324 CALIFORNIA ACADEMY OF SCIENCES. [Proc. 3D Ser. 



which destroy the cells in which they occur can benefit the 

 larger member of the association. The bacteria are known 

 to fix free nitrogen from the air. By this means they form 

 in the cells of their host-plants nitrogenous compounds which 

 the host may use. Apparently the bacteria form more than 

 enough of these valuable nitrogen compounds in the cells 

 of their hosts to compensate for the extra material used in 

 forming and maintaining tubercles the cells of which are 

 ultimately destroyed by the bacteria. This implies a mar- 

 velous balance of profit and loss, the more remarkable since 

 the profit apparently exceeds the loss. 



One point more needs to be made clear. Miss Dawson 

 (1899, P- ^4) ^^y^ ^^'^^ ^^ ^^ difficult to conceive how 

 such strictly aerobic bacteria as these can flourish in 

 the cells of such compact tissue as composes the tuber- 

 cle. This difficulty is of her own conceiving, for do not 

 the cells of the tubercles respire and are they not neces- 

 sarily supplied with oxygen for respiration? Again, inter- 

 cellular spaces in the infected tissue do occur, as figures 

 14 b, 14 c, 13, 12, 15, 8, and 9 plainly show. Even if 

 intercellular spaces did not occur, as asserted by Schneider 

 (1893, pp. 786 and 787), the existence of the living cells of 

 the tubercle tissue would prove the presence of sufficient 

 quantities of oxygen, if not of nitrogen, in the tissue and 

 therefore in the cells. Unless we are to imagine anaerobic 

 respiration for these cells, it is unnecessary to assume it for 

 the bacteria which infest them. Fischer (1897, note 63 to 

 p. 92), shows this clearly, and my sections reveal the pres- 

 ence of intercellular spaces through which a diffusion of 

 gases could take place even if the diosmosis of solutions of 

 the gases concerned were inadequate to supply the demand. 



Summary. 



I. Though the bacteria which form root-tubercles on 

 leguminous plants are usually only slowly motile, if 

 motile at all, in artificial cultures, this proves nothing 

 as to their motility in the soil. 



