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Page:Popular Science Monthly Volume 54.djvu/209

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tive and tonic effect of light. Otherwise expressed, the influence of variations of light upon plants causes adaptive reactions, and disturbances of the nutritive processes and growth.

In consequence of these facts the reaction of any given organ to changes in the intensity of the illumination will depend upon its specific functions and relation to the remainder of the organism.

The stems formed by seedlings and awakening underground organs are usually surrounded by plants or other objects which cut off more or less sunlight. The developing shoot can not spread its leaves to the light advantageously until it has outstripped or grown beyond the objects intervening between it and the light. This necessity is one of the most important conditions in the struggle for existence. To meet it, a very great majority of seed-forming plants have acquired the power of accelerated elongation of the stems when deprived of their normal amount of light.

Very striking examples of this reaction are offered by the awakening corms of the Jack-in-the-pulpit (Arisæma triphyllum). The corms usually lie at a distance of five or six centimetres below the surface of the soil, and when the growth of the large bud begins in the spring the heavy sheathing scales elongate and pierce the soil, opening when the surface is reached at the distance of a few centimetres. If the corm should have been buried deeper in the substratum by floods or drifts of leaves, the growth of the bud scales will continue until the light is reached, though it may be a distance of twenty centimetres. Such growth may be seen if the corms are grown in a deep layer of sphagnum moss, or in a dark room.

After the stems emerge from the "drawn" buds they show a similar attenuation, attaining a length of twice the normal. The excessive elongation of stems is accompanied by variations in the structure and contents of the tissues. The cells are generally longer, while the walls are thinner. In consequence, organs grown in darkness are very weak and easily bent or broken. Growth in darkness is attended by the non-formation of chlorophyll. This is replaced by etiolin, giving the plant a pale, waxy, yellow appearance.

The adaptive elongation is not shown by all species, however. It has been found that stems of beet, hop, dioscorea, and a few others show no adaptations to diminished light. The adaptive modification of stems elongating in darkness is developed from the retarding influence exercised by light upon growth. Thus it is a well-known fact that the action of certain portions of the sun's rays actually impedes or checks the increase in volume known as growth, though it does not influence actual division of the cells to any great extent. When this retarding action is eliminated excessive elongation ensues.