ABSORPTION OF FOOD-SALTS BY LITHOPHYTES. 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 under these conditions, and these choose, by preference, the 
very spots where they are most exposed to the dash of the water. Thus, certain 
Nostocinee (Zonotrichia, Scytonema) are to be found constantly in waterfalls at 
the parts where the most violent fall occurs. Lemanea, Hydrurus, and many 
mosses and liverworts, grow by preference in the foaming cascades of rapid 
torrents. Amongst flowering plants we only know of the Podostemacez as choosing 
a habitat of this kind. Podostemacez 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 Lophogyne 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 LITHOPHYTES. 
Nothing would seem more natural, as to the absorption of mineral salts by 
lithophytes, than that 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 
quartz, 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 
plants, the stones to which they cling by their dises 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 substratum? 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 
entertained 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 paths 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 places of that kind or in 
the air of marshes, lakes, or seas. This notion has certainly some warrant if we 
regard as dust only the coarser particles which are raised from loose earth and 
