270 
PHYSIOLOGY. 
BOOK II. 
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 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 pf trees and 
shrubs, the leaves were killed very quickly by preventing 
absorption 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- 
