230 ABSORPTION-CELLS ON LEAVES. 
coated with varnish. Many plants which have their zoots buried in crevices of 
rock and no small number of herbaceous steppe-plants are quite thickly covered 
with glandular hairs of the kind. Centawrea Balsamita (see fig. 53°), a plant 
occurring on the elevated steppes of Persia, may be selected as an example of the 
latter group. The advantage of the structure of capitate hairs is not far to seek. 
In dry weather the thick cuticle (Pelargonium) or the varnish coating (Centawrea 
Balsamita), as the case may be, prevents desiccation of the cells and groups of cells 
in question. But as soon as rain or dew falls, the cuticle and the coat of varnish 
take up water, and it is by their instrumentality that water reaches the interior of 
the cells. Thus, whilst the exhalation of water is hindered, its absorption is not. 
Other epidermal cells of foliage-leaves besides trichomes are capable of acting as 
absorption-cells, although this action, for reasons already given, is very restricted, 
and is only had recourse to when the turgidity of the cells of the foliage-leaves has 
diminished, and the water exhaled by those cells is not being restored by the 
ordinary apparatus of conduction from the roots. If branches are cut from plants 
which bear no glandular or other form of hair on their leaves or stems—as, for 
and the cut ends are closed with 
instance, the leafy stem of Thesiwm alpinum 
sealing-wax, and the branches left to wither, and, when quite withered, are 
immersed in water, they freshen up speedily and the leaves become tense again, the 
cells having recovered their turgidity. Here, then, decidedly absorption has taken 
place through the ordinary cuticularized epidermal cells. Certainly these epidermal 
cells in Thesiwm are not protected against wetting. Wherever the epidermal cells 
are not susceptible of being wetted owing to a coating of wax or any other 
contrivance there could naturally be no question of water being absorbed. This 
very circumstance, however, leads to the supposition that an important part in 
water absorption is to be attributed to the alternation of wettable and non-wettable 
parts on one and the same leaf. In the case of many foliage-leaves one can see that 
only those cells of the epidermis which lie above the veins of the leaf retain the 
water which comes upon them, that is to say, are wetted by it, whilst the water rolls 
off the intervening areas of the lamina. Indeed, there are in many instances 
contrivances obviously designed for the purpose of conducting water from parts of 
the epidermis not liable to be wetted to parts that can be moistened. 
DEVELOPMENT OF ABSORPTION-CELLS IN SPECIAL CAVITIES AND 
GROOVES IN THE LEAVES. 
The contrivances last described are all only adapted to rather a casual appropri- 
ation of water from the atmosphere. But besides these we find a number of other 
contrivances, which render it possible for every rolling dewdrop and every 
passing shower to be made of use to the utmost extent. These contrivances 
consist of a variety of depressions and excavations, in which rain and dew are 
collected and protected against rapid evaporation. Some species have deep hollows 
or channels, others little pits, whilst others again have basins, vesicular or bowl- 
