Growth in Plants – Biology Class 11

🌱 Growth in Plants

Growth in plants refers to a permanent and irreversible increase in size, mass, or volume. It occurs in different dimensions such as length, width, or girth. Plants grow throughout life by forming new organs like roots, shoots, leaves, and branches.

In lower plants, the entire body can grow, but in higher plants, growth is restricted to specific regions called meristems, which contain actively dividing cells.

🌿 Types of Meristems

There are three main types of meristems:

Apical Meristem: Found at the tips of roots and shoots. Responsible for primary growth (increase in length) and formation of the primary plant body.
Intercalary Meristem: Located at the bases of internodes and leaves (e.g., grasses). Enables regrowth and regeneration after cutting. Temporary in nature.
Lateral Meristem: Found along the sides of roots and stems. Responsible for secondary growth (increase in thickness). It includes:

Vascular Cambium – forms secondary xylem and phloem.
Cork Cambium (Phellogen) – forms the outer protective tissues (periderm).

🌳 Types of Growth

Plants exhibit two major types of growth:

1. Primary Growth

It causes an increase in plant length due to the activity of apical meristems. It occurs in roots and shoots and forms the primary plant body.

Phases of Primary Growth:

Cell Division: Cells of apical meristem divide actively, forming new small, non-vacuolated cells. (Occurs in the zone of cell division.)
Cell Elongation: Newly formed cells absorb water, enlarge, and become vacuolated. (Occurs in the zone of elongation.)
Cell Differentiation: Cells attain final shape and function such as xylem, phloem, and cortex. (Occurs in the zone of differentiation.)

2. Secondary Growth

It results in an increase in the thickness or girth of roots and stems. It is caused by the activity of lateral meristems, mainly the vascular cambium and cork cambium.

A. Vascular Cambium

Located between xylem and phloem. It produces:

Secondary Xylem (on the inner side)
Secondary Phloem (on the outer side)

Continuous formation of secondary xylem increases the thickness of stems and roots.

B. Cork Cambium (Phellogen)

It develops in the outer cortex during secondary growth and produces the protective outer layers called the periderm.

🪵 Structure Formed by Cork Cambium

Phellogen (Cork Cambium): A meristematic layer that divides to form cells both outward and inward.
Phellem (Cork): Formed outwardly; dead, suberized cells that are waterproof and protective.
Phelloderm: Formed inwardly; living parenchyma cells used for storage and sometimes photosynthesis.

Periderm = Phellem + Phellogen + Phelloderm

It replaces the epidermis in woody plants and provides protection against desiccation and infection.

🌰 Bark

Bark is the outermost protective covering of woody stems and roots. It includes all tissues outside the vascular cambium — secondary phloem, cork cambium, and periderm.

Functions of Bark:

Protects inner tissues from injury and temperature extremes.
Reduces water loss by evaporation.
Allows gas exchange through lenticels.

🌲 Annual Rings

Each year, the vascular cambium forms two types of wood:

Spring Wood (Early Wood): Formed in favorable conditions; vessels are wide and thin-walled.
Autumn Wood (Late Wood): Formed in less favorable conditions; vessels are narrow and dense.

Each pair of spring and autumn wood forms one annual ring. Counting these rings helps determine the age of a tree — a process called dendrochronology.

🌿 Summary

Growth Type	Meristem Involved	Direction	Result
Primary Growth	Apical Meristem	Lengthwise	Formation of primary plant body
Secondary Growth	Lateral Meristem	Widthwise	Formation of secondary tissues and bark

Key Points: