THE BACTEROIDS. 351 



already remarked, the bacteroids which, when they exhibit the 

 aforesaid vesicles, are frequently termed vesicular bacteroids are 

 finally dissolved by the surrounding plasma, which is thus en- 

 riched with albuminoids and is then diffused through the plant. 

 The cell contents of the (formerly reddish but now greenish-grey) 

 bacteroidal tissue gradually vanish and are dispersed into other 

 parts of the plant, and the nodule consequently shrivels up. The 

 commencement of this process of evacuation of the cells is indi- 

 cated by the appearance of a central vacuole of gradually increasing 

 size (Fig. 62). 



Neither the transition of the nodule bacteria into bacteroids, 

 nor the final dissolution of the latter, goes on simultaneously in all 

 parts of the individual nodules, which, moreover, are themselves of 

 different ages. Some of the bacteria escape the converting in* 

 fluence of the cell plasma by remaining within the protecting 

 mucinous capsule (membrane) of the infection threads. Hence, it 

 happens that in the autumn large numbers of the bacteria are still 

 present in an active condition within the nodules. In the subse- 

 quent putrefaction of the latter the organisms are set at liberty, 

 pass the winter in the soil, and then act again as nodule-formers in 

 the following spring. 



The bacteroid stage is not reached in every case, the plasma of 

 the nodule cells being unable in many instances to utilise the 

 microbes. In such event it swarms with bacteria alone, which 

 then act solely as parasites towards their host, and consequently 

 the latter derives little or no benefit from the formation of nodules. 

 This phenomenon is termed by Beyerinck "an overgrowth of 

 bacteria." According to the observations of NOBBE and HILTNER 

 (I.), it occurs when the inoculation is performed with bacteria that 

 have been grown on artificial media for a long time. 



196. Clostridium Pasteurianum. 



The fact recorded in 1 94, that absorption of nitrogen is not 

 effected by the superior (aerial) parts of ^ the Leguminosce, led us to 

 investigate the root nodules more closely. We then observed that 

 the possession of these appendages enables the plant to grow well 

 and ripen even in soils destitute of nitrogen, and we furthermore 

 learnt that the production of these nodules is directly connected 

 with the activity of certain special bacteria the nodule bacteria. 

 However important and satisfactory this result may be to agri- 

 cultural practice, it still leaves unsolved the (from a theoretical 

 standpoint) main question, " By what is the free nitrogen fixed?" 

 Is this effected by the nodule bacteria themselves, or do they 

 merely exert a stimulative action on the plasma they inhabit, 

 which is thereby empowered or spurred to unwonted activity ? 



The latter question cannot be answered by experimental means, 

 since for that purpose the chemical activity of the bacteria would 



