ABSORPTION OF WATER BY LICHENS AND MOSSES. 



219 



leaflets. In many instances this felt of rhizoids does not come into contact at all 

 with the soil, rock, or bark (as the case may be), but is surrounded by air alone, 

 and is able to condense or attract, to use a common expression, the aqueous vapour 

 of the air like a piece of cloth or blotting-paper. In dry weather, it is true, mosses, 

 like lichens, lose their water, but they part with it much more slowly than the 

 latter. This is chiefly due to the fact that the moss-leaflets at the commencement 

 of a drought wrinkle, curl up, become concave, and lay themselves one above the 

 other, so that the water is retained at the bottom for a longer period. 



A very remarkable contrivance for the absorption of water from the atmos- 

 phere is also exhibited by the white-leaved Fork -mosses (Leucobryum) and Bog- 

 mosses (Sphagnace^). Although they possess chlorophyll, and assimilate under the 



Fig. 49.— Porous Cells. 



1 Of the white-leaved Fork-moss (Leucobryum) ; x 550. 2 of the Bog-moss (Sphagnum); x 230. ^ of the root of an Orchid 



(Lwlia gracilis) ; x310. 



influence of sunlight, yet they look like parasitic and saprophytic plants destitute 

 of chlorophyll. They are of a whitish colour and always grow in great cushion- 

 like sods, so that the spots where they grow are deficient in verdure, and stand 

 out conspicuously from their surroundings in consequence of their pale tint. 

 Microscopic investigation at once explains this appearance. The cells containing 

 chlorophyll and living active protoplasts are relatively small, and, as it were, 

 wedged and hidden between other cells many times as great, which have entirely 

 lost their protoplasm by the time they are mature, and then cause the paleness of 

 colour appertaining to the plant as a whole. The walls of these large colourless cells 

 are very thin, and in the Bog-mosses have spiral thickening-bands running round 

 them, being thus secured against collapse. After remaining for a time in a dry 

 environment they are full of air only; but the moment they are moistened they 

 fill with water. If there were an actively absorbent protoplast at work in the 

 interior, the water would be able to pass into the cell-cavity through this easily 

 moistened wall, as in the case of other mosses, owing to the delicacy of the cell- 

 membrane. But the air which fills the cells is not absorptive, and in the case of 

 LeucobryuTn and Bog-mosses the water reaches the interior, not in consequence of 



