THE PHYSIOLOGY OF COLORS. 689 



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 condi- 

 tions of existence in them. Plants yield themselves more conven- 

 iently 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 radia- 

 tions. 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 thermo- 

 tropism. A plant grows less rapidly in the light than in the darkness, 

 but with good effect on its general nutrition and transverse develop- 

 ment. In this case the different colors have a very marked specific ac- 

 tion. 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 mech- 

 anism 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 inter- 

 mittent, and taking place only in the light, the result of which is to 

 borrow from the cai'bonic 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 analo- 

 gous to that which Fraunhofer has given for the distribution of lumi- 



TOL. XXTII. 44 



