ABSORPTION OF RAIN AND DEW BY THE FOLIAGE-LEAVES. 227 
to the leaf-surfaces, and they are distinctly wetted. But even cells and groups of 
cells of this kind usually act but for a short time as absorption-cells, and only when 
the necessity and craving for water is very great, or when there is an opportunity 
of acquiring nitrogenous compounds at the same time as the water; and here, 
again, special contrivances are always present which regulate this kind of water- 
absorption, and render it impossible whenever it is not truly advantageous. 
At first one would suppose that amongst the cells composing the epidermis of 
foliage-leaves, those are best adapted to the absorption of water from the atmosphere 
which take the form of hairs. The superficial area being as great as possible, and 
the contained matter relatively little, one can scarcely in fact conceive a conforma- 
tion better suited to the purpose of water-absorption. As, moreover, the area of 
contact between the cells of the leaf and of a hair is small, there would afterwards 
be but very little evaporation through the surface of the hair of the water once 
sucked up by it and conducted into the interior of the leaf. In a word, these hairs 
on the surface of a leaf appear to be peculiarly adapted to the taking up of water, and 
not at all favourable to its exhalation. The hypothesis based on these observations 
is indeed entirely applicable to the case of hairs occurring on the leaflets of mosses, 
as has been already stated. But it does not hold in the case of the hair-like struc- 
tures which spring from the leaf-surfaces of flowering plants. These are frequently 
not wetted at all by water; rain and dew roll off them in drops, and cannot, there- 
fore, be absorbed by them. This is true even of many soft triehomes (hair-structures) 
which form investments upon leaves, and which seem to be more than any fitted 
for the absorption of water. For instance, experiments upon the woolly leaves of 
the Great Mullein (Verbascum Thapsus) have shown that they neither condense 
aqueous vapour nor take up water in liquid drops. Small importance must be 
attributed to the thickness of the cuticle, for sometimes it is the very cells which 
are equipped with a cuticle of considerable stoutness that are adapted to admit 
water, under certain circumstances, through their walls. On the other hand, much 
depends upon the presence of wax in the cuticle and upon the contents of the cells; 
that is to say, upon whether those contents in particular have a strong or weak 
affinity for water. If the cells of the hairs are full of air they are not adapted to 
the absorption of water. 
If a hair is septate, ö.e. consists of a simple series of cells, only the undermost or 
else only the uppermost cells of the series absorb water. Instances wherein it has 
been observed that the lowest cells alone in hairs of the kind become absorption- 
cells are afforded by the Alfredia, represented in fig. 14, by Salvia argentea, and 
several other steppe-plants. The same statement is made concerning the widely- 
distributed Stellaria media, the common Chickweed. This last has hairs on the 
internodes of the stem, running down in ridges from node to node. Usually only 
one side of the stem exhibits a ridge of hairs of the kind, and the ridge always 
terminates at the thickened node, whence springs a pair of opposite leaves. The 
stalks of these leaves are somewhat hollowed out and have their edges beset with 
hairs like lashes. The hairy ridges on the segments of the stem are readily wetted 
