332 The Physiology oj Plants BOOK in 



What exactly was included in the term humus was not 

 very clearly defined. Organic matter in various stages of 

 decay exists in the soil, and the number of constituents, 

 many of them in very small traces, is large and varied. 

 It may conveniently be held to have included all such organic 

 nitrogen-containing bodies as are present in the soil, com- 

 pounds on their way apparently to furnish by further pro- 

 cesses of decay the ammonia and ammonia compounds which 

 can be detected there, or by other kinds of decomposition to 

 furnish nitrous or nitric acid. Complex nitrogenous bodies, 

 such as urea, uric acid, glycin, were in 1865-7 shown by 

 Hampe to nourish plants in water cultures, and even to 

 carry them to the point of producing seeds. In 1868 

 Wagner added to the list kreatin and others, and Wolff 

 found tyrosin similarly utilizable. 



It seemed at the outset, however, hardly probable that 

 the bulk of the nitrogen of plants is taken from organic 

 combinations, especially in the light of Boussingault's 

 results. Liebig had laid stress in 1840 on ammonia as the 

 chief source of the plants' supply. Boussingault had found 

 that rain carries to the soil nitric acid, formed no doubt 

 in the atmosphere. 



It was not, indeed, till the latter part of our period that 

 the compounds of nitrogen we have mentioned were care- 

 fully investigated and compared, when certain relations 

 were found to exist between them, and the soil was ascer- 

 tained to be the scene of considerable chemical and bio- 

 logical activity in order to present the combined nitrogen 

 to the green plant in the most advantageous form. 



Certain isolated observations on somewhat collateral 

 points may be mentioned, as having a bearing on the 

 question of determining the latter. In 1874 Schloesing 

 showed that the leaves can absorb gaseous ammonia when 

 traces of it are present in the air, and in 1887 Kerner 

 found that under certain conditions this source of supply 



