15. PLANT GROWTH AND DEVELOPMENTAll plant cells are descendants of the zygote (fertilized egg).
The zygote develops into a mature plant through growth and differentiation forming roots, leaves, branches, flowers, fruits and seeds. Then they eventually die.
Growth is an irreversible permanent increase in size of an organ or its parts or an individual cell.
It involves metabolic processes that consume energy.
Plant growth continues throughout the life due to the presence of meristems.
Meristematic cells have capacity to divide & self-perpetuate.
The growth where new cells are always added to the plant body by the meristem is called open form of growth.
- It occurs due to root apical meristem & shoot apical meristem.
- It causes the elongation of the plants along the axis.
Secondary growth (In gymnosperms & dicots):
- It occurs due to lateral meristems, vascular cambium & cork-cambium.
- It causes increase in the girth of organs.
At cellular level, growth occurs due to increase in the amount of protoplasm.
Increase in protoplasm is difficult to measure directly. So growth is measured by parameters like increase in fresh weight, dry weight, length, area, volume & cell number. E.g.
- Cell number: E.g. A maize root apical meristem can produce more than 17,500 new cells per hour.
- Cell size: E.g. Cells in a watermelon can increase in size by up to 3,50,000 times.
- Length: E.g. Growth of a pollen tube.
- Surface area: E.g. Growth in a dorsi-ventral leaf.
3 phases: meristematic, elongation & maturation.
- Meristematic phase: It occurs in the meristems at the root apex & the shoot apex. Here, cells have rich protoplasm and large nuclei. Cell walls are primary, thin & cellulosic with abundant plasmodesmata.
- Elongation phase: It occurs in cells proximal (just next, away from the tip) to the meristematic zone. The cells have increased vacuolation, size and new cell wall deposition.
- Maturation phase: It occurs in the cells further away from the apex, i.e., more proximal to the phase of elongation. The cells attain maximal size in terms of wall thickening and protoplasmic modifications.
Detection of zones of elongation by the parallel line technique.
Zones A, B, C, D immediately behind the apex have elongated most.
It is the increased growth per unit time.
The growth rate may be arithmetic or geometrical.
Lt = length at time ‘t’L0 = length at time ‘zero’r = growth rate / elongation per unit time.
Here, both daughter cells continue mitotic cell division.
In most systems, the initial growth is slow (lag phase), then it increases rapidly (log or exponential phase).
If nutrient supply is limited, the growth slows down leading to a stationary phase.
On plotting the parameter of growth against time, we get a typical sigmoid (S) curve.
A sigmoid curve is a characteristic of living organism growing in a natural environment. It is typical for all cells, tissues and organs of a plant.
Exponential growth is expressed as W1 = W0 ert
- W1 = final size (weight, height, number etc.)
- W0 = initial size at the beginning of period
- r = growth rate (relative)
- t = time of growth
- e = base of natural logarithms
Quantitative comparisons between the growth can also be made in 2 ways:
- Absolute growth rate: Measurement & comparison of total growth per unit time.
- Relative growth rate: Measurement of growth of the given system per unit time expressed on a common basis, e.g., per unit initial parameter.
Diagrammatic comparison of absolute & relative growth rates
Conditions (essential elements) for Growth
- Water: Essential for cell enlargement. Turgidity of cells helps in extension growth. Water provides medium for enzymatic activities needed for growth.
- Oxygen: It helps to release metabolic energy for growth.
- Nutrients: Macro & micro elements are needed for the synthesis of protoplasm and act as source of energy.
- Temperature: At optimum temperature, growth is maximum. Deviation from this may harm the plants.
- Light & gravity: Affect certain phases/stages of growth.