436 • Transactions. — Botany. 



conducted with even greater rigour by Laurent and Schloesing, 

 show that this view is untenable. Frank is its only promiuent 

 advocate at the present day, and the experiments on which he 

 rehes are generally held to be inconclusive. Another view 

 was that the Leguminosae, with their abundant and deeply 

 penetrating root-system, could take up nitrogenous compounds 

 from the lower layers of the soil which other plants could not 

 reach. On this view we cannot understand why the presence 

 of numerous well-developed tubercles in the roots should be a 

 necessary condition of the abundant assimilation of nitrogen. 

 It is, moreover, directly refuted by the abundant fixation of 

 nitrogen by plants grown in shallow culture-vessels and in 

 soil devoid of nitrogenous compounds of any kind. 



Two other views have been entertained, and both are more 

 or less consistent with the results of experimental inquiry. 

 But before dealing with these let me describe somewhat more 

 fully the nature of the nodules and their contained organisms, 

 and of the relations of the latter to the living plant wdiich 

 they affect. 



In 1887 Professor Marshall Ward showed that a perfectly 

 definite organism from the soil penetrates the root-hairs, and 

 invades the cortical tissues of the roots. The organism lives 

 in a sort of helpful association (or in symbiosis as the 

 specialists have it) within the tissues of the root-tubercles, 

 and not merely as a hurtful parasite. Instead of injuring the 

 leguminous plant, the nodules and their living invaders in the 

 long run add to its health and vigour. "What follows the 

 infection is briefly as follows : The invading organism causes 

 the cells deep down in the cortical tissues (the tissues that 

 correspond to the bark in stems) to divide and form a 

 delicate tissue of well-nourished cells. As these enlarged cells 

 multiply the organisms keep pace with them, and send 

 brandies into each new cell. Eventually myriads of minute 

 bacterium-like bodies fill the cells. The cells are not, how- 

 ever, killed by the bacteroids ; on the contrary, they show all 

 the signs of intense physiological activity, accompanied by the 

 destruction of copious supplies of carbo-hydrates brought to 

 them from the leguminous plant. After a time this metabolic 

 activity abates, and the myriads of bacteroids begin to get 

 disorganized. Then the now victorious leguminous plant 

 absorbs the disintegrated contents of the interior of the 

 nodules, taking up everything that is capable of solution and 

 absorption, and leaving the nodule a nearly empty, limp, 

 collapsed shell, in which such bacteroids as have passed into 

 a sort of resting-stage alone remain alive, to be scattered in 

 the soil as the debris of the exhausted nodule rots away. 



It looks as if the invading organism acted at first as a 

 parasite, but though it lives at the expense of the cells which 



