SOURCE OF THEIR ORGANIC CONSTITUENTS. 181 



326. As respects the nitrogen, nearly seventy-nine per cent of 

 the atmosphere consists of this gas in an uncombined or free state, 

 that is, merely mingled with oxygen. And, being soluble to some 

 extent in water, every rain-drop that falls tln-ough the air absorbs 

 and brings to the ground a minute quantity of it, which is therefore 

 necessarily introduced into the plant with the water which the roots 

 imbibe. This accounts for the free nitrogen wliich is always pres- 

 ent in plants. 



327. The plant also receives nitrogen in the form of ammonia 

 (or hartshorn), a compound of hydrogen and nitrogen, which is 

 always produced Avhen any animal and almost any vegetable sub- 

 stance decays, and which, being very volatile, continually rises into 

 the air from these and other sources. Besides, it appears to be 

 formed in the atmosphere, through electrical action in thunder-storms 

 (in the form of nitrate of ammonia). The extreme solubility of am- 

 monia and all its compounds prevents its accumulation in the atmos- 

 phere, from which it is greedily absorbed by aqueous vapor, and 

 bi'ought down to the ground by rain. That the roots actually ab- 

 sorb it may be inferred from the familiar facts, that plants grow 

 most luxuriantly when the soil is supplied witli substances which 

 yield much ammonia, such as animal manures ; and that ammonia 

 may be detected in the juices of almost all plants. That the am- 

 monia in the air, and the nitre almost everywhere formed in a fertile 

 soil, and not the free nitrogen of the atmosphere, take the principal 

 part in the formation of the protoplasm and other quaternary ele- 

 ments of plants, is demonstrated by Boussingault's experiments, 

 showing that a seedling from which all nitrogen is excluded except 

 the free nitrogen of the air, as it vegetates does not increase the 

 amount of azotized matter it originally had in the seed, but dimin- 

 ishes it.* Rain-water, therefore, contains the third element of 

 vegetation, namely, nitrogen, both in a separate form and in that of 

 ammonia, &c. 



328. The source of the remaining constituent, cai'bon, is still to 

 be sought. Of this element plants must require a copious supply, 

 since it forms much the largest portion of their bulk. If the carbon 

 of a leaf or of a piece of wood be obtained separate from the other 

 organic elements, — which may be done by charring, that is, by 

 heating it out of contact with the air, so as to drive off the oxygen, 



* Comptes Rendiis, November 28, 1853, and Ann. Sci. NatureUes, ser. 4, Vol. 

 1 & 2 (1854) ; also Vol. 7 (1857), showing the part which nitre plays. 



16 



