CHAP. XII. 
DIGESTION. 
379 
same day and night. That if fresh Mushrooms are placed in 
an atmosphere of pure oxygen, a great part of that gas dis- 
appears at the end of a few hours. One portion of the oxygen 
which is absorbed combines with the carbon of the plant to 
form carbonic acid ; whilst another part appears to be fixed 
in the vegetable, and to be replaced, at least in part, by 
nitrogen disengaged by the Mushroom. That when fresh 
Mushrooms remain some hours in an atmosphere of nitrogen, 
they modify very slightly the nature of that gas. The sole 
effect produced is confined to the disengagement of a small 
quantity of carbonic acid, and sometimes to the absorption 
of a very small quantity of nitrogen.” 
But, although the experiments of phytochemists have led 
to these general conclusions, it is not at all probable that the 
respiratory functions of plants are limited to the decompo- 
sition and recomposition of carbonic acid. It has already 
(page 360.) been stated that Messrs. Edwards and Colin 
have proved that water is decomposed in the act of germi- 
nation, the hydrogen being fixed and the oxygen set free ; and 
there can be little doubt that this phenomenon occurs in 
plants during other periods, perhaps all periods, of their 
vigorous growth. 
Theodore de Saussure found that germinating seeds absorb 
nitrogen. It has been shown by M. Boussingault, that plants 
abstract from the air a quantity of this gas, which they fix in 
their tissue. But under what circumstances, or in what state, 
this element is fixed in plants is unknown at present. Ni- 
trogen may enter directly into plants if their green parts are 
fit to fix it; it may pass into plants with the aerated water 
absorbed by the roots ; and it may be possible, says M. 
Boussingault, that, as some suppose, there exist in the atmos- 
phere very small quantities of ammoniacal vapour. M. Payen 
has also ascertained that this gas exists in abundance in plants. 
He finds it most plentiful in nascent organs, in those in 
the act of first developement, and in cambium ; but he meets 
with it in wood generally. If a large quantity of water is 
passed through a stick of elder wood recently cut, the wood 
loses all its azotised matter, which is carried off by the water : 
this and some other observations satisfy him that wood gene- 
