ABSORPTION OF llAIN 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 ofl'them in drops, and cannot, there- 

 fore, be absorbed by them. This is true even of many soft trichomes (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 {Verhascum 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, i.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 

 intemodes 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 



