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



It would seem, then, from the results just described, that 

 the tubercle meristem is preserved from loss of its meris- 

 matic qualities by escaping infection, and that the presence 

 of bacteria in the cells ultimately costs them their power 

 of division (see pp. 322-323). The meristem near the tip 

 of the tubercle is a survival, as is the vegetative point at 

 the root-tip, of the merismatic cells which constitute the 

 tubercle and the root in their embryonic condition. 



The result of imbedding an infected tissue, a root-tuber- 

 cle, in plaster, is different from that described by Newcombe 

 (1894), who worked on healthy plants. He says that the 

 external mechanical resistance causes developing cells to 

 attain their definitive condition more slowly, continues the 

 merismatic power and activity of the cambium cells, and 

 prolongs the life of such cells as ordinarily die early. 

 Newcombe worked on healthy plants, and his results exhibit 

 the effect of pressure only on growing cells and organs. In 

 my experiment the pressure which checked the growth of 

 the tubercle-cells may not have mechanically affected the 

 bacteria. Since the bacteria and the cells of the tubercle 

 are competitors, the plaster investments handicapped the 

 latter to such an extent that the ultimate results of bacterial 

 activity appeared earlier than in unconfined tubercles, the 

 bacteria gaining an advantage. To the action of the bac- 

 teria rather than of the plaster cast are due the early loss 

 of merismatic power and the early assumption of their 

 definitive condition by the cells of the tubercle. 



III. The Form and Distribution of Root-tubercles. 



The form and distribution of root-tubercles merit some 

 discussion, and since those of Bur Clover are typical I will 

 continue to describe them. The tubercles grow both in 

 length and in thickness at the ends, not at their bases, 

 and thus become club-shaped. They may and often do 

 branch. The growth takes place solely at the tip, because 

 the only meristem is there. The tubercle tissue is supplied 

 with food through vascular bundles which are neither large 



