THE CIRCULATION OF NITROGEN IN NATURE. 61 
So deeply rooted v/as this view that we find it persist even 
in the most advanced text-books of the present time. 
A very different complexion, however, was put on the 
matter by some experiments carried out by Professors 
Hellriegel and Willfarth at the experimental station at 
Dahme. Very contradictory results had been obtained in a 
series of experiments on the nutrition of plants belonging to 
the natural order Leguminosse and recourse was had to 
cultivation in sterile soil. It was found that growth was 
normal for some days, but that unless a proper supply of 
available nitrogen (in the shape of nitrate) was kept up, the 
plants did not as a rule arrive at maturity. In exceptional 
cases, however, the plants developed normal^, and on examin- 
ation were then found to have attached to the rootlets a 
number of small nodules, which were observed to contain 
bacterium-like bodies to which the name of rhizobes has 
been given. Such plants when analyzed contained much 
more nitrogen than could be accounted for by the materials 
supplied in the soil. On repeating the experiments with a 
number of leguminous plants, some of w^hich were dosed with 
an extract of these nodules, it was found that those plants 
only came to maturity which had been so treated, and which 
in consequence developed tubercles on the roots. Hence it 
was concluded that leguminous plants at least can, under the 
influence of the organisms which give rise to nodule forma- 
tion, absorb and assimilate the free nitrogen of the air. 
These experiments have been repeated and confirmed by 
numerous investigators, including Sir John Lawes and Sir 
Henry Gilbert at the Rothamstead experimental farm. 
Microscopic examination of the tubercles shows that they 
consist of tissue resembling and continuous with that of the 
root itself, but that the cells are permeated by a branched 
mycelium which at a later stage gives rise by budding to 
