ABSORPTION OF WATER BY LICHENS AND MOSSES. 217 



weighed 0"126 grm. after desiccation, so that when alive it must have contained 

 94 per cent, of water. Bog-moss, weighing 25'067 grms. before the abstraction of 

 the water was reduced to 2'535 grms. afterwards, showing that the percentage of 

 water was 90. Similar results are obtained in the cases of succulent leaves and 

 stems of flowering plants, Cucurhita, and other fruits. The least proportion of 

 water is contained by mature seeds, solid stony seed-coats, wood, and bark; but even 

 in these an average proportion of 10 per cent of water has been detected. We shall 

 not go wrong in assuming, on the evidence of the weights determined, that most parts 

 of plants, when fresh, consist of dry substance only as regards a third, and as 

 regards two-thirds, of water of imbibition, which passes over into the surrounding 

 air in the form of vapour when desiccation takes place. 



From all this it follows that water is absolutely necessary to plants as food- 

 material, that it is indispensable as a medium of transport of other substances, and 

 that the demand for water on the part of all plants is very great. Further, we may 

 infer that the importation and exportation of water must be regulated with exacti- 

 tude if the nutrition is not to be disturbed and development hindered. 



Water-absorption is at its simplest in hydrophytes. In this case it coincides 

 with the absorption of the rest of the food-materials, and there is therefore nothing 

 material to add to the statements already made on that subject. 



As regards land-plants, lithophytes, and epiphytes, we may likewise refer to 

 what has been already said in so far as these plants suck up water at the same time 

 as food-salts, by means of absorption-cells, from the substratum to which they are 

 attached, or the earth in which they are rooted; but to the extent that they take 

 also water direct from the atmosphere, and have the power of absorbing that water 

 immediately they require it, must be discussed in the following pages. 



ABSOBPTION OT WATEE BY LICHENS AND MOSSES, AND BY 

 EPIPHYTES FUENISHED WITH AERIAL ROOTS. 



The plants which absorb water direct from the atmosphere may be classified in 

 several groups with reference to the contrivances adapted to the purpose. Of all 

 plants lichens are most dependent on atmospheric moisture. Many of them, 

 especially the Old Man's Beard Lichens, which hang down from dried branches of 

 trees, and the gelatinous, crustaceous, and fruticose lichens, which cling to dead 

 wood, and on the surface of rocks and blocks of stone, do in fact derive their 

 necessary supply of water entirely from the atmosphere, and that by absorbing it, 

 not in a liquid but in a gaseous form. The latter circumstance is of the greatest 

 importance to those species in particular which occur on receding rocks, or on the 

 under face of overhansfing slabs of stone. Rain and dew cannot reach such places 

 directly, but only by some of the water trickling down from the wet top and sides 

 of the rocks on to the receding wall, and this happens but seldom. Accordingly, 

 lichens occurring in situations of the kind are entirely dependent upon the water 

 contained in the air in the form of vapour. Lichens, however, are also, of all plants, 



