Xylem
Inside the xylem, dissolved minerals are lifted with water from the root and distributed to all
parts of the plant. The conducting tubes of xylem are the vessels and tracheids. Vessel
elements are more efficient than tracheids in conduction. Vessels are found only in
angiosperms,
the
flowering
plants.
In the xylem, the solution of water and minerals rises by the combined action of capillarity,
root pressure and transpirational pull.
a) Capillary Action (or Capillarity) is the movement of a liquid in a tube of a narrow
diameter. How far water rises depends on the diameter of the tube. The
narrower the tube, the higher the water rises. There are two factors
involved
in
capillary
action:
1. ADHESION is an attractive force between unlike molecules. Thus,
there is an attractive force between water molecules and the cell walls
of
xylem
tube.
2. COHESION is a force between identical or similar molecules as a
result of H-bonding. There is an attractive force between water
molecules in the xylem tubes.
Thus, water moving up the sides of the tube pulls other water molecules up with it because of
cohesion. However, the most that capillary action can do is to raise water only for several
centimeters.
b) Root Pressure is the pressure in the xylem resulting from the inward movement of water.
Root pressure can be explained by osmosis. Root cells contain a higher concentration of
solutes (both organic and inorganic) than does soil water. As a result, water from the soil
enters the root by osmosis and creates the root pressure. This osmotic potential is sufficient to
move water a short distance up the stem. On its own, however, it is never strong enough in
moving water to the tops of tall trees. At most, root pressure can force water upward only a
few meters, and many plants including some of the tallest trees, generate no root pressure at
all.
When transpiration (Transpiration is the evaporative water loss from plant tissues mainly
through the stomata) is limited, as in high humidity or when stomata are closed, water moves
by root pressure in some plants.
During the night, when
transpiration rates are extremely low, drops of water may be forced out of the leaves of
some plants in a process known as GUTTATION.
20
b) Transpiration Pull (Transpiration-Cohesion&Tension): For many years, one of the
major questions in plant
physiology concerned
the
mechanism by which water
reaches the top of trees, over
100
m
tall.
The transpiration-cohesion &
tension theory, proposed in
1890s and supported by
experimental studies since then,
provides the most widely
accepted explanation. Of the
water moving into a leaf, more
than 90 % is usually lost
through transpiration. About 2
% of water conserved in the
leaf is used in photosynthesis
and other metabolic activities.
The
heat
energy
evaporation of H2 O vapor from the leaves is provided by the sun. (As in figure below)
for
When water is lost by transpiration from the leaves, this affects the contents of the xylem, a
continuous column of water. Water that evaporates from the surface of one mesophyll cell
results in the osmotic movement of water across from the next-door cell, and so on to the
xylem
itself.
When molecules of water leave the xylem to enter a cell by osmosis, this creates tension in
the column of water in the xylem, which is transmitted all the way down to the roots. This is
due to the cohesion of water molecules. As a result the column of water has a high tensile
strength
it
is
unlikely
to
break.
The water molecules also adhere strongly to the walls of the narrow xylem vessels and to the
millions of tiny channels and pores within the cellulose cell walls of the leaf cells.
The adhesive force is sufficient to support the entire column of water. Thus, the combination
of adhesive & cohesive forces allows the whole column of water to be pulled upwards. More
water is continuously moved into the roots from the soil to replace that is lost from the leaves
by transpiration.
The following factors affect the rate of transpiration:
1. Light: It has a major effect on transpiration. Stomata usually open in light for
photosynthetic gas exchange & close in the dark. Thus transpiration rates increase
with light intensity until all the stomata are open and transpiration is at maximum.
2. Temperature: It is the next important factor affecting transpiration after light. At a
given light intensity, an increase in temperature increases the amount of evaporation
from the mesophyll cells and also increases the amount of water vapor the air can
contain before it becomes saturated. Both of these factors increase the water potential
gradient between the air inside and outside the leaf increasing the rate of transpiration.
21
3. Wind (air movement): It increases the rate of transpiration because it reduces the
layer of still air around the stomata and so increases the water potential gradient
between the leaf and air.
4. Air humidity: High air humidity lowers the rate of transpiration because of reduced
water potential gradient between the leaf and air. Very dry air (low humidity) has the
opposite effect.
5. Water supply: If there is little soil water, the plant is under water stress and
transpiration is reduced.
6. Atmospheric pressure: Water vapor pressure decreases as the atmospheric pressure
decreases with increasing altitude. The lower the atmospheric pressure, the greater is
the rate of transpiration.
Translocation In The Phloem
Through the use of radioactive elements, phloem has been shown to carry food & water, both
up and down stems. Phloem is a living tissue consisting of sieve tubes & companion cells
together
with
some
parenchyma
&
sclerenchyma.
The fluid inside phloem tissue is SAP, which is made up mostly of water & dissolved
substances.(like
sucrose,
amino
acids
and
minerals)
The movement of sugars and other organic solutes through phloem tissue can be explained by
SOURCE-to-SINK pattern. The SOURCES of the solutes are the leaves & also storage
tissues. All plant parts unable to meet their own nutritional needs may act as SINKS when
they are importing solutes. The most widely accepted current explanation for the Source-toSink movement in translocation is the pressure-flow hypothesis.
According to this hypothesis, the solutes move in solutions because of differences in water
potential caused by concentration gradients of sugar.
Pressure Flow in A Sieve Tube:
1) Loading of sugar into the tube at the source reduces the water potential inside sieve-tube
members. This causes the sieve tubes to take up water from surrounding tissues by Osmosis.
2) This absorption of water generates a hydrostatic pressure that forces the sap to flow along
the tube.
3) The gradient of pressure in the tube
is reinforced by the unloading of sugar
and the consequent loss of water at the
sink. Sugar does not accumulate in the
sink cells because it is either consumed
in metabolism or converted to storage
compounds as starch.
4) Xylem recycles water from sink to
source.
The speed of transport depends on the
differences in concentration between
source & sink.
Transport in xylem
Upwards & sideways (unidirectional)
Transport in phloem
Upwards & downwards (bidirectional)
22
Transport of water and minerals
Transport of organic compounds
Water transport is passive but mineral uptake is
active
Active or passive transport
REPRODUCTION IN ANGIOSPERMS
The reproductive organ in angiosperms is the flower. The figure above shows flower parts.
The entire female part of the flower is called the carpel. In some cases, the term “pistil” is
used to refer to a single carpel or a group of fused carpels. The entire male part of the flower
is called the stamen. There are various colors, shapes and types of flowers. Complete flowers
contain all four basic flower parts (sepals, petals, stamen and carpel); incomplete flowers
lack at least one of these parts. Staminate flowers have only stamens, not carpels; and
carpellate plants have only carpels.
In flowering plants, the cells that are produced by meiosis have no ability to be fertilized.
These cells that are produced by meiosis should undergo further cell division which is
endomitosis. This phase of growth is called as monoploid growth phase.
Formation of egg cells and pollen
In all flowers, each of the ovules contains
a mother cell called a megasporocyte,
which is diploid. It undergoes meiotic
division to form four haploid cells called
megaspores. Only one of these
megaspores survives to grow. The
megaspore undergoes mitosis three times
to form eight haploid cells. One of them
becomes the egg cell, the two are the
polar nuclei, and the other five serve no further
function.
The diploid cells (microsporocytes) within the
anther pollen sacs divide meiotically to form
haploid microspores. The nucleus of each
microspore divides mitotically (without
23
cytokinesis) to form a generative nucleus and a pollen tube nucleus. The microspores then
develop into pollen grains. The outer surface of the thick walls on pollen grains is covered
with spines and ridges that are characteristic of the given species.
Pollination and fertilization
When the anthers are mature, they open and release pollen. The pollen may then be
transferred to the stigma, in a process called pollination. In self- pollination, pollen is
transferred to the stigma of the same flower. When pollen is transferred to the stigma of
another flower of the same species, cross- pollination occurs. For cross- pollination to occur,
the pollen must be carried by pollinating agents, which include wind and insects.
Pollination is complete when the pollen has
been transferred from an anther to a stigma.
Fertilization in flowering plants is double
fertilization, which is a characteristic of this
group. A chemical (sucrose) is secreted by
the stigma and this stimulates the pollen to
form a tube. The growth of this tube is
thought to be controlled by the tube nucleus.
The pollen tube grows down the stigma and
style into the ovary. Within the pollen tube,
the generative nucleus undergoes mitosis to
form two sperm nuclei. Enzymes produced
by the pollen tube digest a small part of the
wall of the embryo sac. The pollen tube then
enters the embryo sac through the micropyle.
Inside the embryo sac, the two sperm nuclei function differently. One sperm nucleus unites
with the egg nucleus to form a diploid zygote, which develops into the embryo plant. The
second sperm nucleus joins with the two polar nuclei in the center of the embryo sac, forming
the triploid primary endosperm nucleus. This nucleus develops into the endosperm.
The endosperm contains tissues that are capable of storing proteins and other food materials.
When the functions of the two sperm nuclei are complete, the embryo that is contained in the
ripened
ovule
(seed)
uses
the
stored
food
for
growth.
While the fertilized ovule develops into a seed, the surrounding ovary matures into a fruit. A
fruit is an enlarged, ripened ovary that contains one or more seeds. Many fruits, such as the
peach, cherry, grape, and the orange, are formed just from the enlarged ovaries (true fruits). A
fruit such as the apple, pear or strawberry, is formed from the ovary along with other parts of
the flower (false fruits).
DEVELOPMENT IN ANGIOSPERMS
All methods of reproduction are similar in the sense that each parent contributes a part of
itself to the new individual. The process by which the cells contributed by the parent, or
parents grow into complete new individuals is called development. The development of the
new individual is achieved by the growth processes of cell division, cell enlargement and
differentiation.
24
Development in Higher Plants
Although plants have evolved reproductive mechanisms that are quite different from those in
animals, the same basic developmental steps are present. The development of the plant begins
almost immediately after fertilization.
The seed consists of the seed coat, the
embryo and endosperm. The tough,
protective seed coat develops from the wall
of the ovule. On the outside of the seed,
there is a scar called hilum, which marks
where the ovule was attached to the ovary.
The embryo develops by mitosis from the
fertilized egg. The endosperm, which is a
food storage tissue, develops by mitosis
from the endosperm nucleus. The nutrients
stored in the endosperm cells are obtained from the parent plant. The plant embryo consists of
one or two cotyledons, and the epicotyl, the hypocotyl, and the radicle.
Cotyledons are modified leaves. Plants with one cotyledon are called monocots (corn, rice,
wheat, sugar cane, bamboo), and those with two cotyledons are called dicots (All the legumes,
the common trees and shrubs except evergreens, and most flower and vegetable garden
plants). In some plants, during seed development the endosperm is digested and its nutrients
are incorporated into the cotyledons. In this case the mature seed lacks an endosperm. In other
plants, where an endosperm is present in the mature seed, the cotyledons are thin and leaf like
in appearance. The part of the embryo above the point of attachment of the cotyledons is
called the epicotyl. It generally gives rise to the terminal bud, leaves, and the upper part of the
stem of the young plant. The hypocotyl is the part of the embryo below the point of
attachment of the cotyledons. The radicle is the lower most part of the embryo. In some plants
hypocotyl gives rise to the lower part of the stem, while the radicle gives rise to the roots. In
other plants, the stem forms entirely from the epicotyl, and the root arise from both the
hypocotyl and radicle. (E.g. Corn)
Seed Dispersal: Seed Dispersal is the transport of seeds away from the parent plant. This is of
great importance in the survival of the species. When seeds are dispersed to places far from
the parent plant, the developing seedlings face less competition from the parent and other
offspring for light, water and minerals. Thus, many different adaptations arise for the
dispersal of seeds. These adaptations ensure dispersal by a variety of agents. These dispersal
agents include mechanical propulsion, wind, water and animals.
Mechanical Propulsion: Some plants have pods that explode when mature, causing seeds to
be thrown some distance away from the parent plant. E.g. Snapdragon, and dwarf mistletoe.
Wind: disperses the seeds of a great number of plants. Maple and ash trees produce winged
seeds; dandelions have fluffy, parachute- like structures attached to their seeds.
Animals: Many fruits are equipped with barbs so that they cling to the fur of animals. Other
fruits, such as cherries, berries are attractive to animals as food. Undigested seeds are
deposited in places far from the parent plant. Birds and bats are also important in seed
dispersal.
25
Seed Dormancy: After their dispersal from their parent plant, many seeds undergo a period
of dormancy. This resting period may last for a few weeks, for a winter, or for many years.
There are several ways in which dormancy is brought about:




In some species, dormancy occurs because the seed coat does not permit water and
oxygen to reach the embryo.
In some species, the seed coat is so strong that the embryo cannot break through it.
In some, the embryo must undergo further development before growth can occur.
In some, chemical inhibitors are present that prevent germination.
Germination
Germination is development of an embryo into a seedling. Germination will not take place
unless certain conditions are present.
1. Many seeds require periods of dormancy before germination will take place. Other
seeds do not require a dormant period.
2. If germination inhibitors are present, they must be overcome. The inhibitors must be
inactivated by special whether conditions or by being dissolved away.
3. Resistance of seed coat to penetration of water and/ or oxygen must be overcome.
4. Sufficient water must be available.
5. Sufficient oxygen must be available.
6. The temperature must be within certain favorable limits. For most seeds, the optimum
temperature range is 150- 270C.
The first phase of germination involves the absorption of sufficient water by the seed. The
enzymes of the seed embryo cannot begin to operate until sufficient water is available. These
enzymes start to change the stored food of the seed into chemical forms that the embryo can
use for growth. During this water absorption phase, most seeds swell, and their seed coats
soften. When enough water is present inside a seed, rapid cell division begins in the embryo.
A supply of oxygen is also required because respiration of the developing cells is an oxygenconsuming process. For this reason, seeds germinate best in well-aerated soil.
Many enzymes are involved in germination. An important enzyme is amylase, which converts
stored starch into usable sugar. Enzymes can operate only within a narrow range of
temperatures. That is why certain temperatures are required for germination of seeds.
26
During the germination of most seeds, the radicle is the first structure to emerge from the seed
coat. The radicle produces the primary root. This root in turn develops root hairs and
secondary roots. Adventitious roots may also appear. In bean seedling development, the
hypocotyl lengthens and arches, lifting the seed out of the soil. The seed coat drops off. The
cotyledons separate, and two small leaves appear between them. As these leaves develop, a
terminal bud emerges between them to produce main stem. The cotyledons shrivel as their
stored food supply is used. In corn seedling development, the radicle grows and breaks
through the water- softened seed coat. It grows downward to form the primary root. The
epicotyl breaks through the seed coat and grows upward. The leaves of the epicotyl remain
tightly rolled in the seed's single coleoptile, a protective sheath enclosing the leaves. After the
coleoptile breaks through the soil's surface, the leaves unroll and grow upward. A stem grows
from between these leaves. Unlike the bean seed, the corn seed remains underground.
Nourishment for the seedling comes primarily from the endosperm. The cotyledon absorbs the
food from the endosperm and transfers it to the growing part of the embryo.
27