6g 
into the system of the plant? The extreme insolubility of Nitro- 
gen shows that the agency of water is not accountable for the ab- 
sorption of this element even when it is remembered' that in order 
to produce one pound of dry vegetation the enormous quantity of 
four hundred pounds of water is necessary. 2 
In order to satisfy the difficult question of the assimilation of 
Nitrogen by plants, many theories have at times been considered. 
Of these, digestion by means of the sap juice cannot be held to 
be the cause of the absorption of insoluble elements, for such a 
process would necessitate the 'breaking up' of Nitrogen into an 
assimilable form — a process at variance with the eminently con- 
structive function of sap. It is now, however, generally held 
that the appropriation of Nitrogen by vegetation is due to the 
agency of bacteria, which infest all fertile soils and exist upon most 
plants. By means of these minute organisms, the insoluble 
Nitrogen is, as it were, decomposed or predigested, and rendered 
assimilable as plant food. 
In the sequence of vegetation growing upon soil undergoing 
the process of weathering from primitive rocks, until late years, 
the lichens were considered as appearing first. The action of 
these simple structures after many generations prepared the way 
for mosses, which again rendered conditions suitable for plants 
of a higher order. It is now known, however, that preceding 
the lichens are the minute bacteria to which reference has been 
made, whose function is to render the inorganic constituents of 
the soil capable of supporting plant food. The origin of the 
bacteria themselves is a more profound question, which has not 
been solved, but their presence to the agriculturist appears to be 
as necessary as the working of the yeast plant is to the brewer. 
This renders the distinction between a sterile and a fertile soil 
better understood, for of two soils almost similar in chemical 
and physical characteristics, one may be extremely fertile and the 
other non-productive. The question of the presence of suitable 
bacteria in a soil is therefore a very important one, and ranks 
equally with those affecting its chemical constituents and physical 
properties. 
The action of the beneficial organisms referred to, as regards 
their function of supplying plant nutrition is principally confined 
to the nitrogenous matter already in the soil, although these or- 
ganisms may possess to a small degree the power of tapping the 
air reservoir itself and obtaining a supply of Nitrogen direct 
therefrom. 
2 The same problems affect the absorption by the plant of the extremely 
insoluble elements, phosphorus and silicia. The latter element, inert 
alike to the action of water and all acids (except hydrofluoric) is first 
broken up' and rendered assimilable by the plant root and then recon- 
verted within the system of some plants where it exists as minute 
crystals. 
