All cells of multicellular plants and animals are derived initially from a single cell (the fertilized egg or zygote) through growth, division (mitosis) and differentiation. All of these form the three important aspects of embryonic development. Differentiation is the qualitative distinction between the cells, tissues and organs of an organism. Example- xylem cells and phloem cells in plants; muscle cells, nerve cells and blood cells in human beings and higher animals. Growth concerns the permanent and irreversible increase in shape, size and bodyweight of an organism. It refers to an increase in cell size and number or total mass.
Growth is a manifestation of greater construction work at the expense of nutrients. Nutrients are the chemicals and they are processed by the organisms through two different metabolic routes. In the first route, called catabolism, nutrients are decomposed so that the energy contained in them becomes available for other activities. In the second route, called anabolism, they are utilized for the construction of large molecules such as proteins, lipids, polysaccharides and nucleic acids (all of which go into the building of body components or of protoplasm). When the construction and the breakdown are in equilibrium, there occurs no growth of the organism. When the construction processes (anabolism) outpaces the breakdown process (catabolism), the organism will grow.
In the initial stages, the body weight may even decrease as it occurs in germinating seed. But when the seedling grows, the body weight again increases. The rate of growth is slow in its initial stage but its pace soon increase and tends to continue until the final stage.
You may observe growth both in plants and animals, but there are some basic differences between the two.
A plant continues to grow throughout its life.
In animals, the growth is regulated in time.
Only meristem-containing growth regions, such as tips of roots and stems, grow.
Growth is uniform throughout the animal body.
Growth is due to meristematic tissue.
No meristematic tissue is present.
Types of Growth:
Three major patterns of growth occur in organisms which are as follows-
In unicellular organisms, such as Amoeba, Paramecium, Euglena, Algae, fungi and bacteria, growth means an increase in the synthesis of material, proliferation in the number of cellular organelles and ultimately an expansion in cell size. Cell division is responsible for an increase in their population. This pattern of growth is called diffused growth.
In higher species (multicellular organisms), a zygote is first formed by the fusion of the male and female gametes which are products of meiosis. The initial growth starts from the zygote. The zygote divides by repeated mitotic cell divisions of cleavage and the number of cells increased but the cell size gets smaller. Hence, at the end of cleavage, the total mass and size of the organism remain the same, even though cell multiplication, morphogenesis (i.e. mode of development of the shape of cells and organs) and differentiation may have occurred. This type of growth is called multiplicative growth.
In higher plants, growth is confined to certain fixed regions such as meristems of root and shoot tips, so it is called localised growth. Here, growth takes place in a sequence of activities such as cell division, cell elongation and cell maturation (differentiation).