COLOR, IN PLANTS. The great majority of plants show distinct coloration, especially in aerial or aquatic organs. The absence of color is an index of parasitic or saprophytic life, but it must be borne in mind that many parasites and saprophytes are highly colored. The most common coloring matter in plants is chlorophyll, which manifests itself in various shades of green. Light of some strength has been shown to he necessary for the development of chlorophyll, though recent experiments show that it may develop in various seedlings germinated in per fect darkness, and that the synthesis of car bohydrates may take place vigorously under a dense layer of cork. The presence of plastids and favorable conditions of nutrition are neces sary for the proper development of chlorophyll. Yellow coloration in plants is also commonly associated with plastids, and is due to the rela tive abundance of xanthophyll or carotin present as compared with the true chlorophyll. The phenomenon of yellowness is most common in dying leaves, and is especially well shown in autumn. However, in many young leaves, especially where the nutrition conditions are un favorable, yellow leaves also appear. Unfavor able nutrition is probably the cause of yellow ness in most cases. In dying leaves the part near the veins usually remains green longest. In young leaves the green parts arc longer and much better developed internally than are the yellow or white parts.
Red or blue coloration in plants is not di rectly associated with plastids, but is due to pigments that are scattered through the cell sap. The coloring substanees are called antho cyans, erythrophyll if red, cyanophyll if blue. Blue colors occur most commonly in flowers, while the reds occur abundantly in leaves, as well as in flowers. The red coloration of leaves has been much discussed in literature, and de serves further mention. While in sonic plants, as coleus, red colors are more or less permanent, in most eases redness is periodic. Perhaps the
three most common examples of color display are: (1) in the young actively growing leaves of seedlings or perennial shoots; (2) in winter ing leaves, especially of rosette plants; and (3) in dying leaves, especially in autumn leaves. All kinds of causes have been assigned to account for periodic coloration, but by far the most satis factory one proposed by Overton in 1899. He has shown experimentally that an excess of sugar in nutrient solutions causes an early and rich development of color, while an absence of sugar retards this development. In the summer the products of a day's photosynthesis are com monly carried off before another day begins; but in the cool autumn nights this transfer is checked, and sugars accumulated in the leaf unite with tannin substances and cause the pro duction of the pigments. A similar explanation, plus the great flow of sap, would account for red leaves in spring. Mechanical injury, which pre vents the carbohydrate transfer, also causes an excess of sugar and gives rise to red colors. Light seems to favor color development, perhaps because it favors the increased production of carbohydrates.
Much has been said as to the ecological signif icance of red colors. Stahl and limy, as a result of experiments, hold that red colors increase the available supply of heat, and thus probing the leaf activities in fall and enlarge them in spring. Kerner has also held the 'protective' theory of color. If red colors do have any such function— and this is by no means proved—it must probably be regarded as quite incidental. In no case can the need for protection be regarded as a cause of the development of pigment, as one might suppose from reading various treatises on the subject of color. See CIHLoR0PAYl.L; PHOTO SYNTHESIS; LEAF. Consult Overton, "Beohaclu tungen and Versuche fiber das Auftreten von rothem Zellsaft bei Ptlanzen," in J oh rb urh f iir wissenschaftliche Bofanik, vol. xxviii. (Berlin, 1899).