BOT.— Vol. II.] PEIRCE— ROOT-TUBERCLES. 317 



growing-point of the root. The cells toward the center 

 differentiate somewhat diversely according as they become 

 infected by bacteria or continue free from them. The un- 

 infected cells remain comparatively small, and present the 

 characters of ordinary parenchyma cells, the protoplasm 

 becoming vacuoled and containing numerous starch grains. 

 There may be several vacuoles in these cells or one tra- 

 versed by strands of cytoplasm. 



The infected cells grow larger and in their definitive con- 

 dition are from half as large again to twice as large as the 

 uninfected cells. This increase in size may be attributed 

 to one of three causes: First, to the stimulation of the 

 protoplasm by the bacteria and the substances produced 

 by them in the cells; second, to the actual irritation (in- 

 flammation) of the protoplasm; and third, to the increased 

 pressure set up in the cell by the rapidly multiplying bacteria. 

 By the plaster of Paris method we can test the relative 

 value of two of these influences; the third must be de- 

 termined by ascertaining microscopically the actual con- 

 dition of the protoplasm of infected cells. On imbedding 

 the young tubercles of growing roots, as above described, 

 the pressure normally or abnormally developed in the cells 

 will be resisted by the plaster, the cells expanding against 

 the plaster will be subjected to compression. Nine days 

 after the tubercles were enclosed in plaster, I opened the 

 casts and sectioned the tubercles by hand. There were 

 many more starch-grains in the uninfected cells than in 

 ordinary tubercles; I saw no infection threads (I did not 

 fix and use the triple stain previously described and hence 

 infection threads might have been present which escaped 

 my notice), the bacteria and bacteroids were smaller, and 

 the general appearance suggested that the leguminous cells 

 were better able to bear confinement than the bacteria 

 were. Increase in size both of the tubercle and of its com- 

 ponent cells being prevented by the plaster investment, the 

 bacteria have no increasing space in which to grow, and 

 continuing to multiply, for a time at least, they remain 



