Introduction
Plants grow and develop from the seedling stage to the reproductive stage. Many factors help in this growth and development.
The germination of seeds and the development of the plant help in the formation of body organisation that produces roots, leaves, flowers, fruits, and seeds.
Growth
Growth is an irreversible and permanent increase in the size or mass of a cell, organ, or whole organism.
Plants show growth due to the presence of meristematic cells in different parts of the plant. Meristematic cells continuously divide and thus help in plant growth.
Some meristematic cells later become permanent and differentiate to form different tissues and organs.
Phases of Growth
1. Cell Division
In this phase, cells continuously divide. These cells are metabolically active and possess large amounts of protoplasm and well-defined conspicuous nuclei.
Certain cell organelles duplicate during this phase. The cells have thin primary cell walls with plasmodesmatal connections.
Cells present at the root and shoot apex remain in the meristematic phase.
2. Elongation
Elongation occurs in the cells close to the meristematic zone.
The cells enlarge due to increased vacuolation. New cell wall deposition also occurs to support enlargement of the cells.
3. Maturation
This phase occurs in cells located away from the apex.
The cells attain maximum size, and the cell wall and protoplasm become fully modified.
These cells differentiate to form specialised tissues.
Example: Formation of root hairs.
Growth Rates
Arithmetic Growth
- Meristematic cells divide by mitosis to form two daughter cells.
- Only one daughter cell continues dividing and remains meristematic.
- The other daughter cell becomes permanent and differentiates.
Geometric Growth
- Meristematic cells divide by mitosis to form two daughter cells.
- Both daughter cells continue dividing.
- This results in rapid increase in the number of cells.
Differentiation
Differentiation is the process of change in a cell, tissue, or organ during embryological development that results in the formation of different mature structures.
The plant body differentiates into root and shoot systems, which further differentiate into specialised tissues and organs.
Example: Meristematic tissue divides and forms tissues that differentiate into vascular tissues such as xylem and phloem.
Xylem and phloem perform specialised functions of water and food conduction.
Development
Development includes all the changes in a plant from seed germination to senescence.
During growth and development, plants show different responses to environmental conditions and develop different structures. This ability is known as plasticity.
Examples of Plasticity
- Heterophylly is seen in cotton, coriander, and buttercup plants.
- In cotton and coriander plants, the leaves of juvenile and adult plants are morphologically different.
- In buttercup plants, leaves submerged in water differ morphologically from terrestrial leaves.
Factors Controlling Development
Development is controlled by intrinsic and extrinsic factors.
Intrinsic Factors
- Genetic control
- Hormonal control
Extrinsic Factors
- Oxygen
- Temperature
- Water
- Nutrients
- Environmental conditions
Written by Sarah Samarawickrama