Importance of ‘ Nitragin ! 513 
referred to in my earlier papers 1 on the subject of the nodules 
of Leguminous plants, but further details were reserved in 
order to secure the advantage of a comparison of the experi- 
ments of three consecutive years, before any definite opinion 
was offered upon so complicated a subject. 
The experiments referred to in this paper were of two 
distinct types : — 
1. The plants were grown in media previously sterilized 
by heat [approximately 200° C. for twenty-four hours], and 
throughout the experiment every precaution was taken to 
prevent any chance infection of the roots. 
2. The plants were grown in the open air on unsterilized 
media. 
In every case the same species was chosen for investigation, 
viz. Pisum sativum . 
In the experiments on sterilized media the plants were 
grown in large pots containing respectively ordinary garden 
soil, a gravelly subsoil, pure silver-sand devoid of nitro- 
genous compounds, and a similar sand supplied with potassium 
nitrate. In each case one half of the pots were sown with 
seeds, previously inoculated with c Nitragin/ whilst the other 
half contained control plants without inoculation. Before 
sowing, the seeds themselves were sterilized by immersion 
in a one per cent, solution of mercuric chloride for fifteen 
minutes 2 . 
Experiments of this kind were carried out in three con- 
secutive years, and in each year the crops were allowed to 
grow for about three months, that is until the pods were 
ripened, and throughout the time were watered with boiled 
distilled water only. At the end of this time the plants were 
carefully removed from the soil, and the roots thoroughly 
washed. A record was then made of the number of plants 
bearing nodules, the number of nodules formed and the dry 
3 Phil. Trans. Roy. Soc., 1899 and 1900. 
2 The use of this re-agent for cleansing the seeds was justified by the results of 
a preliminary experiment in which I found that 15 minutes’ treatment with a one 
per cent, solution of mercuric chloride was without injurious effect on pea seeds, 
though a longer application killed the embryo. 
