Previous Topic: Cytoplasmic Division
Note : this is a continuation on the topic, Growth In Plant And Animal, so you might want to consider reading that before reading this, but if you think you are good to go, then who am I to complain.
Regulation Of Growth By Hormones
Regulation of growth simply means co-ordination and control of growth, so that different parts of an organism grow in proportion and in a balanced manner to one another. For example, in plants, as the shoot system grows, the root system grows to match it.top↑
In both plants and animals, growth is regulated by hormones.
In animals, hormones are substance secreted by an endocrine gland or a tissue, which is transported by the bloodstream to other parts of the body, and which causes a response in a specific organ or tissue.
In plants, hormones are chemical substances produced in one part of the plant, in small quantities, which is capable of producing effects in other parts of the plant.
Hormones That Regulate Growth In Plants
Hormones that regulate growth in plants are, auxins, cytokinis, gibberelins, abscisc acid and ethylene.
Note : cytokinis are different from cytokines.
Cytokines are various small regulatory proteins that regulate the immune system of a cell.top↑
Now let analyze the hormones list above one by one.
First let start with auxins
1) Auxin : In plants, e.g wheat, maize, there is a sheath, called the coleoptile, which covers and protects the plumule (the first bud of a growing plant) as the seed germinates. The coleoptile pushes through the soil and the the plumule bursts through the coleoptile. Growth in the coleoptile is mainly by cell elongation. The seedlings bends towards light coming from one side.top↑
In 1880, Charles Darwin studied this response of seedlings towards light, by using coleoptile, and he found out that, if the tip of the coleoptile was cut off or covered with metal foil, the bending of seedlings towards the light will not occur. This show that, the tip of the coleoptile received the stimulus of light coming from one side, but the response occurred at a little distance behind the tip.
In 1982, it was later found that, if the tip of the coleoptile was cut off or removed, the growth of the coleoptile stopped but if the cut off tip was later replaced, the coleoptile started growing again.
If the cut off tip was placed in contact with the cut surface with a small block of agar for sometime and the agar block was placed on the cut coleoptile, the agar stimulated growth. This hence shows that, the tip of the coleoptile thus produces substances which Cell And Its Environment (Diffusion) downwards into the stem and stimulated the aspect of growth represented by cell elongation. This natural chemical substances has been isolated, and it is called auxin which has a scientific name of indole acetic acid (IAA). Several other substances that perform similar action as IAA are known as indole butyric acid, where some of these substances have been synthesized artificially.
Actions Or Effects Of Auxin In Plants
a) Suppression of lateral buds : auxin, which is produced by cells at the shoot tip and which Cell And Its Environment (Diffusion) downwards in the stem, inhibits the development of lateral buds as a result of difference in the concentration of auxin that stimulated the stem or the lateral buds. Auxin concentration which stimulates cell elongation in the stem is too high for the lateral buds and hence, hinder the growth of lateral buds. In this way, the apical bud continues to grow while the lateral buds are suppressed and this process is called apical dominance. In order to suppressed the apical buds growth, the hedge is cut, and when the hedge is cut, the apical buds are cut, then the apical dominance is removed and lateral buds started to grow.top↑
b) Leaf fall : auxin formed by young leaves diffuses downwards through the petioles into the stem and prevents the leaves from falling off. When a leaf grows old and its ability to produce auxin drops or stops, the leaf falls off. Leaf fall is preceded by the death of a layer of cells at the base of the petiole called the abscision layer.top↑
Abscision layer is a layer of parenchyma cells. when the leaf of a plant gets old, auxin production stops, and the parenchyma cells in the abscision layer die and dry up, the leaf can then fall.
c) Stem elongation : auxin produced by cells at the shoot tip diffuses downwards in the stem and brings about that aspect of growth represented by cell elongation. This elongation of cells behind the stem tip makes the stem to increase in length.top↑
d) Phototropic response : auxin makes stems to bend towards light rays that comes to a plant from one side. When light rays reach a stem from one side, the unilateral light rays causes unequal concentration of auxin on different sides of the stem. The shaded side has a higher concentration of auxin and grows faster than the lit side, hence the bending towards the source of light.top↑
e) Root elongation : auxin produced by cells at the root tips stimulates cell elongation behind the root rip and brings about growth in length of the root. The concentration of auxin that stimulates cell elongation in the root is smaller than the concentration of auxin which stimulates cell elongation in the stem.
f) Renewal of cambium activity : In the dry season, many trees become dormant and shed their leaves. At the beginning of the raining season, new shoot buds are formed which produce auxin that diffuses into the stem and stimulates cambium cells to become active.top↑
g) Root initiation : when a stem cutting is planted, such as stem cutting of cassava, buds appear on the stem. It is this buds on the stem that produce auxin which diffuses into the stem and stimulates cambium cells to become active.
h) Geotropic response : when a root is placed horizontally, more auxin accumulates on the side than on the upper side. In the root, the high concentration of auxin on the lower side hinder cell elongation, while the lower concentration of auxin on the upper side promotes cell elongation. Hence, this implies that, the roots bends towards gravitytop↑
2] Cytokinis : are kind of plant hormones that are produced in actively growing tissues such as embryos and developing roots and fruits.
Functions Or Effects Of Cytokinis In Plants
a) Cytokinins and auxins interact to produce cell Division And cell Enlargement. In cells growing in the laboratory in tissue culture, an auxin with a low concentration of cytokinins produces rapid cell enlargement leading to the formation of few large cells. On the other hand, cytokinins with low concentration of auxin results in rapid cell division forming a large number of small cells. And note that, growth depends on a balance between the two hormones.top↑
2) Cytokinins promote the growth of lateral buds.
3) Cytokinins retard ageing of plant organs.
4) Cytokinins stimulates cell division and cell enlargement.
5) Cytokinins stimulates growth in leaves.
6) Cytokinins break dormancy in seeds.
7) it also stimulates the development of shoot buds from cells.
8) Cytokinins induce flowering in some plants.top↑
3] Gibberellins : are plant hormones that stimulates both cell division and cell elongation.
Effects Of Gibberellins On Plants
a) During germination in grass seeds, the embryo secretes gibberellin which stimulates the aleuron(protein content) layer to produce enzymes such as alphamylase, which hydrolize the starch in the endosperm.top↑
b) Gibberellins cause stem elongation in normal non-dwarf plants.
c) Gibberellins aid growth in dwarf plants. When gibberellins are applied to a genetic dwarf plants, the plants grow to normal size.
d) In some plants, such as cabbage, form leaf rosettes before flowering. In a rosette, the leaves develop and remain clustered together because the stem does not elongate. These plants can be induced to flower by exposure to long days or to a period of cold as in case of temperature countries.top↑
Note : Application of gibberellin causes the plants to grow tall and flower.
Abscisic Acid Or Dormin
4] Abscisic acid or dormin are produced in many parts of the plant such as stem, fruit, seed and leaf.
Functions Or Effects Of Abscisc Acid Or Dorman
a) Abscisc acid stimulates the formation of the abscision layer which leads to friut, leaf and flower fall.
b) It hinder mitosis in meristematic cells such as vascular cambium.
c) Abscisic acid hinder bud development and growth of plants.
d) Abscisic acid causes dormancy of seeds.top↑
5] Ethylene : is a gaseous regulator which is produced by various parts of the plants.
Functions And Effects Of Ethylene On Plants
a) Ethylene is associated with the process of ageing of plant organs.
b) Ethylene accelerates the abscision of leaves, flower and friuts.
c) It hasten the ripening of fruits.
d) Ethylene hinders stem elongation.
e) And it also causes the changes that arises during ripening of fruits.
Hormones That Regulate Growth In Animals
There are two important hormones that regulate growth in animals, pituitary gland and thyroid gland, let analyze them one after the other.top↑
a] Pituitary gland : are endocrine gland that sits in a small, bony cavity at the base of the brain. The pituitary gland has a very important effect on growth. If the pituitary gland is removed from a young animal, the animal ceases to grow and fails to develop.
b] Thyroid gland : is an endocrine gland located on the front of the neck. The thyroid gland also has a stong effect on animal growth.top↑
Functions Or Effects Of Thyroid Gland On Animal
1) In frog, if tadpoles do not produce enough thyroxine as it should, the tadpole will not metamorphose into adult frogs and if the tadpole produces too much thyroxine, metamorphosis will be quicker than normal.
2) If an animal does not produce enough thyroxin ( C9H11NO3) , such animal will not grow to a normal size.
3) Too much of thyroxin in an animal system causes hyperactivity.
Hyperactivity is when an individual (might be animal or man) eats too much food, too active and loses weight.
4) It is the thyroxin that stimulates mental growth and development of an animal.top↑
Thanks for joining us, we are now through with Growth In Plants And Animals our next topic is cell reactions to its environment, thank you ones again.
Feel free to ask our tutors any question via the comment box or use our ask question page and we will be happy to answer your question and take the session again if necessary, your comment notifications is a motivation to us, so don't forget to write something.