GROWTH

Growth is defined as an irreversible increase in volume, size, numbering of parts, length and weight of an organism. It is an organic process which takes some time to accomplish. Growth however occur faster in young organisms than older ones.

The three processes involve in growth

(i)Cell division is a process by which cell increases in number and is achieved by cell division called mitosis. The division of cell commences with synthesis of new protoplasm leading to the doubling of the chromosomes number in a process called replication before cell actually divide into two, with each daughter cell having the same chromosome as parent cell.

There are two types of cell division

(i)Mitosis which is a cell division that lead to growth

(ii)Meiosis is a cell division that lead to formation of gametes

CELL ENLARGEMENT

This is a process which follows cell division. After mitotic division in animals, the daughter cells absorb nutrients from their surrounding which it uses to increase in mass and size. Part of the nutrient is used for respiration to generate energy while the remaining is assimilated resulting in the enlargement of the cells.

CELL DIFFERENTIATION

This process takes place after cell enlargement in which each cell develops into a special type of cell by changing its shape and structure in order to carry out a specialized or particular function. Cell differentiation is important in the growth and development of a mature multi cellular organisms.

MITOSIS

Mitosis is a division of cell which produces two identical cells with the same number of chromosomes and characteristics as those of the parent cell. Mitosis is a cell division that lead to growth and it occurs in somatic cell(body cells) such as skin, bone marrow, lymph nodes and injured places and meristimatic tissues of plant. Mitosis occurs in five stages namely interphase, prophase, metaphase, anaphase and telephase.

• Interphase- This is a resting stage of the cell: At this stage, the cell has normal appearance of non-dividing cell condition. chromosomes are not clearly visible.
• Prophase- The chromosomes become visible as chromatin threads. The chromosomes become shorter, thicker and clearly visible. Each chromosome now forms two district chromatrids joined by a centrometre.

Nucleolus is gradually disappearing and formation of spindle fibres commence.

• Metaphase- At metaphase the chromosomes (now parried) called chromatids arranged themselves along the equator or middle of the spindle. The chromatids are attached to the spindle by centrometre.
• Anaphase- The chromatids of each chromosome separate. The start migrating to the poles of the cell by elongation of the spindle axis. The chromatids eventually reach the pole.
• Telophase (cytokinesis)- The cell
• starts dividing into two by line of division at the equator. The chromosome looses their thick appearance and the nuclear material, nucleus and nuclear membrane reformed. The spindle structure disappear and the cell split completely into two daughter cells having the same condition as interphase. The division of the cell at telophase into daughter cells is called cytokinesis

Growth does not occur any how in all parts of the plant. It takes place in certain tissue and places in a plant bodies. The tissue where growth takes place plant is called meristem or MERISTIMATIC TISSUE.

MERISTIMATIC TISSUE.

A meristem is a tissue that retain their ability to divide by mitosis and turn out new cells. Such meristimatic tissues include root apex, stem or shoot apex base or internodes and vascular cambium.

Growth which occur in the apical stem and root meristem is called apical growth while growth which occurs at meristem base of internodes is called auxiliary growth

Hormones or chemical or biochemical substances produced in small quantity by cell of the body of plant and animal that have profound effect on other part of their body where they are needed for some form of growth

Plant hormones are produced only at the growing parts such as apical meristem of shoot or root i.e. shoot tip or root tip and transported to other part by diffusion.

Example of plant hormones are auxin, gibberellin, cytokinin, abscissic acid and florigens. Plant hormones help plant in the following ways.

1. It promotes stem enlongation
2. It makes plant to respond to tropism

iii.        It causes root and stem apical division

1. It causes fruit growth and ripening
2. It causes flower bud and lateral root initiation