. Explain the Phases of Growth
Phases of Growth:
Growth is not a very simple process. Before completion of this process a meristematic cell has to pass through three phases:
i. Cell division,
ii. Cell enlargement, and
iii. Cell maturation (Cell differentiation)
i. Cell division (formative phase). The plane of cell division is an important factor in determining the shape of an organism. If cell divides in one plane continuously, elongated organs like stem and root are formed. If divisions occur in two planes, flat structures like leaf are formed. In case division takes place in three or more planes, bulky and isodiametric structures appear. The outstanding characteristic of cell division is the reduplication and division of nucleus by mitosis.
The final stages of cell division are accompanied by the formation of a new cell wall, dividing the cell contents into new cells. The cell division is generally completed in two stages: (a) Karyokinesis (division of nucleus) and (b) Cytokinesis (division of cytoplasm).
The mechanism controlling the cell division differs in plants and animals. The hormones like auxins, cytokinins and gibberellins play important roles in cell division of plants, but they have no influence on the cell division in animals. Cytokinins have been found to influence the cell division in callus in culture.
ii. Cell enlargement. Cell enlargement is dominant and most obvious phase in the growth of plant. Increased vacuolation, cell enlargement and new cell wall deposition are the characteristics of the cells in this phase.. It is caused mainly by absorption of water. Due to endosmosis, divided cells elongate, as a balloon expands when it is filled with air under pressure. Elongation is accompanied by formation of large central vacuole and extension of cell wall. Due to endosmosis and development of turgidity, thin elastic and young cell walls expand several times its original length. The first event in the cell elongation is plasticization of the cell wall due to hormones like auxins. Cell enlargement manifests itself in the visible sign of growth— increase in size and weight of the organ or the whole plant. Thus, there is a marked difference between the small, rich protoplasmic cells of the meristem and the large vacuolated cells resulted from cell enlargement.
iii. Cell differentiation. Cell enlargement finally leads to the development of specialized cells such as fibres, tracheids, vessels, sieve tubes, cork cells, collenchyma, sclereids etc. Changes in shape takes place in fibres, tracheids, vessels etc. The cells of this zone, attain their maximal size in terms of wall thickening and protoplasmic modifications. Deposition of cutin and suberin occurs in cork cells. Loss of nucleus and other cell organelles take place in sieve tubes. In parenchymatous cells, the entire proto¬plasm remains alive and functional.
In this phase of maturation cells start maturing and become permanent. The time interval between formative phase and maturation phase is called grand period of growth.