204 ABSORPTION BY LEAVES. [BOOK n. 



the upper stratum perform a function analogous to that of 

 the stomach in animals, digesting 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 

 parenchym 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 cannot 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 epidermis, 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, has been doubted. 

 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 Kidneybean, 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 absorption 

 by the lower surface. But others contend that Bonnet's 

 experiments merely produced a hindrance of evaporation in 

 some cases, and of respiration in others ; and that leaves have, 



