The Encyclopedia Americana (1920)/Color in Plants
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Color in Plants
|Edition of 1920. See also Biological pigment on Wikipedia, and the disclaimer.|
COLOR IN PLANTS. The prevailing color of vegetation is green, owing to the presence of chlorophyll in all external tissues not turned to wood or bark. This is a green substance produced in and necessary to all the growing parts of plants exposed to sunlight; but the flowers and fruits of many families as well as the whole form of seaweeds, lichens and fungi, often exhibit brilliant hues; and some lowly groups, as the molds, etc., are white, or nearly so, because they do not contain chlorophyll. Chlorophyll is green because, as the spectroscope shows, it absorbs the red and blue rays in the sunlight, and leaves only the remainder of the visible rays of the spectrum to come to our eyes, and these, in combination, affect us with a sense of green. The red and blue rays stopped by the chlorophyll are the ones useful to the plant, for it is only their vibrations that are able to break up the carbon dioxide breathed in by the plant-leaves, and to separate its constituents into sustenance (carbon) and waste (oxygen). Chlorophyll, then, is not a true pigment, although it produces the most universal color in nature.
Many leaves and other surfaces of plants do contain, however, a true, reddish pigment, sometimes so abundantly as to redden a whole part or even an entire plant, as is seen in seaweeds. The excessive development of the property by artificial cultivation gives the gardener such gay vegetables as red cabbage, the gaudy foliage-plants, and such shrubs or trees as the copper-beech. This red pigment is erythropkyll (or anthocyan), and when dissolved out in hot water it shows a beautiful rose tint. It appears strongly in leaf-buds as they begin to shoot forth in spring, and in the earliest of the growing twigs that in March give a ruddy tone to thickets in advance of the leafing. Several explanations of its supposed usefulness to the plant have been offered, while others treat it as merely a chemical incident of no particular utility.
The same sort of red reappears in the rich colors that beautify our woods in autumn, when there also becomes strongly manifest in the “turning” leaves another coloring-matter, the bright yellow pigment, anthophyll, which is always present in plants, but in very minute quantity. These reds and yellows show themselves plainly in the fall because stimulated by chemical reactions as the chlorophyll disappears with the slow decay of the leaf; and their presence seems to have little if any utility in the economy of the plant.
Turning now to the variegated and often vivid coloring of flowers and fruits, we enter the domain of the relation between plants and insects. The blossoms of primæval vegetation seem to have been virtually colorless, and this condition persists in those, like the grasses, etc., which are fertilized by means of the wind. Night-blooming flowers, and those growing in shaded places, are usually white — the most conspicuous hue in a dusky light. Most flowers, however, are now more or less brilliant in color, and there is evidence that this is a characteristic that has been gradually acquired, along with other characteristics, the effect of which is to increase the likelihood of the flowers being visited by insects, and thus obtaining an exchange of pollen with other plants of the same kind. Colored flowers become conspicuous amid green leaves, and, aided by odor, serve as signs of the presence of the sweet food (nectar) that insects seek and must have. Many simply constructed blossoms have only a uniform tint, as the plain yellow of the evening primrose; but more, probably, have a complicated structure and are varicolored, especially in a striped way on the inside of the corolla. It is found that in these cases the lines of color lead straight to the nectary and thus indicate the pathway for the insect to follow. The benefit to the plant is, of course, that by following that route the insect is sure to deliver any pollen it may have brought, and will be dusted with more for delivery to the next blossom visited.