Importance of 1 Nitragin ! 5 1 7 
tion to the conversion of the ‘bacteria 5 into bacteroids ; 
unchanged c bacteria 5 may be harmful rather than useful to 
the plant. They also state that if the plants are strengthened 
by the addition of small quantities of nitrogenous food to 
the soil, then nodules produced by artificially cultivated 
organisms are quite normal in their behaviour as nitrogen 
collectors. My own work has led me to an entirely opposite 
conclusion, viz. : that the more favourable the supply of 
nitrogenous food within the culture-media, the less likely 
is inoculation with ‘Nitragin 5 to have any beneficial effect 
upon the crops. 
In a general discussion of the kind of experiments referred 
to in this paper, it might justly be argued against the use 
of sterilized media that the conditions are here so altered 
that the results obtained cannot be regarded as comparable 
with those obtained under natural conditions. 
On the other hand, the value of, and need for, experi- 
ments of this kind must be realized when we consider the 
unusual complexity, of the conditions existing in any ordi- 
nary soil. In media, thoroughly sterilized by heat, these 
conditions are to a large extent under control, and the dis- 
turbing action of soil-Bacteria removed, though even here, 
as wq have seen, the difficulty of guarding against infection 
by air-borne germs is immense. In ordinary soils in the 
open-air, the conditions regulating the supply of nourishment 
and in particular of nitrogenous food to the plants are in 
the highest degree complicated, for along with the action 
of the nodule-organisms is involved the action of the various 
nitrifying and denitrifying Bacteria at work within the soil, 
besides frequent variations in the environment due to changes 
in the atmospheric conditions. 
In this connexion reference should be made to Richter's 1 
investigations upon the effect of sterilization by heat, which 
causes considerable changes In the soil, in regard to its function 
as the main food-supply of plants — in particular, that the in- 
1 Richter, 1 Uber die Veranderungen welche der Boden durch das Sterilisieren 
erleidet.’ Landwirthsch. Versuchsst., 1896. 
M m 3 
