LIGHT AND LIFE. 113 



nomenon. If twining plants are put in a room near a win- 

 dow, 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 window. 

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

 five hours and twenty minutes, the half-circle toward the 

 window employed 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 independent of light in 

 twisting. 



The sleep of plants, in connection with light particular- 

 ly, 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 droops and dies without closing. In 

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

 occurs only once, and its sleep marks its death. Linnaaus 

 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 special substance called chlorophyll, which forms 

 microscopic granulations 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 tis- 

 sues. Sometimes they are closely pressed together, cover- 

 ing the whole inner surface of the cell ; sometimes the 



