An organism starts its life as a single cell. Unicellular organisms grow and reproduce without increasing number of body cells but multicellular organisms undergo a far more complex process of growth and development.
Growth in an organism consists of a permanent and more or less irreversible increase in size, commonly accompanied by an increase in solid matter, dry mass and amount of cytoplasm. Growth in living beings is intrinsic or internal in contrast to extrinsic growth observed in non-living objects. Plant growth is generally indeterminate i.e., continued throughout life. In lower plants, growth is diffused i.e., every cell can divide and enlarge but in higher plants, special body regions called meristems cause body growth.
Plant growth takes place in three phases – formative, enlargement and differentiation.
(i) Formative phase: It is the phase of cell division. As the formation of new cells requires biosynthetic activity, the respiration rate of cells is high in this phase. (ii) Phase of enlargement: In it, the newly formed cells, produced in formative phase undergo enlargement.
In many parts cell enlargement may occur in all directions e.g., isodimetric parenchymatous cells. In many parts, cell enlargement takes place prominently in the linear direction. So much so that this phase is also called phase of cell elongation.
(iii) Phase of differentiation or maturation: In this phase the enlarged cells develop into special or particular type of cells by undergoing structural and physiological differentiation.
Growth can be arithmetic (cells divide in such a fashion that one daughter cell divides again while the other cell differentiates) or geometric (each daughter cell divides). Geometric growth causes rapid increase in size, weight, etc. and is generally observed in unicellular organisms or at early embryonic stage.
Development is the sequence of events that occur in the life history of a cell, organ or organism which in case of angiospermous (flowering), plants include seed germination, growth, differentiation, maturation, flowering, seed formation and senescence.
Development involves differentiation, the process by which unspecialised structures become modified and specialised for the performance of specific functions.
‘Nature vs. Nurture’ in growth and development The processes of growth and development are influenced by many factors, some internal to the organism and some environmental or external. From observation of living things it may be difficult to determine whether a particular feature of growth such as body height is determined largely by internal factors, namely the inherited instructions each organism receives in its genes, or by some environmental factor such as the food supply available to the organism. This is because the effects of internal and external factors interact. The interaction of internal and external factors in growth and development are sometimes referred to in literature as ‘nature vs. nurture’. The distinction is one of ‘genetic vs. environmental’ influences. The whole process of interaction of factors and the expression of characters can be summarised as: