182 THE FOOD AND NUTRITION OF PLANTS. 



hydrogen, and carbon, — although a small part of the carbon is 

 necessarily lost in the operation, yet what remains perfectly pre- 

 serves the shape of the original body, even to that of its most 

 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 ga-:, which consists of carbon united with oxygen. This 

 gas makes up on the average one 2500th of the bulk of the at- 

 mosphere ; 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 veliicle of the others, conveying 

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

 supply of nitrogen, either as such 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 he absorbed by the leaves directly from the air, in the state of 

 gas or vapor. Doubtless most plants actually lake in no small part 

 of their food in this way. Drooping foliage may be revived by 

 sprinkling with water, or by exposure to a moist atmosphere. A 

 vigorous branch of the common Live-for-ever (Sedum Telephium), 

 or of many similar plants, it is well known, -will live and grow for a 

 whole season -when pinned to a dry and bare wall ; and the Epi- 

 phytes, or Air-plants (149), as they are aptly called, must derive 

 their whole sustenance immediately from the air ; for thej' have no 

 connection with the ground. That leaves absorb carbonic acid 

 directly from the air is readily shown (348). 



331. But, as a general statement, it may be said that plants, al- 

 though they derive their food from the air, receive it mainly through 

 their roots. The aqueous vapor, condensed into rain or dew, and 



