224 METABOLISM 



purpurin, making it a substitute for chlorophyll. Further, the need for sun- 

 light in the assimilatory activity of the sulphur-bacteria is thus explained, 

 while according to Winogradsky's view, its significance is only indirect. It 

 is to be noted, however, that Engelmann's view is by no means established, 

 and that Winogradsky's criticisms (1888) still remain unanswered. It is to 

 be hoped that these interesting problems may soon receive a final investigation 

 and explanation. 



WiNOGRADSKY believed that another biological group of Bacteria should 

 be associated with the sulphur-bacteria, to which he gave the general name 

 of iron-bacteria. They are able to turn iron protoxide into iron oxide, and to 

 benefit by means of this oxidation just as the sulphur-bacteria do from the 

 oxidation of sulphuretted hydrogen. Unfortunately Winogradsky has not 

 followed up his short preliminary note by a detailed treatise, and meanwhile 

 MoLiscH has failed to confirm his results ; hence at present we can present 

 no definite data on the subject of iron-bacteria. [Compare also Rullmann, 

 1904.] For our present purpose, however, this is of little consequence, since 

 iron-bacteria play by no means so important a part in nature as do the 

 sulphur-bacteria. These latter work for the benefit of the higher plants, inas- 

 much as they enable the sulphuretted hydrogen formed in the process of 

 putrefaction or otherwise to become once more available for the nutrition of 

 the green plant. 



We saw at the conclusion of the preceding lecture, that in addition to sulphur, 

 another even more important element was similarly transformed in the process 

 of putrefaction into a condition in which it was of no service to the higher 

 plant. We found that free nitrogen and ammonia were among the final pro- 

 ducts of putrefaction, and that the nitrogen was never used by the green plant, 

 and the ammonia much less frequently than nitrate. We have yet to answer 

 the question whether these materials, like sulphuretted hydrogen, undergo 

 alteration by the activity of definite micro-organisms, so that they become 

 once more available as food, and we will first deal with the question as far as 

 regards ammonia. We may note, to begin with, that all the ammonia does 

 not arise from the fermentation of proteid, but that there are many other 

 important sources of this substance. 



Only rarely are katastates containing nitrogen formed in the plant which are 

 unable to undergo further elaboration (compare p. 200, the formation of ammonia 

 by Hyphomycetes) ; on the other hand, animals regularly give off nitrogen, 

 especially in urine, which contains it in abundance in the form of urea, uric acid, 

 and hippuric acid. It has long been known that these substances form un- 

 suitable sources of nitrogen for autotrophic plants, and it is all the more im- 

 portant to know that they may undergo alteration in the soil into nutrients 

 that are of value. In these processes micro-organisms also play a great part. 

 The best known is the alteration of urea into carbonate of ammonia, a process 

 which has often been termed * urea fermentation '. This takes place according 

 to the formula : — 



C0(NH3)a + 2H,0 = COsCNH^o 



the process being thus a simple case of hydration, such as accompanies the 

 action of many enzymes, but without any splitting. If we limit our conception 

 of fermentation to such processes as result in a gain of energy, then the forma- 

 tion of carbonate of ammonia must be excluded from fermentative actions. 

 If, however, we consider the hypothesis referred to at p. 213, we may speak here 

 also of fermentation, since in all probabihty the significance of the process 

 in the organism is biological only. We can at least suggest that the markedly 

 alkaline reaction acts in the same way as acids and alcohol do (i. e. as a poison) 

 in preventing the presence of other concurrent organisms, since it is a fact 



