94 COMPOUND ORGANS OF PLANTS. 



atmospheric cryptogamia flourish and die, forming a humus 

 for the growth of grasses, ferns, and more highly organized 

 plants ; until at length there is formed on the surface of that 

 once barren rock a sufficiency of humus for the nutrition of all 

 the varieties of vegetable organisation found in the fertile 

 meadow, the tangled thicket, and the widely extended forest. 

 Finally man comes to take possession of the new domain 

 which nature has thus been carefully preparing for him, and 

 life reaches its highest stage of development. 



The inorganic matter constituting the ash which remains after 

 the combustion of the plant, is wholly absorbed from the soil, 

 and enters the plant in a state of solution by the pores of the 

 roots. Some persons have supposed that these mineral matters 

 were produced by the plants themselves, and not derived from 

 without. It is true that the earths, such as silica or sand, 

 alumina or clay, are insoluble by themselves in water, and that 

 the subdivision of the matter of which they are composed must 

 be carried to an almost infinite degree of minuteness, before 

 they can pass into the system of the plant through the minute 

 pores of the roots; but all the earths are soluble with the 

 alkalies, such as potash, which enters largely into the composi- 

 tion of all rocks, and as these earths are furnished to the soil 

 by the slow decomposition or disintegration of rocks, there can 

 be no doubt that the water, as it percolates the soil impreg- 

 nated with potash and carbonic acid, effects their solution to 

 such an extent that they pass unimpeded into the system of 

 the plant along with the water which is imbibed by the root. 



Each species of plant, according to its peculiar constitution, 

 retains a greater or less amount of one or more of these earthy 

 ingredients. Thus, nearly all plants retain a quantity of 

 potash ; wheat, a certain amount of silex ; some aquatic plants 



