82 ABSORPTION OF FOOD-SALTS BY LAND-PLANTS. 
the usual mineral salts in about such proportion as is necessary for the cultivation 
of cereals in fields has actually an injurious effect on these lithophytes and soon 
kills them. 
At the end of this section we shall consider what happens to dust which is 
brought to earth from the air by rain and snow but is not dissolved, and the 
important part it plays in clothing the naked ground and in changes of vegetation. 
Here, however, it must be noted that most lithophytes are true dust-catchers, that is 
to say, they are able to retain, mechanically, dust conveyed to them by wind, rain, 
and snow, and to use it in later stages of development by extracting nutriment from 
it. Many mosses are completely lithophytic in early stages of development whilst 
later they figure as land-plants. 
ABSORPTION OF FOOD-SALTS BY LAND-PLANTS. 
In no class of plants is the absorption of mineral food-salts accomplished in 
so complicated a manner as in land-plants. Moreover, this absorption is by no 
means uniform in different forms of plants, and we must beware of generalizing 
with regard to processes which have only been traced and studied in isolated 
groups—perhaps only in the commonly distributed cultivated plants. On the other 
hand, with a view to synoptical representation, it is not desirable to enter into too 
great detail or to attempt to describe all the various differences minutely. 
At the outset, it is difficult to give an accurate account of the soil which 
constitutes the source of nutriment in the case of land-plants. From the dark 
graphitic mass composed of sun-motes, which is deposited in the place of a melted 
layer of snow, to coarse gravel, there is an unbroken chain of transition stages; 
loam, sand and gravel are only specially-marked members of this chain. Again, 
just as earth varies in respect of the size of its component parts, so also it 
varies in the mineral salts it contains, in the amount of admixture of decaying 
vegetable and animal remains, in the nature of the union of its constituents, 
and in its capacity to absorb, to retain, or to yield up water. Compare the sand 
composed of quartz on the bank of a mountain stream with that of calcareous 
origin which is found impregnated with salt on the sea-shore, or with the sand 
at the foot of mountains of trachyte, which has an efllorescence of soda-salts. 
Or compare the granite bed of a desert, bare of soil, with the loam on the granitic 
plateaus of northern regions where there is an intermixture of the remains of a 
vegetation for centuries active. How great is the difference in each case! But 
whatever the kind of earth, it is only of value as a source of nutriment for a 
plant when the interstices of its various particles are filled with watery fluid 
for the time during which the plant is engaged in the construction of organic 
substances. 
But how is the earth supplied with water? 
“ Das hat nicht Rast bei Tag und Nacht, 
Ist stets auf Wanderschaft bedacht.” 
