158 TURNING OF LEAVES TO THE LIGHT. 



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 posi- 

 tion, the leaves resume 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 mpch 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 ob- 

 served to be peculiarly sensible to light, while other 

 plants, as Mallows, according to Bonnet, are much 

 less so. The Miseltoe, Viscum alburn^ Engt. Eot. 

 t. 1470, the two sides of whose leaves are alike in ap- 

 pearance, 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 upright 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, pro- 

 vided the experiment be made with sufficient care and 



