213 
The histology of the tubercle may be briefly considered as 
follows:—The tubercle consists of a collection of abnormally 
large parenchyma cells whose protoplasm and nuclei are much 
modified: The nuclei are much enlarged, chromatine prominent, 
Rhizobia imbedded in the cell protoplasm, all division very active, 
thus producing new and infected cells. Beginning on the outside 
in examining a section of tubercle, the following layers may be 
described. Epidermal layer, no root cap; non-infected area one, 
two or more layers thick; immediately below that is the starch- 
bearing zone entirely surrounding the infected area, and continu- 
ous with the starch-bearing layer of the vascular bundle. The 
remainder of the tubercle is the infected area filled with Rdzzobia 
and some starch granules. No starch is present in cells entirely 
filled with the RAizobia. The plant no doubt requires the proto- 
plasm of the R/zzodia from which to build starch. Between the 
infected and non-infected area, and including the starch-bearing 
zone, the cells are partially filled with R/Adzobia, (see fig. 1, plate 
CXXIX). 
Tubercles had been noticed by some of the older botanists. 
Malphighi took them for animal galls; Treviranus (Bot. Zeit., 
1853) for adventitious buds; DeCandolle for pathological growths; 
Clos for lenticels. Their real nature was not suspected. 
SYSTEM OF “RHIZOBIA.” 
SCHIZOMYCETES. 
MYCODOMATI~. 
Rhizobia developing and living in root tubercles of variable 
size and form. Tubercles always on main root or its branches, 
and decaying at the close of vegetation. 
Rhizobium mutabile, n. sp. : 
Syn.: Bakteria, Woronin. 
Bakterotden, Brunchorst. 
Spores or gemmules, Ward. 
Bacillus radicicola, Beyerinck in part. 
This species was first discovered and probably the most com- 
mon. It may be that this species, above all others, has the power 
to assimilate free nitrogen. Experiments will be necessary to 
determine that point. As is seen from the synonyms, some 
