A living organism, whether plant or animal is what it is by virtue of its specific structure and function. Life-sustaining functions can be carried out within the framework of specific structure. One of the key activities of an organism is nutrition which provides the raw materials to be used for such activities as are needed to remain alive. Nutrient, which are chemicals, are processed by the organisms through two different routes.
In the firsts they are decomposed so that the energy contained in them becomes available for other activities. In the second, they are utilized for the construction of large molecules. When the construction and the breakdown are in equilibrium the engine of life will be chugging along without any growth. When the construction outpaces the breakdown process, the organism will grow. Growth is, therefore, a manifestation of greater construction work within the organism at the expense of the nutrients.
Growth increases the size of the organism, not only as a whole but in parts as well. In this process the number of cells increased by cell division. But such activities must ultimately create a species-specific form or shape. This is called morphogenesis, which is the first component of development. Growth does not only mean production of a large number of identical cells, but a series of cells capable of performing specialized functions. This specialization is called differentiation, which is the second component of development. In unicellular individuals like Amoeba, Paramecium, bacteria, growth means the increase in the cell size. Multicellular organisms begin their life from the zygote which is formed by the union of sperm with the ovum. Successive cell divisions in the zygote give rise to a large number of cells which rearrange, differentiate and enlarge to form tissues and organs. But beyond a point the process of growth comes to a halt.
In plants, however, growth is a continuous process, usually restricted to the tips of the shoots and roots and also near the bark region. In animals growth is often restricted to a period much before the death. It usually takes place throughout the entire body
In the beginning all cells look alike but gradually they acquire different shapes and sizes and become differentiated to form tissues and organs. This phase of growth is called development. Development is a qualitative term while growth is a quantitative term. The processes of growth and development are interlinked. Initially, each living organism shows a period of slow growth (lag phase) which is followed by a period of maximum growth (exponential phase or log phase), a period of declining growth (senescent phase) and finally a period of no growth (steady phase). An S-shaped or sigmoid growth curve is obtained when these phases of growth are plotted on a graph.
The various phases of growth can be seen when a gram seed germinates into a new plant.
The growth and development is an integrated and co-coordinated phenomenon which involves the following events (i) cell division (multiplication), (ii) cell enlargement, (iii) cell movement, (iv) cell differentiation and (v) cell death.
(i) Cell Division:
Cells increase in their number by mitotic cell division. In all the organisms’ cell division is the basis of development. All the divisions during growth and differentiation are mitotic. In plants, mitosis is restricted to the tips of root and shoots and continues till death.
In case of animals where growth becomes slow, mitosis is reduced. The division, however, continues slowly in the digestive tract, skin, bone marrow and gonad and the female reproductive tract.
(ii) Cell elongation:
Cell division is generally followed by cell elongation and enlargement. Cell elongation takes place as a result of two processes: (a) absorption of water which stretches the cell wall due to increased turgor pressure, (b) accumulation of cell wall material on the stretched wall.
This phase (elongation phase) also results in appearance of large vacuoles. The growth in plants is also due to an increase in protoplasmic mass. Thus, cell elongation increases the surface area of the cell.
(iii) Cell movement:
Cells migrate to their proper positions during development and growth by movement. They occupy their predetermined positions in the embryo for the development of form or structure. For this reason, the movement is also called morphogenetic movement. This process gets completed within a limited space and time and in an organized manner.
(iv) Cell Differentiation and Maturation:
In this stage cells are differentiated into specific types of tissues namely parenchyma, collenchymas, sclerenchyma, xylem, phloem, etc. cells now acquire a definite shape and perform a particular faction. They include deposition of secondary walls, maturation and differentiation of cells into permanent tissue. During differentiation, cells become different from each other and assume permanent functional characteristics. They finally form different tissues and organs of the organism.
(v) Cell death:
Mature and differentiated cells die after performing specific functions for some time. The dead cells are replaced by new cells. Living beings have a unique characteristic of reproducing their own kind before death. When the number and rate of cell deaths is more than that of cells replaced, the organism grows old and ultimately dies.