Plant Growth and Development

The first step in the process of plant growth is seed germination. Development is the sum of two processes: growth and differentiation. To begin with, it is essential and sufficient to know that the development of a mature plant from a zygote (fertilised egg) follow a precise and highly ordered succession of events. 

During this process a complex body organisation is formed that produces roots, leaves, branches, flowers, fruits, and seeds, and eventually they die.

1.0What is growth?

Growth is regarded as one of the most fundamental and conspicuous characteristics of a living being. 

Growth can be defined as an irreversible permanent increase in size of an organ or its parts or even of an individual cell. 

Generally, growth is accompanied by metabolic processes (both anabolic and catabolic), that occur at the expense of energy. Therefore, for example, the expansion of a leaf is growth.

The given figure shows growth of two leaves over the period of one week. If, 

AGR = absolute growth rate and RGR = relative growth rate, then correct answer is- 

AGR for leaf A - 5 cm2 per week and AGR for leaf B - 50cm2

RGR for leaf A - 100%,, RGR for leaf B - 10% 

Plant Growth Generally is Indeterminate. Plant growth is unique because plants retain the capacity for unlimited growth throughout their life. This ability of the plants is due to the presence of meristems at certain locations in their body.

The cells of such meristems have the capacity to divide and self-perpetuate. The product, however, soon loses the capacity to divide and such cells make up the plant body. 

This form of growth wherein new cells are always being added to the plant body by the activity of the meristem is called the open form of growth.

Growth is Measurable Growth, at a cellular level, is principally a consequence of increase in the amount of protoplasm.

Since increase in protoplasm is difficult to measure directly, one generally measures some quantity which is more or less proportional to it.

Growth is, therefore, measured by a variety of parameters some of which are: increase in fresh weight, dry weight, length, area, volume and cell number.

One single maize root apical meristem can give rise to more than 17,500 new cells per hour, whereas cells in a watermelon may increase in size by upto 3,50,000 times. In the former, growth is expressed as increase in cell number; the latter expresses growth as increase in size of the cell. 

The growth of a pollen tube is measured in terms of its length, an increase in surface area denotes the growth in a dorsiventral leaf.

The period of growth is generally divided into three phases, namely, meristematic, elongation and maturation.

The constantly dividing cells, both at the root apex and the shoot apex, represent the meristematic phase of growth. 

The cells in this region are rich in protoplasm, possess large conspicuous nuclei. Their cell walls are primary in nature, thin and cellulosic with abundant plasmodesmatal connections. 

The cells proximal (just next, away from the tip) to the Shoot apical meristem zone represent the phase of elongation. Increased vacuolation, cell enlargement and new cell wall deposition are the characteristics of the cells in this phase.

Further away from the apex, i.e., more proximal to the phase of elongation, lies the portion of axis which is undergoing the phase of maturation. The cells of this zone, attain their maximal size in terms of wall thickening and protoplasmic modifications.

Graph: Sigmoid (S shape ) growth curve

Growth rate: The increased growth per unit time is termed as growth rate. 

Quantitative comparisons between the growth of living system can also be made in two ways : 

(i) Measurement and the comparison of total growth per unit of time is called the absolute growth rate. 

(ii) The growth of the given system per unit of time expressed on a common basis, e.g., per unit initial parameter is called the relative growth rate.

2.0Differentiation

The cells derived from root apical and shoot-apical meristems and cambium differentiate and mature to perform specific functions. This act leading to maturation is termed as differentiation.

During differentiation, cells undergo few to major structural changes both in their cell walls and protoplasm. For example, to form a tracheary element, the cells would lose their protoplasm. 

They also develop a very strong, elastic, lignocellulosic secondary cell walls, to carry water to long distances even under extreme tension.

3.0Dedifferentiation

The living differentiated cells, that by now have lost the capacity to divide can regain the capacity of division under certain conditions. This phenomenon is termed as dedifferentiation. For example, formation of meristems – interfascicular vascular cambium and cork cambium and wound meristem from fully differentiated parenchyma cells.

4.0Redifferentiation

Meristems/tissues are able to divide and produce cells that once again lose the capacity to divide but mature to perform specific functions, i.e., get redifferentiated.

When these secondary meristematic cells again lose their property to divide, they are said to undergo redifferentiation. The process of development of an entire plant from cells of callus is also termed redifferentiation in plant tissue culture.

Example of Redifferentiation: Formation of secondary xylem and phloem from dedifferentiated cells of the vascular cambial ring.

5.0Development 

Development is a term that includes all changes that an organism goes through during its life cycle from germination of the seed to senescence. 

Plants follow different pathways in response to environment or phases of life to form different kinds of structures. This ability is called plasticity, e.g., heterophylly in cotton, coriander and larkspur. 

In such plants, the leaves of the juvenile plant are different in shape from those in mature plants. On the other hand, difference in shapes of leaves produced in air and those produced in water in buttercup also represent the heterophyllous development due to environment . This phenomenon of heterophylly is an example of plasticity. 

Figure- Leaves from a heterophyllous plant (Hygrophila ) 

shifted from terrestrial to submerged conditions.

6.0Conditions or factors for Plant Growth

• Light– Light controls growth and its phases in plants.
• Water– Water is essential for cell enlargement, extension and for keeping plant cells upright. It also provides the medium for enzymatic activities which is needed for growth. Therefore, plant growth and its phases are highly dependent on water.
• Oxygen– Metabolic energy is needed for plant growth activities. Oxygen helps to release this metabolic energy.
• Nutrients– Macro and micronutrients are sources of energy for plants. They are also needed to make protoplasm.
• Temperature– Every plant has an optimum temperature range suitable for its growth. Changes in this range are harmful to plant growth.

7.0Previous Year Questions (MCQs) (AIPMT / NEET)

Q.1  The process of growth is maximum during:                 (NEET-UG 2020)

(1) Dormancy (2) Log phase (3) Lag phase (4) Senescence

Ans.   (2) Log phase

Q.2  Which one of the following plants does not show plasticity? (NEET-UG 2022)

(1) Coriander (2) Buttercup (3) Maize (4) Cotton

Ans.  (3)  Maize

Q.3 The ability of plants to show different pathways in response to environment leading to formation of different kinds of structures is called: (Re-NEET-UG 2022)

(1) Redifferentiation (2) Development

(3) Plasticity (4) Differentiation

Ans.   (3) Plasticity 

Q.4  In tissue culture experiments, leaf mesophyll cells are put in a culture medium to form callus. This phenomenon is called as:    (NEET-UG 2023)

(1) Dedifferentiation (2) Development

(3) Senescence (4) Differentiation

Ans.   (1) Dedifferentiation