CLOSTIUDIUM PASTEURIANUM 269 



on the basis of the factor 6.25 to an albumen content of 43.4 and 46.5 per cent. 

 As 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 enriched 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 indicated 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 influence 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 subsequent 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 194, 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 agricultural 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 need to be eliminated 

 and only their assumed stimulating action allowed to operate. However, the 

 wished-for decision may be expected from the experimental solution of the 

 previous question, and attempts may therefore be made to ascertain whether 

 the nodule bacteria are of themselves capable of fixing free nitrogen. On this 

 point BEYERINCK (XVII.) has conducted researches in nutrient solutions with 

 pure cultures of Bacillus radicicola, and in this manner ascertained that during 

 a period of two months an increase of 9-18 mgrms. of combined nitrogen 

 occurred per litre. Nevertheless he considers that this discovery leaves the 

 question still undecided, and consequently a repetition and extension of his 

 researches in this direction is highly desirable. 



The exercise of the still imperfectly proved capacity of Bacillus radicicola 

 for fixing free nitrogen is opposed by a considerable obstacle consisting in the 



