SPONTANEOUS MOVEMENTS IN PLANTS. 343 



right. De Candolle attempted a mechanical explanation of this 

 bending of green stems towards the light, connecting it with assimi- 

 lation and growth. He supposed that, as the side upon which the 

 light strikes will fix most carbon by the decomposition of carbonic 

 acid (34G-348), so its tissue will solidify faster, and therefore elon- 

 gate less, than the shaded side (which will become drawn, as the 

 gardener terms it) ; and the stem or branch will necessarily bend 

 towards the shorter or illuminated side. But when the light is 

 equally diffused around a plant, the decomposition of carbonic acid 

 will take place uniformly on all sides, and the perpendicular direc- 

 tion naturally be maintained. Two facts at once demolish this in- 

 genious theory. 1. It is now Avell known that, under the solar 

 spectrum, the decomposition of carbonic acid in the green parts of 

 plants is effected chiefly by the most luminous rays, that is, by yellow 

 light, and next to this by orange and red ; whereas the bending is 

 strongest under the violet and blue rays, the yellow producing little 

 curvature, and the red none at all. 2. When a stem curved under 

 the light is split from the apex downwards, so as to separate the. 

 illuminated from the shaded side, the former curves more than be- 

 fore, while the latter tends to straighten, — showing that it was 

 pulled over by the contraction of the concave side, and not pushed 

 over by its own greater growth. From all this it clearly appears 

 that the turning of parts towards the light, and the other special 

 directions of plants, are independent of growth, and apparently are 

 effected by some inherent power. At least, they have thus far 

 proved no more susceptible of mechanical explanation than the more 

 marked movements of animals. 



670. In leaves it is the denser and deeper green upper surface 

 (262) that is presented to the light, while the paler lower surface, of 

 looser tissue, avoids it. The recovery of the natural position, when 

 the leaf is artificially reversed, is the more promptly effected in pro- 

 portion to the difference in structure and hue between the two strata. 

 This movement is so prompt in some plants, that their leaves follow 

 the daily course of the sun. The leaf is more capable of executing 

 such movements, on account of its extended surface, and its pliancy, 

 and also on account of its usual attachment by an articulation. 

 Here the slender vascular bundles oppose little resistance to lateral 

 motion, while the soft and usually cellular enlargement favors it. 

 Indeed, the efficient cause of the movement appears to be exerted 

 here, and to be connected with the unequal tension or turgescence of 



