Growth and Development In Plants

The permanent irreversible increase in size, weight, shape and structure is called growth. In  two lower plants growth occur in some entire body. In higher plants growth occur only in some regions called the growing plants. These growing points are called meristem. These regions consists of cells, which can divide and produce new cells. These div ding cells are called meristematic cell. The most important meristematic cell are found at the tips of stem and roots, are called apical meristem.

These are responsible for the increase in the length of plant i.e primary Apical men stem also forms leafs and floral plants.

In some cells increase in length also takes place by the activity of intercalary men stem, which are seprate and also left behind as the apical men stem moves on during growth. These meristems are located at the base of inter nodes, below nodes at base of leaf.

The higher plants also increase in diameter i.e the increase in thickness. This is called secondary growth. Secondary growth increase by activity of lateral meristem, which is present between xylem and phloem or below epidermis. Lateral meristem is the cambium and phallogen of cork.

Phase of growth: The permanent irreversible increase in size, shape, structure and weight is called growth.

     In lower plants like Bryophytes growth is only cell division and cell enlargement of plant body. In higher plants like angiosperms increase in length takes place at the tips of roots and shoots, there are three phases of growth in these regions.

1. The formative growth: It is located at the tips of roots and shoots. In this region the cells divide and increase in number. Informative region the cells are closely packed. These cells have thin wall, dense cytoplasm and large nuclei. These cells divide constantly.
2. Elongation Phase: It is located just below the formative region. The cells do not divide they become turgid and elongated and attain maximum size. In the roots the elongation phase of few millimetres. But in stems is existence up to few centimetres.
3. Maturation Phase: It is located behind the elongation phase. It this phase the cell walls of cells become thick and cells are differentiated to form different types of tissues. Their mature cell performs different function.

Secondary growth: The permanent irreversible increase in length, shape, structure and weight. The primary growth results in increase of length of stem and roots. The plant increase in diameter by the process of secondary growth.

The increase in thickness of plants body is called secondary growth. Secondary growth due to activity of secondary men stem called cambium. In the beginning cambium consists of small groups of cells when secondary growth starts then all these groups of cell link together and form a ping of cambium between the xylem and phloem. The xylem is present on the inner side of cambium while phloem is present on the outer side of the cambium. The cell of cambium divide, they form secondary xylem tissues on the in side and secondary phloem on the out side. The cells present between the vascular bundles form secondary parenchyma, they increase the thickness of medullary rays.

Conditions for growth: The permanent irreversible increase in length, size, shape, structure and weight. Growth is due to cell division and cell enargement, differentiation. There are many factors, which effect the rate of growth: these factors can be external or internal.

• External factors: There are many external factors, which effect the rate of growth position or negative. These factors are given below

1. Temperature: The temperature plays important role in the growth of plant body. Most enzymes have their maximum activities at the temperature between 25c to 37c. The temperature effects the metabolic rate function of cells, formation of new protoplasts, and cell division and all other functions occur rapidly. If temperature is suitable, normally rate of growth increases with increase in temperature. At very high temperature the rate of growth stops and plant may die because high temperature destroy the enzymes and water lasts by transpiration.
2. Light: Light plays important role in the growth of plant. It is required for the synthesis and action chlorophyll. So the process of photosynthesis takes place in the presence of light effects the growth in three ways.

(a) Intensity of Light: The intensity of light effects the synthesis of chlorophyll and three plants pigment. The high intensity of light destroy the chlorophyll and rate of photosynthesis decrease and rate of growth also decrease.

(b) Quality of Light: The quality of light also effects the rate of growth. The red light favours the elongation of cells. The blue light increase the rate of cell division but retard cell enlargement.  The ultraviolet light destroy the protoplasts and retard growth.

(c) Duration of Light: The duration of light effects the growth of vegatative and reproduction structure e.g. photoperiodism in which light duration increase or decrease the flowering.

(d) Oxygen: Oxygen is very necessary for growth. Oxygen does not effect the growth of aerial parts of plants. It effects the growth of roots. Energy is needed for growth and it is supplied by respiration. O2 is used in respiration. So oxygen indirectly effect the growth. When O2 is in high concerntration, it inhibits the growth by photorespiration, which is wasteful process.

(e) Carbon dioxide: Co2 is necessary for photosynthesis and food production. If this process will continue, more food and other compounds for the synthesis of new protoplasm will be produced.

(f) Nutrients: Plants need many nutrients for growth like nitrogen, calcium, potassium etc. If nutrients are in less quality, the rate for growth decreases. 
          E.g. If there is deficiency of nitrogen in soil, less amount of proteins will be formed in plants and growth rate will decrease.

    2. Internal factors: There are some internal factors, which control growth in plants. There are two improtant internal factors.

1. Hormones: These influence the rate of growth. The plant hormones. Which effecct rate of growth are Auxins, Gibberellins and cytokinins. These hormones are influenced by light, gravity and water etc.
2. Genectic Constitution: The genes also control the rate of growth under the influence of hormones.