326 
PHYSIOLOGY. 
BOOK II. 
ing 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 pro- 
vided 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 epidermis, as soon as the heat of 
the sun begins to be perceived. 
Whether or not leaves have the power of absorbing atmos- 
pheric 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 Kidneybean, the Lilac, 
the Cabbage, and others, retained their verdure equally long 
whichever side w'as 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 ex- 
periments merely produced a hindrance of evaporation 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 infusions, 
