1893.] Bacteriology in its General Relations. 857 
this class of plants, and seem to be universally absent in all 
other genera.” 
For a long time they were regarded as pathological struc- 
tures, and it was only when it was found that legumes grown 
in sterilized soil were poorly nourished and entirely devoid of 
tubercles, that the causal relation of these structures with the 
peculiar property of the plant was recognized. These observa- 
tions were followed by more systematic experiments by Hell- 
riegel and Wilfarth, who discovered that the production of the 
tubercle depended upon the germ contents of the soil. Beyer- 
inck, Prasmowski and others succeeded in isolating the germ 
which possessed the ability to penetrate the young root hairs 
and the cortical parenchyma of succulent roots. These bacteria 
set up an irritation in the plant tissue that causes the forma- 
tion of a meristem, and thus the tubercles are produced as 
lateral outgrowths from the root. 
The tubercle is filled with peculiar shaped bacteria, which 
possess the ability in some way not yet perfectly understood, of 
fixing the nitrogen of the air in such a condition that it can 
be utilized by the plant. Our knowledge concerning the 
details of the process is far from complete, but this rational 
explanation of the phenomena has opened the way for further 
research. The problem that at present awaits solution is to 
find whether there is any difference between the organisms 
infecting one variety and those that are symbiotic in another. 
The artificial selection and cultivation of these forms that 
have the greatest ability as “ nitrogen collectors” is a question 
of prime importance. Like that of nitrification it involves 
great economic interests, inasmuch as it is the key to the solu- 
tion of the important question of the restoration of impov- 
erished soils. 
It would be treating our subject in a very imperfect manner 
were we to close our résumé of the influence that bacteriology 
? Recent experiments (Landw. Vers. Stat. XLI,138) seem to indicate that the tub- 
ercles on the roots of Z/eagnus angustifolia, a member of the Eleagnacez, a close 
relative to the Leguminosz are developed by bacteria. Infection by means of 
soil washings of sterilized soil planted with these seedlings gave more luxuriant growth 
than controls in sterilized soil. The organism has been isolated in pure culture and is 
found to differ from Bacillus radicicola, the form so common in the legumes. 
