170 TURNING OF LEAVES TO THE LIGHT. 



sition to those on a southern wall over against them. 

 Plants in a hot-house all present the fronts of their leaves, 

 and this influences even the posture of the branches to 

 the side where there is most light, but neither to the 

 quarter where most air is admitted, nor to the flue in 

 search of heat. If the branches of a trained fruit-tree 

 in full leaf be disturbed in their position, the leaves re- 

 sume their original direction in the course of a day or 

 two. The brighter the day, the more quickly is this 

 accomplished. If the experiment be often repeated, 

 they continue to turn, but more weakly, and are much 

 injured by the exertion. Black spots appear about the 

 veins on their under sides, and the cuticle scales off". 

 Succulent leaves, though so thick and firm as many of 

 them are, have been observed to be peculiarly sensible 

 to light, while other plants, as Mallows, according to 

 Bonnet, are much less so. The Miseltoe, Fiscum al- 

 bum, Em-L Bot. t. 1470, the two sides of whose leaves 

 are alike in appearance, and both equally, in general, 

 presented to the light, are not found to turn upon any 

 change in the posture of the branch. Neither do up- 

 right sword-shaped leaves alter their position, because in 

 them both sides must be presumed to perform the same 

 functions with respect to light as well as air. 



Mr. Calandrini found vine-leaves turned to the light 

 when separated from the stem and suspended by a 

 thread. Of this any one may be easily satisfied, provid- 

 ed the experiment be made with sufficient care and del- 

 icacy. It is important, as demonstrating the turning to 

 be accomplished by an impression made on the leaf it- 

 self, and not upon its footstalk. 



