and
promoting the various chemical vital processes, is not a direct source
of the plant's oxygen. The important function played by atmospheric
oxygen in certain stages of the plant's growth has been long recognised.
Malpighi, nearly two hundred years ago, observed that for the process of
germination atmospheric air was necessary; and shortly after the
discovery of the composition of the air was made, oxygen was identified
as the important gas in promoting this process. Oxygen is also
especially necessary during the period of ripening.

_Source of Plants' Hydrogen._

Hydrogen, which amounts to about 6 per cent, is, as has already been
pointed out, chiefly derived from water. It is possible that ammonia
also may form a source.

_Source of Plants' Nitrogen._

When we come to treat of the source of the nitrogen, which is found in
the plant's substance to an extent varying from a fraction of a per cent
to about 4 per cent, we enter on a much more debated question.

What is the source, or, what are the sources, of plant-nitrogen? is a
question to the solution of which more time and more research have been
devoted than to the solution of any other question connected with
agricultural chemistry.

The most obvious source is the free nitrogen, which forms four-fifths of
the atmospheric air. Reference has already been made to this
question.[24] Priestley was the first of the long list of experimenters
on this interesting question.

As far back as 1771 he affirmed that certain plants had the power of
absorbing free nitrogen; and this opinion he supported by the results of
certain experiments he had made on the subject. Eight years
later,--viz., in 1779--Ingenhousz further supported this conclusion, and
stated that all plants could absorb, within the space of a few hours,
noticeable quantities of nitrogen gas. The first to oppose this theory
was de Saussure, who, in 1804, carried out experiments which showed that
plants were unable to utilise free nitrogen.

Subsequent experiments, carried out by Woodhouse and Senebier, supported
de Saussure's conclusions. Mention has already been made of
Boussingault's elaborate researches on the subject.[25] His first
experiments were carried out in 1838. He concluded that plants did not
absorb free nitrogen. Georges Ville was the first to reassert the older
theory, put forward by Priestley and Ingenhousz. His opinion was founded
on experiments he had carried out during the years 184