Plant Growth and Development - Notes | Class 11

Plant Growth and Development - Growth

All 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. Eventually, they die.

Growth

Growth is an irreversible permanent increase in size of an organ, its parts, or an individual cell.
It involves metabolic processes that consume energy.

Plant growth continues throughout life due to the presence of meristems.
Meristematic cells have the capacity to divide and self-perpetuate.
The growth where new cells are always added to the plant body by the meristem is called the open form of growth.

It occurs due to lateral meristems, vascular cambium, and cork-cambium.
It causes an increase in the girth of plants.

At the cellular level, growth occurs due to an increase in the amount of protoplasm.
Since protoplasm increase is difficult to measure directly, growth is measured by parameters such as fresh weight, dry weight, length, area, volume, and cell number. E.g.,

Cell number: A maize root apical meristem can produce more than 17,500 new cells per hour.
Cell size: Cells in a watermelon can increase in size by up to 350,000 times.
Length: Growth of a pollen tube.
Surface area: Growth in a dorsiventral leaf.

Meristematic phase: Occurs in meristems at the root and shoot apexes. Cells have rich protoplasm, large nuclei, and primary, thin, cellulosic walls with abundant plasmodesmata.
Elongation phase: Occurs in cells proximal to the meristematic zone. Cells exhibit increased vacuolation, size, and new cell wall deposition.
Maturation phase: Occurs in cells further from the apex, proximal to the elongation phase. Cells attain maximal size with wall thickening and protoplasmic modifications.

Growth rate is the increased growth per unit time. which may be arithmetic or geometrical.
2 types: arithmetic and geometrical.

In arithmetic growth, following mitotic division, only one daughter cell continues to divide while the other differentiates and matures.
On plotting the length of the organ against time, a linear curve is obtained.
Mathematically, it is expressed as:

L_t = L₀ + rt

L_t = length at time ‘t’

L₀ = length at time ‘zero’

r = growth rate / elongation per unit time

Here, both daughter cells continue mitotic division.
The growth is initially slow (lag phase), then increases rapidly (log or exponential phase).
If nutrient supply is limited, growth slows, leading to a stationary phase.
Plotting growth against time yields a sigmoid (S) curve, characteristic of living organisms in a natural environment.
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.

W₁ = W₀ e^rt

W₁ = final size (weight, height, number, etc.)

W₀ = initial size at the beginning of the period

r = relative growth rate

t = time of growth

e = base of natural logarithms

Here, r is the relative growth rate, also a measure of the plant’s ability to produce new material (efficiency index). Thus, the final size W₁ depends on the initial size W₀.
Quantitative comparisons of growth can be made in two ways:
- Absolute growth rate: Measurement and comparison of total growth per unit time.
- Relative growth rate: Measurement of growth per unit time expressed on a common basis, e.g., per unit initial parameter.

Water: Essential for cell enlargement. Turgidity of cells aids in extension growth. Water provides a medium for enzymatic activities needed for growth.
Oxygen: Helps release metabolic energy for oxidation.
Nutrients: Macro and micro elements are needed for protoplas synthesis and as an energy source.
Temperature: Growth is maximum at optimum temperature. Deviations may harm plants.
Light and gravity: Affect certain phases/stages of growth.

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