The curve can be shown appearing slowly along the line and stabilizing.
During the initial stage, i.e., during the lag phase, the rate of plant growth is slow. Rate of growth then increases rapidly during the exponential phase. After some time the growth rate slowly decreases due to limitation of nutrients. This phase constitutes the stationary phase.
The curve obtained by plotting growth and time is called a growth curve. It is a typical sigmoid or S- shaped curve.
Showing Phases of Cell Division, Cell Elongation and Cell Differentiation
During growth, meristematic cells pass through the following 3 phases:
i) Cell formation phase or Formative phase
ii) Cell enlargement phase or Phase of elongation
iii) Cell differentiation phase or Phase of maturation.
During this phase the meristematic cells divide to form new cells. Division is restricted to the apical meristems, both at root and shoot tips. The newly formed daughter cells are thin walled and have dense protoplasm. The daughter cells have the same number of chromosomes and the same genetic constitution.
During this phase, the newly formed cells absorb water by osmosis resulting in the increased turgidity and expansion and dialation of the elastic cell wall. In the initial stages, the cell enlargement occurs in all directions but is later confined to a specific direction.
This occurs a little lower down the zone of elongation. Here the cells start maturing to obtain a permanent size. The thin stretched cell walls grow in thickness and then gradually undergo structural and physiological changes depending on their location in the plant.
The time interval from the cell formation phase upto the cell differentiation phase is called the grand period of growth.
Take a germinating bean seed with the radicle about 2 cm in length. Dry the seedling with the help of filter paper and mark the root at 2 mm intervals from tip upwards with water proof ink. Place the seedling on moist filter paper in a petridish for 24 hours. Observe the ink marks.
Study of Growth Regions in a Root by a Parallel-line Marking Technique
It will be observed that the lines at a little distance behind the tip become widely separated from each other while those at the tip and those higher up remain more or less intact. This clearly shows that the growth is fastest behind the apex. This region is called the region of cell elongation.
Growth is a natural phenomenon, which occurs in both plants and animals. The growth in length of the plant is measured with the help of equipment called auxanometer or auxograph.
An auxanometer is therefore an instrument used to measure the rate of growth of a plant with respect to shoot length.
A thread is tied to the tip of the growth plant, and the other end of the thread is tied to a weight after passing the thread over a pulley. The needle attached in the centre of the pulley will show deflection, as the plant growth. The deflection can be read on the graduated arc to find out the increase in length of the plant.