Just like any other organism, plants too grow in height and size over time. So how does plant growth occur? Or how flowers and fruits in a tree appear and fall periodically? All events in plants, right from the zygote stage to the full-grown plant take place in an orderly manner. This is called development. Development is the sum of growth and differentiation. Let us understand plant growth and its phases in more detail.

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## Plant Growth And Its Phases

Growth is the most fundamental characteristic of any living organism. It is defined as “an irreversible, permanent increase in the size of an organ or its parts, or even of an individual cell”. Generally, growth is accompanied by metabolic processes (catabolic and anabolic) that spend energy.

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### Characteristics Of Plant Growth

#### Plant Growth Is Indeterminate

Plants have the unique ability to grow indefinitely throughout their life due to the presence of ‘meristems’ in their body. Meristems have cells that can divide and self-propagate. This is called ‘open form of growth’ because new cells are constantly added to the plant body by the cells in the meristem.

Meristems in the roots and shoots of plants are responsible for ‘primary growth of the plant’. These increase the height of the plant. On the other hand, lateral meristems increase the width of the plant. This is known as the ‘secondary growth of the plant’.

#### Plant Growth Is Measurable

At a cellular level, growth simply means an increase in the amount of protoplasm. Since this is hard to measure, growth is measured in a quantity proportional to this increase. Therefore, growth is measured in terms of increase in cell number, area, volume, length etc.

Did you know that the cells in a watermelon can increase in size up to about 3,50,000 times! On the other hand, a single apical meristem in maize roots can produce about 17,500 new cells per hour! In the first case, growth is in terms of the size of the cell whereas in the second case, it is in terms of cell number.

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### Phases Of Plant Growth

#### Meristematic Phase

The cells in the root and shoot apex of a plant are constantly dividing. They represent the meristematic phase of growth. The cells in these regions have large nuclei and are rich in protoplasm and their cell walls are thin and contain cellulose.

#### Elongation Phase

The cells in the zone just after the meristematic region represent the phase of elongation. The characteristics of cells in this zone are cell enlargement, increased vacuole formation and new cell wall deposition.

#### Maturation Phase

Just close to the phase of elongation, but away from the apex lies the phase of maturation. The cells in this region reach their maximum size with respect to their protoplasm and cell wall thickening.

### Rates Of Growth

The growth rate is defined as increased growth per unit time. An organism or its parts can give rise to more cells in a number of ways. The increase in growth may be arithmetic or geometrical and thus, the rate of growth can be expressed mathematically.

#### Arithmetic Growth

Following mitotic cell division, only one cell continues to divide while the other begins to differentiate. A simple example of this is the elongation of a root at a constant rate. When you plot this increase in length against time, you get a linear curve as shown below. This is expressed mathematically as –

L_{t} = L_{0} + rt (where, L_{t} is the length at time ‘t’, L_{0} is the length at time ‘zero’, r is the growth rate)

#### Geometric Growth

Most biological systems show an initial growth phase which is slow. This is the ‘lag phase’. This phase is followed by a period of exponential growth and is called ‘log phase’ or ‘exponential phase’. Here, following mitotic cell division both the daughter cells continue to divide.

However, due to limited nutrient supply, the growth slows down giving rise to the ‘stationary phase’. When we plot this growth, we get a sigmoidal curve or S-curve as shown below. This growth curve is typical for all cells, tissues, organs and is characteristic of an organism living in a natural environment. Mathematically, it is expressed as –

W_{1} = W_{0} e^{rt }(where W_{1} is final size, W_{0} is initial size, e is base of natural logarithm, r is the growth rate and t is the time of growth). Note: Here, r is the growth rate and is also the measure representing the ability of the plant to produce new plant material. This is the ‘efficiency index’.

Growth rate can also be described as absolute or relative. The** absolute growth rate **is the measurement of total growth per unit time. The** relative growth rate **is also the growth of a given system per unit time but relative to another parameter like initial size, weight etc.

### Conditions For Growth

- 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.
- Light – Light controls growth and its phases in plants.
- Temperature – Every plant has an optimum temperature range suitable for its growth. Changes in this range are harmful to plant growth.

## Differentiation, Dedifferentiation And Redifferentiation

**Differentiation** is when the cells have stopped dividing and are beginning to mature and perform special functions. For example, to form tracheids (elongated cells that carry water in the xylem), the cells lose their protoplasm. They also develop strong, elastic cell walls to carry water across long distances.

**Dedifferentiation** is the phenomenon where differentiated cells that have lost their ability to divide, regain the capacity to divide under specific conditions. Example – fully differentiated parenchyma cells can go back to their earlier meristem form and divide.

**Redifferentiation** is the phenomenon where dedifferentiated cells divide and once again produce cells that can no longer divide but mature to perform specific functions. Example – the meristems obtained after dedifferentiation (described above) can divide and again produce cells that stop dividing but go on to mature.

Just like growth, differentiation in plants is also open. This is because cells that arise from the same meristem have different structures once they have matured. Also, the final structure of the cells at maturity is dependent on the location of the cell. For example, cells that are away from the root apical meristem become root caps, whereas cells pushed to the periphery become epidermis.

## Plant Development

It is defined as all the changes that an organism goes through during its life cycle, right from seed germination to senescence. Development of plants (i.e. growth and differentiation) is influenced by extrinsic factors (light, temperature, water) and intrinsic factors (genes and plant growth regulators).

Plants respond in different ways to environment and phases of life and give rise to different forms of structures. This ability of plants is called ‘plasticity’. Example – Leaves of a young cotton plant are different in shape from a mature cotton plant. Also, leaves of the buttercup plant that grow in the air have a different shape than those that grow in water. This phenomenon of producing different forms is called ‘heterophylly’.

## Solved Example For You

Question: What is the initial, slow phase of geometric growth called?

- Elongation phase
- Lag phase
- Log phase
- Exponential phase

Solution: The answer is ‘b’. In geometric growth, the initial phase of slow growth is called ‘lag phase’.