Plant Growth

                                                                  Plant Growth

Characteristics of growth:

• Growth increases in protoplasm at cellular level.
• Stem and roots are indeterminate in growth due to continuous cell division and is called open form of growth. The primary growth of the plant is due to the activity of apical meristem where, new cells are added to root and shoot apex causing linear growth of plant body.
• The secondary vascular cambium and cork cambium add new cells to cause increase in girth.
• Leaves, flowers and fruits are limited in growth or of determinate or closed form growth.
• Monocarpic annual plants produce flowers only once during lifetime and dies. Example: Paddy and Bean
• Monocarpic perennials produce flowers only once during life time but the plants survive for many years. Example: Bamboo.
• Polycarpic perennials produce flowers every year during life time. Example: Coconut.

Indication of growth:

Growth in plants can be measured in terms of,

i. Increase in length or girth (roots and stems)

ii. Increase in fresh or dry weight

iii. Increase in area or volume (fruits and leaves)

iv. Increase in number of cells produced.

Phases of growth:

There are three phases of growth,

1. Formative phase

2. Elongation phase

3. Maturation phase

1. Formative phase:

Growth in this phase occurs in meristematic cells of shoot and root tips. These cells are small in size, have dense protoplasm, large nucleus and small vacuoles. Cells divide continuously by mitotic cell division. Some cells retain capability of cell division while other cells enter the next phase of growth.

2. Elongation Phase:

Newly formed daughter cells are pushed out of the meristematic zone and increases the volume. It requires auxin and food supply, deposition of new cell wall materials (intussusception),addition of protoplasm and development of central vacuole take place.

3. Maturation Phase: 

During this stage cells attain mature form and size. Thickening and differentiation takes place. After differentiation, the cells do not grow further.

Kinetics of growth:

It is an analysis of the motion of cells or expansion.

1. Stages in Growth rate:

The total period from initial to the final stage of growth is called the grand period of growth. The total growth is plotted against time and ‘S’ shaped sigmoid curve (Grand period curve) is obtained. It consists of four phases.

They are:

i. Lag phase

ii. Log phase

iii. Decelerating phase

iv. Maturation phase

i. Lag phase

In this phase new cells are formed from pre-existing cells slowly. It is found in the tip of the stem, root and branches. It is the initial stage of growth.

ii. Log phase or exponential growth

Here, the newly formed cell increases in size rapidly by deposition of cell wall material. Growth rate is maximum and reaches top because of cell division and physiological processes are quite fast. The volume of protoplasm also increases. It results in rapid growth and causes elongation of internode in the stem.

iii. Decelerating phase or Decline phase or slow growth phase

The rate of growth decreases and becomes limited owing to internal and external or both the factors because the metabolic process becomes slow.

iv. Steady state period or maturation phase

In this phase cell wall thickening due to new particle deposition on the inner surface of the cell wall takes place. The overall growth ceases and becomes constant. The growth rate becomes zero.

2. Types of growth rate

The increased growth per unit time is termed as growth rate. An organism or part of an organism can produce more cells through arithmetic growth or geometric growth or both.

i. Arithmetic Growth Rate:

If the length of a plant organ is plotted against time, it shows a linear curve and this growth is called arithmetic growth.

The rate of growth is constant and it increases in an arithmetic manner. Only one cell is allowed to divide between the two-resulting progeny cell.

One continues to divide but the other undergoes cell cycle arrest and begins to develop, differentiate and mature.

After each round of cell division, only a single cell remains capable of division and one new body cell forms. For example, starting with a single cell after round 1 of cell division there is one dividing cell and one body cell. After round 2 there are two body cells, after round 3 there are three and so on. The plants single dividing cell would undergo one million rounds of nuclear and cellular division. If each round requires one day, this type of arithmetic increase would require one million days or 2739.7 years. This arithmetic rate is capable of producing small number of cells present in very small parts of plants. For example; the hair on many leaves and stems consists of just a single row of cells produced by the division of the basal cell, the cell at the bottom of the hair next to other epidermal cells. Hair may contain 5 to 10 cells by the division of the basal cell. So, all its cells could be produced in just five to ten days.

ii. Geometric growth rate:

This growth occurs in many higher plants and plant organs and is measured in size or weight. In plant growth, geometric cell division results if all cells of an organism or tissue are active mitotically.

iii. Arithmetic and Geometric Growth of Embryo:

Plants often grow by a combination of arithmetic and geometric growth patterns. A young embryonic plant grows geometrically and cell division becomes restricted to certain cells at the tips of roots and shoots. After this point, growth is of the slower arithmetic type, but some of the new cells that are produced can develop into their mature condition and begin carrying out specialized types of metabolism. Plants are thus a mixture of older, mature cells and young, dividing cells

3.Conditions of growth:

Plant growth is influenced by a variety of external and internal factors. A brief account of these factors is given below:

I. External Factors:

a. Water:

Water is essential for cell enlargement as well as growth in the size of the cell. Turgidity of cells helps in growth extension. Water provides the medium for enzymatic activities needed for growth.

b. Nutrition:

Nutrition plays an important role in the formation of protoplasm. Macro and micro elements are very important as sources of energy. For example, carbon and oxygen in carbon-di-oxide and hydrogen in water are assimilated in photosynthesis.

c. Temperature:

Temperature plays a significant role in the growth of the plant. Proper growth of a plant occurs at a about 28⁰C to 30⁰C temperature and above 45⁰C will damage the protoplasm and hinders the growth.

d. Oxygen:

Oxygen has a vital role in the growth of the plant. It helps in releasing metabolic energy essential for growth activities. It is necessary for respiration.

e. Light:

Light has its own contribution in the growth of the plant. Light is important for growth and photosynthesis. Light stimulates healthy growth. Absence of light may lead to yellowish in colour. This is called etiolation.

II. Internal Factors:

a. Genes are intracellular factors for growth.

b. Phytohormones are intracellular factors for growth. Example: auxin, gibberellin, cytokinin.

c. C/N ratio.

The ratio of carbohydrates and nitrogenous compounds regulate the specific pattern of growth in plants. For example, if a plant contains more nitrogenous compounds as compared to carbohydrates it produces more protoplasm less mechanical tissues and vigorous vegetative growth. On the other hand, less nitrogenous compounds and more carbohydrates favour the synthesis of more wall material, less protoplasm, and more mechanical tissues.

4. Measurement of growth:

5. Sequence of developmental process:

in a plant cell Development is a term that includes all the changes that an organism goes through during life cyle from germination of a seed to senescence. Diagrammatic representation of the sequence of processes which constitute the development of a cell of a higher plant. It is also applicable to tissues/organ.

1. Differentiation:

The process of maturation of meristematic cells to specific types of cells performing specific functions is called differentiation.

2. Dedifferentiation:

The living differentiated cells which had lost capacity to divide, regain the capacity to divide under certain conditions. Hence, dedifferentiation is the regaining of the ability of cell division by the differentiated cells. Example: Interfascicular cambium and Vascular cambium.

3. Redifferentiation:

Differentiated cells, after multiplication again lose the ability to divide and mature to perform specific functions. This is called redifferentiation. Example: Secondary xylem and Secondary phloem.

4. Plasticity:

Plants follow different pathways in response to environment or phases of life to form different kinds of structures. This ability is called plasticity. Example: Heterophylly in cotton and coriander. In such plants, the leaves of the juvenile plant are different in shape from those in mature plants. On the other hand, the difference in shapes of leaves produced in air and those produced in water in buttercup also represent the heterophyllous development due to the environment. This phenomenon of heterophylly is an example of plasticity.