Secondary Growth

Growth is the most fundamental characteristic of any living being. Like animals, plants also grow and change their size over time. Plant growth and development depend on multiple intrinsic (internal) and extrinsic (external) factors. External factors include humidity, temperature, light, and nutrients and internal factors include plant growth regulators.

Types of Plant Growth

There are two main types of growth – primary and secondary.

1.Primary growth

Apical meristems (tips) increase the size of the roots and shoots of plants, resulting in the subsequent expansion of cells. In simple words, primary growth in plants begins at the roots and extends to the shoots, giving rise to a primary plant.

2.Secondary growth

Secondary growth happens due to cell division in the lateral meristems, increasing the diameter (girth) of the plant. It increases girth by growing vascular and cork cambium (which appear in later stages).

Other types of plant growth include:

• Unlimited growth: The period of plant growth from germination to death is known as unlimited growth.
• Limited growth: Plants do not grow further after acquiring a fixed size.
• Vegetative growth: During this, plants grow stems, branches and leaves.
• Reproductive growth: It involves the flowering of a plant.

Stages of Plant Growth

There are three stages of plant growth-

1.Meristematic

It is also known as the formative phase, where plants grow by mitosis (cell division). It is carried out in two steps – nucleus division (karyokinesis) and cytoplasm division (cytokinesis).

2.Elongation

It is also known as the enlargement phase. Formation of protoplasm, absorption of water, vacuoles development and formation of cell walls make the plant body thicker.

3.Maturation

It is the final stage of plant growth when the cell wall thickens. During maturation, cells undergo protoplasmic modifications and acquire a definite shape.

Factors Affecting Secondary growth

Since Secondary growth is a metabolic process, it is affected by several factors.

Note that only the factors affecting the activity of protoplasm can alter the growth of a plant.

These factors are broadly classified as:

• External. 
• Internal.

External Factors 

1.Light: 

The intensity, duration and quality of incident light play an important role in plant growth. 

2.Temperature: 

A plant requires optimum temperature for its development stage. Also, the optimum temperature depends on the temperature zone of the plant. The minimum temperature for plant growth based on temperature zone are:

• 5°C in the Arctic zone. 
• 20°C in the temperate zone.
• 35°C in the tropical zone.

3.Water: 

If not watered regularly, the plant may exhibit a stunted appearance.

4.Oxygen: 

Oxygen plays a direct role in the initial stage of cell division. Oxygen deficiency may result in stunted growth of a plant. Optimum oxygen concentration in the soil is essential for respiration.

5.Mineral Salts:

Like oxygen, mineral salts like nitrogen are essential to avoid abnormal growth in plants.

Internal Factors 

1.Nutrition: 

Plants, being autotrophs, require raw materials for growth. Nutrients like carbon and nitrogen directly affect their growth. 

2.Growth Regulators:

These include phytohormones and some synthetic substances crucial for the growth and development of plants.

Characteristics of Plant Growth Regulators

Plants require chemical substances for their growth and development, called plant growth hormones or growth regulators. Plants themselves produce these chemical substances.

Let us look at some characteristics of these chemical substances.

• They lead to the differentiation and elongation of cells.
• They lead to the development of leaves, stems, and flowers.
• They are responsible for the wilting of leaves.
• Fruits ripen due to plant growth hormones.
• Growth regulators can also lead to seed dormancy.

Plant hormones are divided into 2 major groups:

1. Plants growth promoters. 
2. Plant growth inhibitors.

Promoters: As the name suggests, this group of hormones ‘promote’ plant growth by their action. They fall under the category of Promoters.

• Auxins.
• Gibberellins. 
• Cytokinins. 

Inhibitors: These phytohormones ‘inhibit’ the growth of plants by inducing the phase of dormancy. They fall under the inhibitors group.

• Ethylene. 
• Abscisic Acid.

Note: Ethylene is considered a promoter as well as an inhibitor. Ethylene readily promotes transverse growth but restricts longitudinal growth. 

Conclusion

Growth is the most fundamental characteristic of any living being. Like animals, plants also grow and change their size over time. Primary growth is the increase in the length of the plant, and it occurs due to the mitotic cell division in the root and shoots apex. In comparison, secondary growth involves the increase in the diameter of a plant body due to the secondary meristem.