188 THE FOOD AND NUTRITION OF PLANTS. 



attained the fullest development of which it is capable under these 

 circumstances, it will be found to weigh (after due allowance for 

 the silex it may have taken up) perhaps fifty or one hundred times 

 as much as the original seed. There can be no question as to the 

 source of this vegetable matter in all these cases. The requisite ma- 

 terials exist in the air. Plants possess the peculiar faculty of draw- 

 ing them from the air. The air must have furnished the whole. 

 This conclusion is amply confirmed by a great variety of familiar 

 facts ; such as the accumulation of vegetable matter in peat-bogs, 

 and of mould in neglected fields, in old forests, and generally 

 wherever vegetation is undisturbed. Since this rich mould, instead 

 of diminishing, regularly increases with the age of the forest and 

 the luxuriance of vegetation, the trees must have drawn from the 

 air, not only the vast amount of carbon, &c., that is stored up in 

 their trunks, but an additional quantity which is imparted to the 

 soil in the annual fall of leaves, &c. 



334. Still it by no means follows, that each plant draws all its 

 nourishment directly from the air. This unquestionably happens 

 in some of the special cases just mentioned ; with Air- plants, and 

 with those that first vegetate on volcanic earth, bare rocks, naked 

 walls, or pure sand. But it is particularly to be remarked, that 

 only certain tribes of plants will continue to live under such cir- 

 cumstances, and that none of the vegetables most useful as food 

 for man or the higher animals will thus thrive and come to matu- 

 rity. In nature, the races of plants that will grow at the entire 

 expense of the air, such as Lichens, Mosses, Ferns, and certain 

 succulent tribes of Flowering plants, gradually form a soil of veg- 

 etable mould during their life, which they increase in their decay ; 

 and the successive generations live more vigorously upon the in- 

 heritance, being supported partly upon what they draw from the 

 air, and partly upon the ancestral accumulation of vegetable 

 mould. Thus, each generation may enrich the soil, even of those 

 plants that draw largely upon vegetable matter thus accumulated ; 

 for it annually restores a portion by its dead leaves, &c., and when 

 it dies it bequeathes to the soil, not only all that it took from it, but 

 all that it drew from the air. It is in this way that the lower tribes 

 and so-called useless plants create a soil, which will in time sup- 

 port the higher plants of immediate importance to man and the 

 higher animals, but which could never grow and perfect their 

 fruit, if left, like their humble but indispensable predecessors, to 



