18i THE FOOD AND NUTRITION OF PLANTS. 



seed of known weight to germinate on powdered flints, or on a soil 



wliicli has been heated to redness, and watering it with rain-water 



alone. "When the young plant has attained all the development it is 



capable of 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 materials exist in the air. Plants possess the peculiar 



facility of drawing them from the air. The air must have furnished 



the whole. This conclusion is amply confirmed by a great variety of 



familiar facts ; sucli as the continued accumulation of vegetable mat- 



' ter in peat-bogs, and of mould in neglected tields, 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 whicli is im- 



// parted to the soil in the annual fall of leaves, &c. 



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

 nourislmient directly from the air. This unquestionably happens 

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

 witli 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 

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

 , table mould during their life, which tliey 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. 

 ' Tlius, each generation may enrich the soil, even when consisting of 

 plants that draw largely upon vegetable matter thus accumulated ; 

 for these annually restore a portion by their dead leaves, &c., and 

 when they die they may bequeatli to the soil, not only all that they 

 took from it, but all that they 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 support the higher plants, of immediate importance to 



