Plant-growth And Development Class 11 Notes Biology Chapter 15 - CBSE

Chapter : 15

What Are Plant-growth And Development ?

Growth

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 Is Generally Indeterminate

Plant growth continues throughout the life due to the presence of meristems.

Meristematic cells have capacity to divide and self-perpetuate.
The growth where new cells are always added to the plant body by the meristem is called open form of growth.
Primary growth: It occurs due to root apical meristem and shoot apical meristem. It causes the elongation of the plants along the axis.
Secondary growth (In gymnosperms and dicots): It occurs due to lateral meristems, vascular cambium and cork- cambium. It causes increase in the girth of the organs.

Growth Is Measurable

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 and cell number.

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.

Phases of Plant Growth

Meristematic Phase

It occurs in the meristems at the root apex and the shoot apex.

Elongation Phase

It occurs in cells proximal (just next, away from the tip) to the meristematic zone.

Maturation Phase

It occurs in the cells further away from the apex.

Growth Rate

(It is the increased growth per unit time)

Arithmetic Growth

In this, 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’

Geometric Growth

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. A sigmoid curve is a characteristic of living organism growing in a natural environment.

W₁ = W₀ e^rt

Differentiation

In this process, cells derived from root apical and shoot apical meristems and cambium differentiate and mature to perform specific functions.

Dedifferentiation

Process in which living differentiated cells regain their capacity to divide.

Redifferentiation

Process in which differentiated cells that have lost their ability to divide are reformed from dedifferentiated cells and have the ability to perform specific functions.

Development

It is a process that includes all changes in the life cycle of an organism from seed germination to senescence.

It is the sum of growth and differentiation.
Plants follow different pathways in response to environment or phases of life to form different kinds of structures. This ability is called plasticity.
Heterophylly due to phases of life: E.g. In cotton, coriander and larkspur, the leaves of the juvenile plants and mature plants are different in shape.
Heterophylly due to environment: E.g. Difference in shapes of leaves produced in air and water (e.g. buttercup) nutrition, etc.

Factors Controlling The Development

Plant Hormones

Intrinsic factors: Include intracellular (genetic) or intercellular factors (such as plant growth regulators).
Extrinsic factors: Include light, temperature, water, oxygen, nutrition, etc.

The term hormone was first used by William Bayliss and Ernest Starling in 1902.