All organisms build up their living material, protoplasm, from similar materials and by a series of similar chemical processes, yet such is the complexity and infinite variety of this mysterious substance that not only has each organism its own characteristic protoplasm but this differs from cell to cell in the various tissues of multicellular organisms, reaching its highest degree of differentiation in vertebrate animals, e.g. muscle, nerve and bone tissues.
Provided that food is available, and all conditions are favorable, the building up or assimilation of protoplasm proceeds more rapidly than that unstable substance breaks down, so that there is an increase in the total quantity of protoplasm ; in other words, growth occurs. Since assimilation is an anabolic process, whereas the breaking down is a katabolic process, we can say that when anabolism exceeds katabolism, growth results. Growth generally involves cell division, accompanied by differentiation of tissues and often by development of new organs.
There is a limit to the growth of most organisms, even in perennial plants which grow for an indefinite period. When the maximum size is reached we say that the organism is adult and it can then reproduce its kind. We ought perhaps to say that the organism is adult when it is fully developed, since this definition covers all forms of life, including perennials which continue to grow after they begin to reproduce.
Cell Division
The division of a cell is always preceded by the division of the nucleus. This is a complicated process which results in the nuclei of the new cells being exact replicas of that of the parent cell. Briefly it occurs as follows :—
The nucleus is bounded by a thin nuclear membrane. Within this is a definite number of fine threads called chromosomes and a body called the nucleolus. In man there are 48 chromosomes in each of the nuclei of the cells of the body but the number varies in different types of plants and animals e.g. the cells 344 of the Bluebell have 16 chromosomes in their nuclei. The chromosomes are invisible until the nucleus is about to divide. The nucleolus breaks up and its material forms a coating on each chromosome which can be stained to make the chromosomes visible under the microscope. Under very high magnification the chromosomes are seen to be split longitudinally into two threads called chromatids except at one point. The nuclear membrane now disappears and a double-cone shaped structure called the spindle appears in the cytoplasm.
The divided chromosomes become attached by their centromeres to the equator of the spindle. The centromeres next divide and the chromatids of each chromosome separate and move to opposite ends of the spindle. The two groups of chromatids now revert to the ordinary nuclear condition, becoming surrounded by nuclear membranes. They lose their coats and become invisible while nucleoli reappear. The cell now divides across the middle of the spindle. This type of nuclear division is called mitosis.
When germ cells are produced, a different type of nuclear division called meiosis occurs. The chromosomes appear in an undivided condition. They unite temporarily in pairs but after attachment to the spindle separate and proceed to opposite poles of the spindle. Each of the two new nuclei formed therefore possesses only half the number of chromosomes found in the nuclei of the cells of the body. When gametes fuse their nuclei unite and the original number of chromosomes is restored.
The Nature of Growth in Animals and Plants
Animal growth
This is usually intercalary, I.e. growth occurs throughout the length of the animal, and proceeds for a limited period only during the life of the animal.
Plant growth
This is apical, I.e. occurs mainly at the extremities, and continues indefinitely throughout the life of the plant. Plant growth results from the division of cells at the apices of the stems and roots. Following the production of new cells in this way at the apical meristems, a period of cell growth occurs. The cells of the meristems are at first without vacuoles and have very thin walls. As they grow the cell wall is thickened and extended and the cells become vacuolated. The extension of the cell wall is produced by the insertion of more and more cellulose molecules produced by the protoplasm between those already built up into the wall. Each cell passes through a grand period of growth during which its wall grows at an increasing speed for a time and then with a declining speed until growth ceases. Since the cells at each horizontal level pass through this period at the same time, it follows that each part of the root and stem undergoes a grand period of growth. Extension of the cell wall depends on the maintenance of turgor pressure.
Experiment 63—To show the Region of Growth in a Root and the Grand Period of Growth
A root of a freshly germinated bean about 1 in. in length is marked with horizontal lines in Indian ink at equal distances of – 1/2- in. The root is left to grow in a damp atmosphere, e.g. by pinning the seed to the under side of a cork with the root hanging vertically downwards and inserting the cork into the neck of a flask with a little water in the bottom. It will be seen that growth in length has occurred for some distance behind the tip as shown by the separation of the lines. The widest separation shows where the cells are at the peak of their grand period of growth. On either side of this, the spacing is less where the cells are either approaching the peak or declining from it.
Similar results are obtained by marking a young stem of a bean plant below the first pair of leaves. The region of growth will, however, be seen to be more extensive than in the root, I.e. the grand period lasts longer in the cells of the stem than in those of the root.
Growth in girth of the stems of woody perennials results from the activity of the cambial ring.
Animals also pass through a grand period of growth, which is best seen by recording their weights at even intervals during their growing period and graphing the results.
