342 SPONTANEOUS MOVEMENTS IN PLANTS. 



fore elongate less rapidly ; and the stem or branch will conse- 

 quently bend towards the light. But when the light is equally dif- 

 fused around a plant, the decomposition of carbonic acid will take 

 place uniformly on all sides, and the perpendicular direction natu- 

 rally be maintained. The same law would regulate the disposition 

 of branches, which are invariably so arranged as to have the great- 

 est possible exposure to the light ; the uppermost branches of a 

 tree growing nearly erect, those beneath them extending more 

 horizontally until they reach beyond their shade, when they curve 

 upwards (unless too slender to support their own weight, as in the 

 Weeping Willow), and the lower being still more divergent, or even 

 turned downwards, when the foliage is dense. The divergence of 

 the branchlets takes place in the same manner. This effect, how- 

 ever, is confined to the green parts of plants, which alone decom- 

 pose carbonic acid under the influence of light (344). The direc- 

 tion of old branches, where the surface has lost its green color, is 

 no longer affected by the light ; and those which creep under 

 ground beyond its influence (173), and have the white color and 

 much the external appearance of roots, show little upward ten- 

 dency so long as they remain in this situation ; but whenever 

 their extremities are exposed to the light, they first acquire a 

 green hue by the formation of chlorophyll, and then tend to as- 

 sume a vertical direction. 



652. In leaves, it is the deeper-colored surface that is always 

 presented to the light. But the turning of this surface towards the 

 light cannot be explained as a mere physical effect of that agent 

 upon the leaf. A leaf cut from its stalk, attached to a hair, and 

 plunged by a bit of lead in a glass vessel filled with water, when 

 exposed in a window, will perform its functions of digestion as well 

 as ever, but it will not turn its upper surface towards the light. 

 The light can produce this motion only by its influence on some 

 power inherent in the vegetable itself. 



653. Still less will purely physical explanations account for the 

 reaching forth of tendrils, or the twining of those stems which act 

 like tendrils ; in which the green parts turn from the light, instead 

 of towards it. We pass to more obvious cases of spontaneous 

 movements. One of the most general of these is what was termed 

 by Linnseus 



654. The Sleep of Plants, namely, the peculiar position which the 

 leaves of many plants assume, either by drooping, or by the fold- 



