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 efflorescence 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 East bei Tag und Nacht, 

 1st stets auf Wanderschaft bedacht." 



