FACTORS CONCERNED IN GROWTH. These are chiefly external, such as abundance of food and other conditions of existence, as heat, light, etc. Hatschek assumes that in growth the simple molecule of living proteid continually attracts elements to itself from the food. Growth is more rapid in a well-fed animal or plant. Develop ment of the higher organisms is usually intro duced by a period of rapid cell-division with al most no growth. This is followed by a period of rapid imbibition of water during which the tis sues are stretched, and growth occurs with great rapidity. Later still, the cell-walls thicken, in tercellular substance is deposited, and the dry weight slowly increases. This is the special period of assimilation and secretion. In the de velopment of the frog the percentage of water rises to 96 when half a month old, and then falls slowly. At six weeks from fertilization the human embryo consists of 97.5 per cent. water, but at birth the proportion of water has fallen to 75 per cent., and in adult man it is 66 per cent. The growth processes may be classified as 'transi tory,' in which growth is followed by a return to the former size, and 'permanent' or develop mental, in which there is a persisting enlarge ment which plays an important part in develop ment. Transitory growth may be illustrated by the enlargement of those cells which cause the leaflets of the sensitive plant to turn; this en largement is only temporary. Developmental growth is illustrated by the increase of the em bryonic plant. Growth phenomena may also be classified as 'diffused' or 'localized.' Diffused growth affects the entire individual or many of its parts, whereas in localized growth the process is confined to a limited region. In early develop ment the whole animal swells by diffused growth; later, the separate organs enlarge as a result of localized growth. Although cell-division usually accompanies growth, it is in no way the cause of growth. The curve of growth is obtained by join ing with a line the tops of ordinates erected from a horizontal base-line at equal time intervals, the height of the ordinates being proportional to the volume of the organism. A study of these growth-curves shows that absolute increments of size are at first small, rapidly increase to a maximum, and then decline to zero. This decline to zero is not a necessary property of protoplasm, but it is an adaptation to the limited size which is advantageous to most animals.
Various external agents influence the rate of growth as follows: (1) Chemical assimilative and secretory are chemical processes. They require certain materials to work upon. These materials are the food of organisms. The principal elements found in the body are car bon, oxygen, nitrogen, calcium, phosphorus, potas sium, sodium, chlorine, magnesium, sulphur, silicon, and iron, which enter the body in various combinations, while each plays a definite role. Since phosphorus is especially abundant in embryonic tissue, its peculiar importance for growth is indicated. Potassium is probably of great importance in imbibition. Iron is essen tial in the early processes of cell-division. Be sides the inorganic elements, organic food is essential to the growth of animals; various chemical agents that are not built up into the organic body may accelerate growth by acting as a stimulus. Certain poisons do this.
(2) imbibition of water is an important growth process. Developing embryos are usually found in moist situations.
(3) Density of the Medium—The rapidity with which water is imbibed depends largely upon the osmotic pressure of the medium, and this is determined by its density. Regeneration of marine animals is hastened in dilute sea-water, retarded in a concentrated solution. Pond-or ganisms develop with abnormal slowness in a salt solution.
(4) Molar facilitates growth, and after a wound excessive growth takes place along the cut edge.
(5) may retard or accelerate growth in different cases. Its effect is usually unfavorable to growth in young plants, and the growing tissue of vegetative plants is usually protected from the sun's rays. So, likewise, among animals the embryo is usually sheltered from the light by being carried in the maternal body, covered by egg-shells, or placed in dark places. Acceleration of growth by light, on the other hand, takes place in certain aquatic algT, especially diatoms, and in certain aquatic em bryos (tadpoles, fish-embryos, and young pond snails). In these processes of retardation and acceleration not all the component rays of white are equally effective. Thus, in general, the long waved rays have no more effect than darkness except in forming starch. It is the active rays which retard the growth of aerial organisms, and it is the actinic rays which accelerate the growth of aquatic organisms.