Growth comprises of several complex biochemical and morphological steps. A meristematic cell has to pass thought all these stages before it attains its mature, shape, size and structure. Broadly speaking, these various stages of growth have been categorized into three phases. These three phases of growth are:-
(i) Phase of cell formation or cell division
(ii) Phase of cell elongation or enlargement
(iii) Phase of cell differentiation or maturation
Phase of cell formation (cell division):
This is the first phase of growth. New cells are formed by the division of meristematic cells. The meristematic cells are thin walled, isodiametric and have dense non vacuolated granular cytoplasm together with a prominent nucleus. Cell division for growth is always mitotic hence it is also called vegetative cell division (meiosis is called reproductive cell division).
Synthetic activity will be maximum before mitotic cell division, as extra materials have to be produced to be distributed between the two daughter cells. The daughter cells immediately after their formation will be smaller than the parent cell and they have to be through the next two stages before they become an integral part in the structure and function of the plant system. phase of cell elongation (cell enlargement):This is the second phase of growth when cells attain their maximum size.
The enlarging cell has a large quantity of solute particles and as a result water enters into the cell increasing its turgidity and volume. Some physiologists believe that increased cellulose deposition in the enlarging cell attracts large quantities of water increasing its turgidity.
Another change that is noticed in the enlarging cell is vacuolation. A large central vacuole appears which pushes the cytoplasm to the periphery. Meanwhile the elastic cell wall dilates and elongation takes place due to stretching and deposition of molecules of cellulose. Cellulose synthesis is maximum at this stage.
There are two views regarding the cell wall elongation brought about by the synthesis of cellulose molecules. According to the first view cell wall elongation is brought about by intussusceptions i.e., new cellulose particles are added in between the older ones. The second view however states that growth is by apposition i.e., new materials are added against the old ones on the inner side of the wall. The major events during cell enlargement may be listed as follows.
(i) Increased synthesis of cytoplasm and wall materials
(ii) Osmotic entry of water resulting in stretching
(iii) Addition of new wall materials
(iv) Appearance of central vacuole bringing about cell enlargement.
Phase of cell maturation (cell differentiation):
This is the final phase of growth when the cell matures physiologically and morphologically. It assumes its final shape, size, and structure. Depending on the requirement, location, genetic factors etc differentiation sets in resulting in the formation of specialization in the mature tissue. Thus a meristematic cell matures into a parenchyma, sclerenchyma, xylem, phloem etc. Secondary cell walls are laid down during cell maturation.