Growth is defined as a permanent or irreversible increase in dry weight, mass or volume of cell, organ or organism.
Plant growth takes place in three steps or phases cell division (meristematic), cell elongation and cell maturation
(i) Cell Division (Meristematic) Phase
It is also called formative phase.
New cells are produced hy mitotic divisions of the pre-existing cells.
The meristematic cells have thin cellulose walls with abundant plasmodermal connections, dense protoplasm and conspicuous nuclei.
In higher plants cell division occurs in meristems or growing points.
As the formation of new cells requires intense biosynthetic activity, the rate of respiration in - the cells of formation phase is very high.
(ii) Cell Enlargement Phase
It is also called phase of cell elongation.
This phase lies just behind the growing points and is mainly responsible for growth of plant parts.
The newly formed cells, produced in formative phase undergo enlargement. '
The cell walls of the enlarging of cell show plastic extension through enzymatic loosening of microfibrils and deposition of new materials.
The enlarging cell also develops a central vacuole, rate of respiration is high but less than that of the cells in the formative phase.
Thus, this phase is characterized by cell enlargement, new cell wall deposition and increased vacuolation.
(iii) Cell Maturation Phase
This phase occurs just behind the phase of elongation.
The enlarged cells develop into particular type of cells by undergoing structural and physiological differentiation.
Hence, at this phase all the diverse tissue types observed in root or stem.