226 ABSORPTION OF RAIN AND DEW BY THE FOLIAGE-LEAVES 
the soil in which the roots are imbedded that a gardener waters on dry days, 
although incidentally he may pour the water over the aérial parts of the plants. 
He sees, however, that the water which falls in the form of rain or dew upon the 
foliage and stems normally runs off them at once, or else collects in drops, which 
trickle down whenever the plant is shaken by the wind, and are sucked up by the 
thirsty ground. This phenomenon must be due to the possession by the leaves of 
special contrivances to prevent their being wetted. It does not in any case support 
the idea that foliage is as well adapted for the absorption of water as experience 
has proved subterranean roots to be. This train of thought, which forces itself 
upon every unbiassed observer of the processes as they take place in nature, is 
certainly warranted in the majority of cases. Each absorption-cell on the roots 
buried in the earth has an easily permeable membrane, and, as is well known, water 
passes from damp earth through the cell-membranes into the interior of a plant 
with great rapidity. The water in the interior of the plant would be equally easily 
withdrawn through these cell-membranes by dry surroundings, but, as it is, this 
scarcely ever happens, in consequence of the roots being situated underground. In 
the case of aérial parts, especially the foliage-leaves, the circumstances are quite 
different. The leaves have to yield up to the air a portion at least of the water 
conducted from the roots, because, as will be more thoroughly explained later on, it 
is only by means of this evaporation that the entire machinery in the interior of the 
plant can be kept in motion. But this evaporation must not go too far; it must be 
in proper relation to the absorption of water by the subterranean roots, and be 
regulated to that end if the plant is not to run the risk of drying up altogether at 
times—an occurrence which flowering plants are unable to survive, although the 
mosses described in former pages have that power. Accordingly, in the case of the 
foliage-leaves of flowering plants, evaporation is confined to certain cells and groups 
of cells, and these, in addition, have contrivances by means of which evaporation 
can be entirely stopped on occasion of great drought. It stands to reason that all 
contrivances which make it impossible for water to pass from the interior of the 
leaves through the walls of the superficial cells into the surrounding air also hinder 
the entrance of water into the leaves from the atmosphere. 
It would be altogether inconsistent with the system of arrangement of the sub- 
ject adopted in this book if we were to discuss here all the contrivances serving to 
regulate the exhalation of water by leaves, and we must, therefore, confine ourselves 
to referring, by way of introduction, quite briefly, to the following facts, namely, 
that those pores on the surface of leaves which are known by the name of stomata, 
and are used as doors of egress by the exhaled water, do not admit rain or dew, or in 
general, any water in the liquid state; that the so-called cuticle covering the exter- 
nal walls of the epidermal cells in leaves is an additional barrier to both egress and 
ingress of water; that when, in particular, this cuticle is furnished with a wax-like 
coating, water does not adhere to the surface of cells so protected; and, lastly, that 
atmospheric moisture can only penetrate into the interior of the plant at parts of 
the leaves where the waxen incrustations are absent, where water remains adherent 
