The quantitative increase in plant body is called growth. Following processes are involved in the growth and development of plants:
- Cell division
- Cell elongation
- Differentiation of cells into tissues and then organs.
Growth is an irreversible increase in size and development. It is a programmed series of stages from a simpler to more complex form. As development proceeds, cellular differentiation of structure and function takes place.
The plants have a growth pattern called open growth. The plant adds new organs such as branches, leaves and roots throughout the life. Plant enlarges its body from the tips of roots and shoots. But the rate of growth is not uniform throughout the plant body. The growth is slow at the beginning. But gradually it becomes rapid and attains a maximum rate. Then gradually it slows down. The growth occurs through the activity of meristems in vascular plants. Meristems are young tissues or group of cells that retain the potential •to divide. The entire plant body is capable of growing in lower plants. On the hand, the entire plant body is not capable of growing in higher plants. Their growth is limited to certaiii regions known as growing points. These growing points consist of groups of cells ‘which are capable of division. These growing points are called meristem. These meristematic cells are located at the stem and root. There are fol,lowing types of meristems:
- Apical Meristems: The meristems present at the tips of roots and shoot are called apical meristems. They are primarily concerned with the division of plant body. The perpetual growth zones are present at the pieces of roots and stems. They increase the number of cells at the tips of roots and stem. Thus they play important role in primary growth
- Intercalary eristems: The meristem situated at the bases of internodes is called intercalary meristem. These are the parts of apical meristem. They get separated from apex meristem by permanent tissues. They play important role in the production of leaves and flower. These are of temporary nature.
- Lateral Meristems: The cylinders of dividing cells present in the vascular and cork tissue of the plants are called lateral meristems. Lateral meristems are present in dicots and gymnosperms. Vascular and cork cambium are the example of lateral meristem. They play an important role in the increase in diameter of stem and root. So they are involved in secondary growth.
There are two types of lateral meristem:
(a). Determinate: Their grovah is determinate i.e. they grow to certain size and then stop. For example leaves, flowers and fruits
(b) Indeterminate: Their growth is indeterminate i.e. they grow by meristems that continually replenish themselves during remaining youthful age. For example. vegetative root and stem.
Types of Growth
Growth is of two types:
(a) Primary Growth: In this growth, primary tissues are added by the apical meristem. It increases the length of the plant.
(b) Secondary Growth: In this growth. secondary tissues are added by the intercalary or vascular cambium. It increases the thickness of plant.
Phases of Growth
Growth of multice!lular plant is divided into four phases. cell division. elongation, maturation and d itlèrentiat ion.
I. Phase of Cell division
The numbers of cells are increased by mitosis during this phase. It occurs at the tip of root and shoot. These cells are small and they • have spherical nuclei bing in the center of cytoplasm. So they are non-vacuolated. After the cell division each daughter enlarges in size. Synthesis of cytoplasm and cell wall material also takes place in this zone.
- Phase of cell elongation
The zone of elongation is present a little distance from apex of root and shoot. It is only of few millimeters in length. The cells uptake of water during elongation and their volume increases up to 150 fold. Plasticity of the cell wall increases and wall pressure is reduced. Synthesis of new cytoplasm and cell wall material remain continue.
- Phase of differentiation and maturation
The cell attains its final size during maturation. The cells of pith, cortex and certain other tissues do not elongate further along the axis. While other cells like fibers and tracheids elongate lengthwise more than in other direction. The walls of many kinds of cells and tissues become pitted. Thickening appears on the walls of xylem vessels. The cells of various tissues differ in spatial dimensions and many new structural features develop in them.