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, isstillless 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. Linnzeus 
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 
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