220 ABSORPTION OF WATER BY LICHENS AND MOSSES. 



a chemical affinity on the part of the cell-contents, but solely by capillary action. 

 All the cell-walls are perforated and furnished with pores, and through these the 

 water rushes into the interior with lightning rapidity. 



This extremely rapid influx of water into an air-filled cavity leads us necessarily 

 to the conclusion that each cell has a number of pores in its walls, and that in 

 proportion as water enters through one of the small apertures the air can escape 

 equally fast through another. This is in fact the case. The large cells not only 

 have pores on their external walls, but communicate one with another by similar 

 holes, and the water soaks in from the one side as it does into a bath-sponge, whilst 

 the air is at the same time forced out on the other. This absorptive apparatus is 

 exceptionally elegant in Leucohryum, which grows abundantly in many woods. 

 In it, as is shown in the illustration above (fig. 49 ■'), the adjacent prismatic cells 

 communicate by highly symmetrical, circular gaps made in the middle of the 

 partition- walls, whilst in the Bog-mosses (the various species of Sphagnum), they 

 are to be seen scattered here and there between the thickening bands on the cell- 

 walls (see fig. 49 ^ ). Now these porous groups of cells possess not only the power 

 of taking up water in the liquid state, but also that of condensing it when in the 

 form of vapour. There is no need of any more proximate proof of the fact that 

 the cells previously mentioned as containing chlorophyll, and lying imbedded 

 between the large perforated cells, take up water supplied by the latter, or 

 perhaps it is better to say that the large perforated cells suck in the water for 

 the living green cells. We have only to ask why it is, then, that these small green 

 cells do not absorb water themselves direct from the environment, as is done in 

 the case of so many other mosses and liverworts. It is diflScult to answer this 

 quite satisfactorily, but thus much seems certain, that the large porous cells, when 

 full of air, afford a means of protecting the small living cells from too excessive 

 desiccation, and that they are in addition preservative of the chlorophyll in the 

 small cells, a matter to which we shall return presently. 



A certain resemblance to these Leucobryums and Sphagnums, in respect of water- 

 absorption, is exhibited by a few Aroidese, and more especially by a whole host of 

 Orchidacese. Of the 8000 difierent orchids hitherto discovered, a good proportion, 

 it is true, are rooted in the earth. But more than half these wonderful plants 

 flourish only on the bark of old trees, and most of them would quickly perish if 

 they were detached from that substratum and planted with their roots buried in 

 earth. A double function appertains to the roots of these Orchidese which inhabit 

 trees. On the one hand they have to fix the entire orchid-plant to the bark, and, 

 on the other, to supply it with nutriment. When the growing tip of an orchid's 

 root comes into contact with a solid body, it adheres closely to it, flattens out more 

 or less, sometimes even becoming strap-shaped (see fig. 15), and develops papilli- 

 form or tubular cells, which grow into organic union with the substratum, and 

 might conveniently be termed clamp-cells. In many cases these cells creep over 

 the bark, divide, interlace, and form regular wefts. The organic connection with 

 the substratum is so intimate that an attempt to separate the two usually results 



