350 The Physiology of Plants BOOKIII 



subject, but its consideration does not come within our 

 present province. 



The processes by which the nitrogen absorbed in these 

 various ways takes part in further chemical changes, or 

 the synthesis of proteins, received much attention during 

 the period under review, but the information which was 

 gained did not at all approach completeness. We have 

 seen that Boussingault, in the course of his researches on 

 the fixation of atmospheric nitrogen, ascertained that for 

 a large number of plants some compound of nitric acid 

 is the most advantageous form of presentation of this 

 element. The subsequent investigations upon nitrification 

 in the soil detailed above bear out this opinion. Still, it 

 must not be concluded that the older writers who said that 

 ammonium compounds are beneficial were altogether wrong. 

 The early part of our period supplies evidence bearing on 

 this point. Sachs in 1860-3, Schloesing in 1874, and Mayer 

 in the same year, showed that ammoniacal gases can supply 

 nitrogen. Later, again, Laurent in 1889, Pagnioul in 1890, 

 and Muntz in 1896, showed that some plants can use 

 ammonium compounds as advantageously as nitrates. 

 While, therefore, both classes of compounds are useful, 

 it is interesting to find that some plants prefer the one 

 and others the other form of combination. Lawes and 

 Gilbert, in 1862, divided the plants of pastures into nitrate 

 lovers and ammonia lovers, showing clearly that the matter 

 is not so one sided as Boussingault thought. 



Muntz showed that the appropriation of ammonia is more 

 vigorous in light than in darkness. That the chlorophyll 

 is not, however, a necessary factor in the process was 

 proved by Laurent, Marchal, and Carpiaux, who ascer- 

 tained in 1896 that the white leaves of mottled species of 

 various plants appropriate it more easily than the green 

 ones. In both cases they found light to be favourable, 

 but determined the effective rays to be the ultra violet. 



