LIGHT AND LIFE. 301 



length. Then he threw the solar spectrum by a prism on this little 

 field. The plants inclined toward a common axis. Those exposed to 

 the red, orange, yellow, and green rays, leaned toward the deep blue, 

 while the part lighted by violet bent in the opposite direction. Thus 

 the crop took the appearance of a wheat-field bowing under two con- 

 trary winds. The turnips placed in the violet-blue region looked 

 toward the prism. Gardner thus determined, as Payer had done, that 

 the most refrangible rays are those which effect the bending of the 

 young stems. He proved also that the plants grow erect again in the 

 dark. 



These experiments, repeated and varied in many ways by Du- 

 trochet and Guillemin, uniformly gave like results, but the phenome- 

 non itself still remains almost unexplained. This remark also applies 

 to the very singular facts of the twisting of running plants. The 

 stems of these plants, in twining about their supports, usually curl from 

 the left to the right. Others follow the contrary course, and some 

 twist indifferently in either way. Charles Darwin inferred, from his 

 investigation, that light has an effect on this phenomenon. If twining 

 plants are put in a room near a window, the tip of their stalk takes 

 longer to complete the half circuit during which it turns toward the 

 darker part of the room than that which is described nearer the win- 

 dow. Thus one of them, having gone through a whole turn in five 

 hours and twenty minutes, the half circle toward the window em- 

 ployed a little less than an hour, while the other was not traversed in 

 less than four hours and a half. Duchartre placed some China yams in 

 full vegetation in a garden, and others in a completely dark cellar. 

 The stems of the plants uniformly lost in the dark the power of twist- 

 ing around their supporting sticks. Those exposed to the sun presented 

 one portion twisting, but when put in the cellar they shot out straight 

 stems. Yet some twining plants are known that seem to be indepen- 

 dent of light in twisting. 



The sleep of plants, in connection with light particularly, is still less 

 understood. The flowers and leaves of certain vegetables droop and 

 wither at fixed hours. The corolla closes, and after quiet inaction the 

 plant again expands. In others, the corolla drops and dies without 

 closing. In others still, as the convolvulus, the closing of the flower 

 occurs only once, and its sleep marks its death. Linnaeus noted the 

 hours of opening and shutting in certain plants, and thus arranged 

 what has been called Flora's clock ; but the relations of these closings, 

 with the intensity of light have not yet been scientifically determined. 



The green coloring of vegetable leaves and stems is due to a spe- 

 cial substance called chlorophyll, which forms microscopic granula- 

 tions contained in the cells which make up these stems and leaves. 

 These grains are more or less numerous in every cell, and it is their 

 number as well as intensity of color that determines the tint of the 

 plant's tissues. Sometimes they are closely pressed together, covering; 



