ABSORPTION OF KOOD-SALTS BY LlTHOl'HYTES. 79 



partly also on the circumstance that mechanical difficulties are opposed to the taking 

 up of saline molecules from water in rapid motion. There are only a few plants 

 that are able to absorb xnider these conditions, and these choose, by preference, the 

 very spots where they are most exposed to the dash of the water. Thus, certain 

 Xostocineaa (Zonotrichia, Scytoneiua) are to be found constantly in waterfalls at 

 the parts where the most violent fall occurs. Lemania, Hydrurus, and many 

 mosses and liverworts, grow by preference in the foaming cascades of rapid 

 torrents. Amongst flowering plants we only know of the Podostemacete as choosing 

 a habitat of this kind. Podostemacea3 are exceedingly curious little plants, which 

 at first glance one would take for mosses or liverworts without roots. Some of 

 them, e.g. the Brazilian species of the genus Lajihogyne and the various species of 

 Terniola growing in Ceylon, exhibit no differentiation into stem and leaves, but are 

 only represented by green fissured and indented lobes attached to stones. They 

 belong without exception to the tropical zone, and occur there in the beds of streams, 

 attached to rocks, over which the foaming water rushes. 



ABSORPTION OF FOOD-SALTS BY LTPHOPHYTES. 



Nothing would seem more natural, as to the absorption of mineral salts by 

 lithophytes, than tliat the stone which constitutes their support should yield the 

 salts, and that the attached plants should suck them up; but, generally speaking, 

 the case is not so simple. There are mosses and lichens which cling to the surfaces 

 of rocks on mountain tops. These rocks are sometimes composed of perfectly pure 

 «[Uartz, and yet the plants in question contain very little silica; they contain, on 

 the other hand, a number of substances entirely wanting in the composition of the 

 underlying rock, and which could not, therefore, have been derived from that 

 source. For many of these lithophytes the rock is, in the main, only a substratum 

 for attachment, and in no way a nutrient soil; just as, in the case of many aquatic 

 jjlants, the stones to whicli they cling by their discs of attachment are anything 

 but sources of nourishment. 



From what source, then, do stone-plants of this kind derive the food-salts which 

 are wanting in their substi-atum ? It may sound paradoxical, but it is nevertheless 

 the fact, that they obtain those salts from the air through the medium of atmospheric 

 precipitation. Rain and snow not only absorb carbon dioxide, sulphuric acid, and 

 ammonia — which occur in air universally, although in extremely minute quantities 

 — but they also collect, as they fall, floating particles of dust. The opinion is widely 

 entez'tained that although the atmosphere is full of dust in the neighbourhood of 

 cities and human settlements generally, where the soil is laid bare and ploughed 

 up, and roads and jjaths have been made for purposes of traffic, and perhaps also 

 over steppes and deserts where large areas of ground are destitute of vegetation, 

 yet that there is no dust in the air over land remote from jjlaces of that kind or in 

 the air of marshes, lakes, or seas. This notion has certainly some warrant if we 

 regard as dust ordy the coarser particles which are raised from loose earth and 



