158 BIOLOGY OF PLANTS. 



fore highly probable, that plants derive their oxygen and hy- 

 drogen, as well as their carbon, from several sources ; and 

 that these two substances enter the vegetable organs in the 

 form of water ; of geine or humic, crenic and apocrenic 

 acids ; of ammonia ; of common air ; and, probably, of sev- 

 eral acids. 



V. Source and Assimilation of the Nitrogen of Plants, It 

 was formerly supposed, that nitrogen existed in only a few 

 plants, but it is now established that it exists in all. " It 

 exists in every part of the vegetable structure."* The quan- 

 tity, however, is very small, compared with the other ingre- 

 dients of the vegetable principles. Hay, dried at 240° F., 

 contains but IJ, oats 2j, and potatoes 1^ per cent. In the 

 ordinary state in which these substances are found, they must 

 contain a much less quantity. 



This quantity is small only in comparison with the other 

 organic constituents, for if we calculate the quantity of nitro- 

 gen in an average crop of hay and grain grown on three hun 

 dred acres of land, it will amount to eight tons.t 



This relatively small, but absolutely large quantity of ni- 

 trogen is of the highest importance to vegetation. In fact 

 the value of manure has been estimated by its power of yield- 

 ing nitrogen in the form of ammonia.| The body which ex- 

 ists in the smallest quantity in the vegetable products, is just 

 as necessary to their formation, as that which is most abun- 

 dant. It has been due to a neglect of this principle that so 

 little effort has, as yet, been made to supply plants directly 

 with this substance. Whence, then, do plants derive their ni- 

 trogen 1 The following are the principal sources. 



* Liebig. 



t A ton of hay contains about 30 lbs. of nitrogen ; but tlic quantity 

 depends very much upon the kind of crop. Red clover contains 

 double the quantity of nitrogen which common hay does ; hence, an 

 acre yielding three tons would require 180 lbs. of nitrogen. 



X Dana, 



