18 



Many other compounds appear to occur occasionally, since 

 they are formed by certain organisms, especially bacteria, 

 and more especially under anaerobic conditions. 



On this conception of respiration the customary 

 occurrence of aerobic respiration in a given organism 

 is no evidence that anaerobic respiration is impossi- 

 ble to it. The fact that certain roots do, under ordinary 

 conditions, respire aerobically does not prove that these 

 same roots might not respire anaerobically under other 

 conditions. Still less is it possible to extend to 

 roots in general, conclusions drawn from the respiratory 

 behavior of the roots of a single species or of a few 

 species. The essential matter would be the possibility 

 of avoiding the production of, or procuring the adequate 

 disposal of, intermediate respiration products which are 

 toxic. The abilities of roots in this connection are 

 probably widely varied. It is known that when the oxygen 

 supply is low, as in swamp soils, the customary excre- 

 tion of carbon dioxide by roots is replaced in part by 

 the excretion of other substances many of which are toxic 



1. The compounds include methane, carbon mono- 

 xide, hydrogen sulphide, amyl, butyl and other alcohols, 

 leucine, tyrosine and many amines, indol, skatol, sever- 

 al mercaptans, etc. For details see Lohnis,- landwirt- 

 schaftliche Bakteriologie, 1910; and Kossowicz,- Boden- 

 bakteriologie, 1912. On the bacteria which reduce com- 

 pounds of sulphur, nitrogen and iron see, e.g., Kossowicz,- 

 loc. cit . pp. 16-74 (1918). 



