Page:Popular Science Monthly Volume 27.djvu/709

Rh been softened so as to cause a predominance of the rose and yellow tones.

The predilection of animals for particular colors is not the result of an artistic preference. If the Daphnias seek the green light and the ants the ultra-violet, it is, doubtless, because they find better conditions of existence in them. Plants yield themselves more conveniently to studies of this kind. A common plant, like those we have habitually under our eyes, increases, develops itself in every quality, adds to its weight, produces leaves, flowers, and fruits, and respires, or keeps up a constant exchange between the elements it contains and the gases of the atmosphere. These different acts of vegetable life are very unequally affected by the various luminous or calorific radiations. The growth of plants, by the elongation and multiplication of cells, takes place mostly under the influence of the calorific rays, and there is for each plant a preferred temperature. If a plant receives heat only from one side, it is more developed on that side, and forms a curve in the opposite direction. This is the phenomenon of thermotropism. A plant grows less rapidly in the light than in the darkness, but with good effect on its general nutrition and transverse development. In this case the different colors have a very marked specific action. With a good light, the retarding action, insensible in the duller rays, exhibits a first maximum toward the red end, a minimum in the yellow, where the light is most intense, and a grand maximum in the violet. The rays of greater wave-length are, therefore, the more active ones. Hence results a very simple explanation of heliotropism, or of the marked tendency of plants to bend toward the light. When a plant is exposed to a lateral light, the illuminated parts lengthen less rapidly than those which remain in the shade, and the plant bends its head toward the light. We are able to go still further into the mechanism of nutrition. Besides the loss of water by evaporation, plants have two kinds of respiration—one which is continuous day and night, disengaging carbonic acid, a kind of combustion correlative with life and quite analogous to the respiration of animals; and the other intermittent, and taking place only in the light, the result of which is to borrow from the carbonic acid of the atmosphere the carbon from which the plant makes sugar and wood, and to disengage oxygen. The coloring-matter of the leaves, chlorophyl, plays the principal part in this nutritive respiration. Now, chlorophyl has to be made first, and then to perform its respiratory functions; and in this, again, the different colors act very unequally.

If we examine the formation of chlorophyl in the plant with a moderate light, we shall find that it takes place through the whole extent of the solar spectrum, very weakly in the infra-red, reaching a maximum in the deep yellow, and undergoing a regular diminution to the ultra-violet. The curve of this action takes a direction analogous to that which Fraunhofer has given for the distribution of