78 SOURCE AND ASSIMILATION 



number of other compounds containing nitrogen. 

 The chemical characters of ammonia render it ca- 

 pable of entering into such combinations, and of 

 undergoing numerous transformations. We have 

 now only to consider whether it really is taken up 

 in the form of ammonia by the roots of plants, and 

 in that form applied by their organs to the produc- 

 tion of the azotised matters contained in them. 

 This question is susceptible of easy solution by well- 

 known facts. 



In the year 1834, I was engaged with Dr. Wil- 

 brand, professor of botany in the university of 

 Giessen, in an investigation respecting the quantity 

 of sugar contained in different varieties of maple- 

 trees, which grew upon soils which were not ma- 

 nured. We obtained crystallised sugars from all, 

 by simply evaporating their juices, without the ad- 

 dition of any foreign substance ; and we unexpect- 

 edly made the observation, that a great quantity of 

 ammonia was emitted from this juice, when mixed 

 with lime, and also from the sugar itself during its 

 refinement. The vessels, which hung upon the 

 trees in order to collect the juice, were watched 

 with greater attention, on account of the suspicion 

 that some evil-disposed persons had introduced urine 

 into them, but still a large quantity of ammonia was 

 again found in the form of neutral salts. The juice 

 had no colour, and had no reaction on that of 

 vegetables. Similar observations were made upon 

 the juice of the birch-tree ; the specimens subjected 



