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 Lewcobrywm, which grows abundantly in many woods. 
In it, as is shown in the illustration above (fig. 491), 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 Sphagnwm), 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 difficult 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 Aroidez, and more especially by a whole host of 
Orchidacee. Of the 8000 different 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 Orchidez 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 
