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Plant Growth
Plant growth occurs by cell division,
elongation and differentiation (specialization of cells). Cell division occurs
primarily in regions of undifferentiated cells known as meristems. Meristematic
cell has cellulose walls, large protoplasm with little
or no vacuoles. It produces several cells which increase in size by absorbing
water and developing large vacuoles. The cells differentiate into different
permanent tissues.
There are two kinds of meristems:
1. Primary Meristems: are responsible for an increase in length or height called primary growth. The primary meristem found at the tips of stems or roots is called the apical meristem. It gives rise to the primary permanent tissues. Apical meristems may differentiate into three kinds:
a. Protoderm: around the outside of the stem and develops into the epidermis.
b. Ground meristem: give rise to the cortex (ground tissues). It produces the cork cambium.
c. Procambium: develops into primary xylem and primary phloem. It also produces the vascular cambium.
2. Secondary meristems: cause secondary growth, or an increase in width. There are two types of secondary meristems:
i. Vascular cambium: produces secondary xylem and secondary phloem, which may continue through the life of the plant. It is what gives rise to wood in plants.
ii. Cork cambium: gives rise to the bark of a tree.
The growth of the roots and stems in length with the help of apical meristem is called the primary growth. Increase in thickness or girth of the stems and roots due to the formation of secondary tissue is called secondary growth and secondary thickness. Secondary growth occurs only in dicot stem and root.
Secondary Growth
The two kinds of tissue involved in
secondary growth: vascular cambium, and cork cambium.
Steps in secondary growth
Formation of cambium ring: The cambium within vascular bundles divide radially to form the intra-fascicular cambium. The parenchymal cells in the medullary rays which lie between the adjacent vascular bundles also divide to form the inter-fascicular cambium. The cells in the intra-fascicular and inter-fascicular cambium fuse and result in the formation of a complete circular ring called cambium ring. The cambium ring forms the secondary tissues in the stelar region.
Formation of Vascular Tissues: the cambium ring undergoes division to form new cells, both on outer and inner sides. The new cells formed on the outer side gradually modify into the elements of secondary phloem. The cells formed on the inner side gradually modify into secondary xylem. Secondary xylem forms the wood.
Cork cambium or phellogen: develops in the cortex. It is made of narrow, thin-walled rectangular cells. Phellogen undergoes division to form outer cells called cork cells or phellem and inner cells called secondary cortex or phelloderm. The cork cells produce suberin in the cell walls making them impermeable to water and gases. Phellogen, phellem, and phelloderm are collectively known as periderm.
Due to the formation of periderm, the epidermis is subjected to pressure and as a result it breaks at several places to form openings called lenticels. Lenticels permit exchange of gases.
Bark: refers to all tissues. exterior to the vascular cambium, including secondary phloem, pericycle, cortex, and periderm.
Coordination and Control in Plants
The growth and development of a plant are influenced by genetic factors, external environmental factors, and internal factors. Plants respond to many environmental factors such as light, gravity, water, inorganic nutrients, and temperature. Example of internal factor is plant hormones.
Plant hormones
Hormone is a chemical released from one cell that affects growth and development of target cells. There are five catagories of plant hormones: Auxins, Gibberelins, Cytokinins, Abscisic acid and Ethylene.
Auxin
Auxin is one of the most important plant hormones. It’s produced by shoot apical meristem tissue. Example naturally occurring auxins is IAA (indoleacetic acid).
1. It promotes cell elongation in stems.
2. It participate in stem or root growth responses to light or gravity.
3. It inhibits lateral bud sprouting (apical dominance).
4. It retards abscission (dropping off) of flowers, fruits and leaves.
5.. IAA applied to fruit trees to promote flowering and fruit development.
Gibberellin
Gibberellin is produced in seeds, roots, shoot, chloroplast and young leaves
1. It promote cell elongation.
2. It stimulates the breakdown of starch which may influence flowering.
3. It causes seed germination/breaking dormancy in seeds.
Cytokinin
Cytokinin is produced by young fruit and root tips
1. It stimulates cell division in root meristems.
2. It promotes leaf expansion and retard leaf aging.
Abscisic Acid
Abscisic Acid occurs in ripe fruit, seeds, old leaves and leaves chloroplast
1. It inhibits cell growth,
2. it prevent water loss by promoting stomatal closure
3. promotes seed or bud dormancy.
4. It promotes leaf fall
Ethylene
Ethylene produced in fruit old (senescent) leaves.
1. It stimulates ripening of fruit.
2. It promotes abscission of leaves, fruit, flowers .
Growth and Movement in Plant
Plant movement is as the results of irritability of protoplasm. It is sensitive and respond to internal or external stimuli. Unicellular (lower) plants e.g. algae can freely move in water. Higher plants move only by change in direction or position of some organs.
Types of Movement in Plants
1. Turgor Movement: is caused by change in the cell volume due to turgor changes and is reversible. Example is rolling up of leaves in dry weather.
2. Growth Movement: is the changes in the position of plant organs due to enlargement of cells or due to increase in number of cells. Growth movement is irreversible.
i. Autonomic or Spontaneous: is movement which is independent of any external stimuli. E.g. in twig plants, the tip of the stem grows at unequal rate in different segments and thus causes the twining around the support. This movement is also called Notation.
ii. Induced or Paratonic: is caused by external stimuli. Paratonic movement can be classified into: tropic movement (tropism), tactic movements (taxes) and nastic movements (nasties).
Tropism in Plants
Tropism is growth movement of part of plants in response to external unilateral stimulus. Plants move in response to environmental stimuli such as: light, gravity and mechanical disturbances.
1. Movement of plant toward an environmental stimulus is called a positive tropism.
2. Movement of plant away from a stimulus is called a negative tropism.
Types of Tropism in Plants
Phototropism
Phototropism is a growth movement of plant organs in response to unilateral effect of light. Stems are positively phototropic and roots are negatively phototropic. Leaves are transversely phototropic as they keep their faces at right angles to the direction of light and are said to be diaphotototropic.
Experiment on Phototropism
Aim: To demonstrate the phototropic response of shoots.
Method
Observation
and conclusion
It will be seen that the shoot is found bending towards the source of light, showing that shoot is positively phototropic.
Geotropism
Geotropism a growth movement
of stems and roots in response to the force of gravity. Primary roots are positively
geotropic. Primary shoots are negatively geotropic.
Lateral roots grow horizontally, at right angles to force of gravity and are
known diageotropic.
Experiment of Geotropism
Aim: To demonstrate the geotropic response of shoots and roots.
Method (1)
Take 3 bean seeds and keep them in different positions on moist cotton in a petrish dish. Placed the set-up on its edges and support it to prevent rolling or movement. Observe them after a few days.
Observation
It will be seen that after a few days all
the seeds have germinated in different positions. In all the cases the roots
are found growing in the downward direction and shoots in the upward direction.
Method (2)
Take a pot with a
young seedling and rest it on its side so that the stem lies horizontally.
Observe after a few days. Arranged
another pot with young seedling on a special apparatus called clinostat
designed to serve as a control. Set the clinostat to rotate the pot. Observe
after a few days.
Observation: It is seen that the shoots are found growing upwards after showing a curvature. The roots have grown downwards.
Conclusion: Roots are positively geotropic and shoots are negatively geotropic. If a
plant is rotated on a clinostat, it neutralizes the effect of gravity and the
shoot continues to grow horizontally without any curvature.
Hydrotropism
Hydrotropism: is a growth movement of plant parts in response to the unilateral source of water. Roots are positively hydrotropic.
Chemotropism
Chemotropism is a growth movement of plant parts in response to a unilateral source of chemicals. Example: Growth of pollen tube through the style towards the embryo sac.
Thigmotropism
Thigmotropism or haptotropic is the growth movement of certain parts of plant in response to touch or contact. E.g., tendrils are positively haptotropic.
Tactic Movement
A taxis is the movement of an entire cell or organism in response to an external unilateral stimulus such as light or the presence of food. The direction of the movement is obtained by the direction of the stimulus. Taxes are classified based on the type of stimulus, and whether the organism move towards or away from the stimulus. If the organisms moves towards the stimulus, the taxis is positive, while if it moves away, then the taxis is negative.
Types of taxes
1. Chemotaxis: is a movement of organism in response to a unilateral chemical. E.g. the antherozoids of ferns, and mosses respond to chemicals secreted by the archegonia.
2. Phototaxis: is the movement of an organism in response to light (either intensity or direction). E.g. Euglena move towards a light source.
3. Thermotaxis: is a migration in response to temperature. E.g. Movement of Slime molds, Amoeba or human in response to temperature gradients.
4. Gravitaxis (geotaxis): is a movement in response to gravity. Both positive and negative gravitaxes are found in a variety of protozoans.Nastic Movement
It is the movement of certain parts of plants in response to diffuse stimuli. Diffuse stimulus is the one that is non-directional. The nastic movements are independent of the direction of the stimulus. Examples are
1. opening and closing of buds to form shoot
2. opening of stomata at day and closure at night
3. sudden closure of Mimosa pudica (Touch-me-not) leaves when touched
4 folding of leaves to prevent excessive loss of water
Experiment
to Demonstrate the Effect of Auxins on Elongation of Shoot
Darwin used coleoptiles (young shoot) to study phototropism. His experiments led him to conclude that "when seedlings are freely exposed to unilateral light, some chemicals (auxins) are transmitted from the upper to the lower part, causing seedling elongation and bending towards the light source".
Procedure
1. The seeds of a suitable plant are allowed to germinate and grow in the dark.
2. The tip of the seedling is cut off just above the zone of elongation and then replaced few hours later.
3. A razor blade is inserted into the tip of the shoot to separate it from the lower part.
4. An agar block is inserted into the tip of another shoot to separate it from the lower part.
5. In another shoot also, a razor blade or mice sheet is inserted into one of the side of the shoot just in front of the zone elongation.
Observation and Interpretation
a. It is seen that no further growth takes place when tip is removed. However, if the tip is replaced, growth begins again.
b. Growth stops when razor blade is inserted.
c. Growth continues when the agar block is inserted. This means a chemical produced at the tip passes through the agar block into the shoot to cause elongation.
d. Growth stops on the side of the shoot where razor blade is inserted. The shoot bends over the side where blade is inserted.
Conclusion
The experiment show that chemical substances
(signals) are made in the tip of the shoot and transported to the growing shoot
where they produce an effect. These
chemical substances are known as plant hormones or growth
factors e.g. auxins.
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