Root-nodules of the Leguminosae . 191 
largely accepted as modified lateral roots, and the organism responsible for 
their formation has been the main subject of research. It has been variously 
described as a fungus by Frank in 1879, Marshall Ward in 1887, and others, 
also as simply proteid matter by Brunchorst, but in 1888 Beijerinck called 
the organism Bacillus radicicola , and showed it to be a true Schizophyte. 
The formation of infection threads led many to regard the organism as 
a fungus, but in 1900 Dawson demonstrated that these are not fungal hyphae 
but masses of little rod -like bacteria, which have divided very rapidly and 
remained associated together, forming zoogleal threads in which they pass 
from cell to cell (PI. XIII, Fig. 7). 
It had long been thought that the nodules on leguminous roots were 
connected with nitrogen assimilation ; this was definitely established to be so 
by Hellriegel and Wilfarth in 1888. A few years later Nobbe and Hiltner 
and also Maze demonstrated that it was the bacteria in the nodule which 
were capable of utilizing the atmospheric nitrogen, and much has since 
been done in Germany, America, and also in this country in an endeavour 
to make a wide practical application of this power which the bacteria 
possess. That the bacteria have a source of nitrogen supply apart from 
that of the host plant is supported by the statement made by Moore, that 
the percentage of nitrogen present in the nodule is greater than in any other 
part of the plant. 
The organism is polymorphic, and different forms occur, not only in 
connexion with different plants but also in different cells of the same nodule. 
Nobbe thought each tribe of the Papilionaceae had its own specific organism. 
This, however, cannot be, because although the organism adapts itself to 
a particular host and will therefore more readily infect another host of the 
same kind, in 1901 the American Board of Agriculture found it possible to 
cross-inoculate any legume from any other except Lupin, and Bottomley 
has since cross-inoculated Tares with the organisms from Acacia , Alnus , 
and Elaeagnus , i. e. the two sub-orders of the Leguminosae, Papilionaceae, 
and Mimosoideae with one another, and a legume with the two non-legumes 
Alnus and Elaeagnus . Beijerinck in 1888 said there were two kinds of 
Bacillus radicicola , distinguished by the medium in which they flourished, 
which was either acid or alkaline. Most legumes do not incline towards an 
acid soil, but Lupin and the particular race of Bacillus radicicola which 
inhabits the nodules on its roots do, and since in an alkaline soil, or a plant 
which normally lived in one, the environment would be so very different, it 
is probable that the adaptation required is too great to be easily obtained. 
This possibly has a further bearing on the structure of the nodules produced 
by plants living under very different conditions. 
The Natural Order Leguminosae is the second largest family of 
Phanerogams, having about 440 genera and 7,000 species, living in every 
kind of soil and climate and showing a variety in habit which includes trees, 
