230 ABSORPTION-CELLS ON LEAVES. 



coated with varnish. Many plants which have their roots buried m crevices of 

 rock and no small number of herbaceous steppe-plants are quite thickly covered 

 with glandular hairs of the kind. Centaurea 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 {Centaurea 

 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 

 instance, the leafy stem of Thesium alpinum — -and the cut ends are closed with 

 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 Thesium 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- 



