270 PHYSIOLOGY. BOOK II. 



dio-estin«f the crude matter they receive from the stem, and 

 that the lower stratum takes up the matter so altered and 

 submits it to the action of the atmosphere, which must enter 

 the leaf purely by means of the stomates. Nor are the 

 stomates and the cavernous parenchyma of the leaf the only 

 means provided for the regulation of its functions. Hairs, no 

 doubt, perform no mean office in their economy. In some 

 cases these processes seem destined only for protection against 

 cold, as in those plants in which they only clothe the buds 

 and youngest leaves, falling away as soon as the tender parts 

 have become hardened ; but it can hardly be doubted that in 

 many others they are absorbent organs, intended to collect 

 humidity from the atmosphere. In succulent plants, or in such 

 as grow naturally in shady places, where moisture already 

 exists in abundance, they are usually wanting : but in hot, 

 dry, exposed places, where it is necessary that the leaf should 

 avail itself of every means of collecting its food, there they 

 abound, lifting up their points and separating at the approach 

 of the evening dews, but again falling down, and forming a 

 layer of minute cavities above the cuticle, as soon as the heat 

 of the sun begins to be perceived. 



Whether or not leaves have the power of absorbing atmo- 

 spheric fluid, independently of their hairs, is a matter of doubt. 

 By some it is believed that they do possess such a power, and 

 that absorption takes place indifferently by either the 

 upper or under surface of the leaf, but that some plants absorb 

 more powerfully by one surface than by the other. Bonnet 

 found that, while the leaves of Arum, the kidney-bean, the 

 lilac, the cabbage, and others, retained their verdure equally 

 long whichever side was deprived of the power of absorption, 

 the Plantago, some Verbascums, the marvel of Peru, and 

 others, lost their life soonest when the upper surface was 

 prevented from absorbing ; and that, in a number of trees and 

 shrubs, the leaves were killed very quickly by preventing 

 absolution by the loicer surface. But others contend that 

 Bonnet's experiments merely produced a hindrance of evapor- 

 ation in some cases, and of respiration in others ; and that 

 leaves have, in fact, no power of attracting fluid. In proof of 

 this it is urged that, if leaves are made to float on coloured in- 



