The New Student's Reference Work/Growth

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Growth (in plants), permanent alteration of form, usually accompanied by increase in size, the latter being the only popular criterion of growth. In plants growth exhibits three features at different times. In the earlier phase the formation of new cells by division is characteristic. This is followed by the rapid enlargement of the cells. In doing this, the cells do not form a corresponding amount of new protoplasm, but take up large quantities of water, which is secreted into the center of the cell. The volume may thus be increased a thousandfold or more. In this feature the growth of plants differs most from that of animals. After enlargement has ceased, thickening (usually irregular) of the walls and sometimes the death and disappearance of the protoplasm ensue. The phase of enlargement is the only easily measurable phase of growth. It is studied by records or observations with the auxanometer (which see). Growth is dependent on a suitable supply of oxygen, food and water. It is greatly influenced in rate and amount by external agents, as heat, gravitation, light, etc. Because the heat and light vary periodically, corresponding to day and night, the rate of growth shows a daily variation, being usually most rapid during the night. Besides variations due to external causes, growth shows variations which are unexplained, but are supposed to be due to internal causes. Even when all the conditions affecting growth are uniform, its rate is at first slow, becoming more and more rapid till it reaches a maximum, after which it declines even more rapidly, until it ceases entirely. The duration of growth of any part is called the grand period to distinguish it from the daily period just mentioned. During both daily and grand periods, there also are minor variations in rate. The cause of the cessation of growth, even under favorable conditions, is not known.