218 ABSORPTION OF WATER RY LICHENS AND MOSSES. 



the best adapted for the absorption of aqueous vapour from the air. If living 

 lichens, which have become dry in the air, are left in a place saturated with mois- 

 ture, they take up 35 per cent of water in two days, and as much as 56 per cent 

 in six days. Water in the liquid form is naturally absorbed much more rapidly 

 still. When Gyrophoras, which project in the form of cups after a long continuance 

 of dry weather, are moistened by a fall of rain, they swell up completely within 

 ten minutes, and spread themselves flat upon the rocks, having in that short 

 space of time absorbed 50 per cent of water. The saying, " Light come, light go," 

 is no doubt true in these cases. When dry weather sets in, evaporation from the 

 masses of lichens goes on at a pace corresponding to the previous absorption. In 

 the Tundra, the lichens, which form a soft tumid carpet when moistened by rain, 

 are liable to be so powerfully desiccated in the course of a few hours of sunshine, 

 that they split and crackle under one's feet, so that every step is accompanied by a 

 crunching noise. 



In the power of condensing and absorbing the aqueous vapour of the atmos- 

 phere, lichens are most analogous to mosses and liverworts, and to those pre- 

 eminently which live on the bark of dry branches of trees or on surfaces of rock, 

 covering places of the kind with a carpet which is often enough interspersed and 

 interwoven with lichens. Like the latter these mosses and liverworts are able to 

 remain as though dead in a state of desiccation for weeks together, but as soon as 

 rain or dew falls upon them they resume their vitality; and similarly if the air is so 

 damp as to enable them to derive sufficient water of imbibition from that source. 

 A specimen of Hypnum molluscum, a moss which covers blocks of limestone in the 

 form of soft sods, was after a few rainless days detached from the dry rock and 

 placed in a chamber saturated with vapour, and it was found that after two days 

 it had absorbed water from the air to the extent of 20 per cent, after six days 38 

 per cent, and after ten days 44 per cent. Many mosses condense and absorb water 

 with the whole surfaces of their leaflets, others — as, for example, the gray rock- 

 mosses clinging to slate formations (Rhacomitriae and Grimmife) — do so especially 

 with the long hair-like cells at the apices of the leaflets, whilst others again only 

 use the cells situated on the upper saucer-shaped or canaliculate leaf-surface. 



In some bearded mosses {Barbula aloides, B. rigida, and B. ambigua) chains of 

 barrel-shaped cells occur closely packed together upon the upper surface of the leaf 

 and at right angles to it, which to the naked eye have the appearance of a spongy 

 dark-green pad. The terminal cells of these short monihform chains have their 

 upturned walls strongly thickened, but the other cells have verj' thin walls and 

 take up water rapidly. It is the same with the various species of Polytrichia/), 

 which are provided on their upper leaf-surfaces with parallel longitudinal ridges 

 likewise composed of thin-walled, highly-absorbent cells. The rhizoids also play 

 an important part in these processes. These brown, elongated, thin-walled cells 

 entirely clothe the moss stems, usually in the form of a dense felt, and often pro- 

 ject from the under surface of the leaves, whilst in a few tropical species they make 

 their appearance, strangely enough, in the form of little tufts at the apices of the 



