Page:Popular Science Monthly Volume 49.djvu/91

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79
THE PHYSIOLOGY OF COLOR IN PLANTS.

end of the spectrum, which, as has been pointed out, is the one which causes disintegration in the cells, as well as the lower red and infra-red rays. The spectrum of the autumnal red coloring of the leaves of the Virginia creeper (Ampelopsis) is shown in Fig. 1, III. It may be safely asserted that the above-described occurrences of coloring matter are undoubtedly marked factors in the physiology of a large number of plants, without reference to the manner in which such coloring screens and shields have arisen. It is also true, however, that a large number of plants contain coloring matters in the interior of organs or disposed in such other manner that they could sustain no possible relation either to light or to animals furnished with a color sense; still, many other occurrences of color are to be noted in which a physiological function is quite possible but is not proved. Among the latter is the color formation which ensues in evergreen leaves on the approach of a cold season, or in other leaves on the approach of a dry season. It must be admitted that in some instances physiologists have been led to conclusions concerning the use of colors quite as little justified as many of those reached by enthusiastic students of "adaptation to insect visitors." It is now somewhat generally admitted that color substances must very often be regarded as simply by-products in the chemical processes carried on by plants; a view which is undeniably valid of color masses in the interior of underground roots or tubers, or massive aerial organs, and also in a large number of instances in flowers. This latter application is further justified by the fact that some flower colors may change during the season without any relation to light conditions, insect visitors, or other ecologic factors. It is quite within the range of possibility that color masses in aerial organs bear an important modifying relation to the forms of irritability to radiant energy acquired by the plant.

As a summation of the foregoing, it may be stated that coloring matters stand in the following relations to the plants:

1. Chlorophyll converts light into energy by the aid of which protoplasm containing it is able to build up foods from carbon dioxide and water.
2. Non-green coloring substances serve as a screen between the chlorophyll and the too violent rays of the sun, at the same time converting the absorbed portion of the rays into useful heat.
3. Non-green coloring substances convert the light which has passed the chlorophyll bodies into useful heat.
4. Coloring substances absorb the blue-violet rays and prevent their disintegrating effect on nitrogenous compounds in situ or in transit in the interior of the plant.
5. Non-green coloring substances are in some instances simply by-products or waste matter from the physiological processes, and in the present stage of development and under ordinary conditions are of no use to the plant containing them.


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