BOT.— Vol. II.] PEIRCE— ROOT-TUBERCLES. 323 



already destructive, effect of the bacteria upon the nucleus 

 and other parts of the cells causes them to lose their power of 

 division. As has been previously pointed out (p. 311), be- 

 tween the tubercle meristem and the infected cells there 

 normally lie two or more layers of uninfected daughter- 

 cells. These divide also and grow. By this means the 

 meristem cells are kept from infection. When, however, 

 the growth of these daughter-cells is prevented, and the 

 rate of division of the meristem cells is reduced by such 

 mechanical pressure as a plaster of Paris investment of the 

 whole tubercle, the infection progresses to and into the 

 merismatic cells and they presently lose their merismatic 

 character and take on their definitive form. 



One result of the division of infected cells is the produc- 

 tion of new cells already infected and therefore not requiring 

 the entrance of infection threads. As shown by Hedlund 

 (1892) and myself (1899), the gonidia of lichens frequently 

 divide in such directions as to exclude the haustoria from 

 some of the daughter-cells. No such result can follow 

 when a cell contains a number of minute parasites distrib- 

 uted in its cytoplasm. The cell may divide under these 

 conditions, but the daughter-cells share the parasites as well 

 as the substance of the mother-cell. 



The figures above referred to, especially figs. 14 a, h, c, 

 d, and fig. 8, plainly show that the presence of tubercle 

 bacteria is not beneficial to the cells which contain the 

 bacteria. So far as the relations of the bacteria and their 

 host-cells are concerned, no one would hesitate to call the 

 association a clear case of parasitism of bacteria in the cells. 

 Whether it is a benefit to the leguminous plant to form 

 tubercles, to harbor bacteria in these, to have the cells of 

 the tubercles destroyed, and all the food supplied to the 

 tubercle consumed, is another question. The most careful 

 experimentation and the critical examination of the results 

 of experiment have so far led to the generally accepted 

 belief that the association of bacteria with the roots of 

 leguminous plants is beneficial to the leguminous plants. I 

 find it hard to understand how association with bacteria 



