186 THE FOOD AND NUTRITION OF PLANTS. 



delicate cells and vessels. ' With the exception of the ashes, this 

 consists of carbon, or charcoal, amounting to from forty to sixty 

 per cent, by weight, of the original material. Carbon is itself a 

 solid, absolutely insoluble in water, and therefore incapable of as- 

 sumption by the plant. The chief, if not the only fluid compound 

 of carbon which is naturally presented to the plant, is that of car- 

 bonic acid gas, which consists of carbon united with oxygen. This 

 gas makes up on the average one two-thousandth of the bulk of the 

 atmosphere ; from which it may be directly absorbed by the leaves. 

 But, being freely soluble in water up to a certain point, it must also 

 be carried down by the rain and imbibed by the roots. The car- 

 bonic acid of the atmosphere is therefore the great source of carbon 

 for vegetation. 



329. It appears, then, that the atmosphere considering water 

 in the state of vapor to form a component part of it contains all 

 the essential materials for the growth of vegetables, and in the form 

 best adapted to their use, namely, in the fluid state. It furnishes 

 water, which is not only food itself, inasmuch as it supplies oxygen 

 and hydrogen, but is likewise the vehicle of the others, conveying 

 to the roots what it has gathered from the air, namely, the requisite 

 supply of nitrogen, either separately or in the form of ammonia, 

 and of carbon in the form of carbonic acid. 



330. These essential elements, the whole proper food of plants, 

 may be absorbed by the leaves directly from the air, in the state of 

 gas or vapor. Doubtless most plants actually take in a portion of 

 their food in this way, at least when other supply is arrested. 

 Drooping foliage may be revived by sprinkling with water, or by 

 placing them in a moist atmosphere. A vigorous branch of the 

 common Live-for-ever (Sedum Telephium), or of many such 

 plants, it is well known, will live and grow for a whole season 

 when pinned to a dry, bare wall ; and the Epiphytes, or Air-plants 

 (132), as they are aptly called, must derive their whole sustenance 

 immediately from the air ; for they have no connection with the 

 ground. 



331. But the peculiar office of leaves is something different from 

 that of absorbing nourishment. As a comprehensive statement, 

 leaving extraordinary cases out of view, it may be said that plants, 

 although they derive their food from the air, receive it chiefly 

 through their roots. The aqueous vapor, condensed into rain or 

 dew, and bringing with it to the ground a portion of carbonic acid, 



