304 VEGETABLE PHYSIOLOGY 
contain principally starch. Fungi which are fed with car- 
bon-compounds that contain relatively little oxygen give 
off relatively less carbon dioxide than others which are 
supplied with food containing a large percentage of this 
constituent. Organs which contain much protein matter 
respire more copiously than others which contain but little. 
The nature of the inorganic salts absorbed also influences 
the process to a certain extent, though probably these only 
act indirectly. 
Respiration is thus to be looked upon as a process very 
largely connected with the utilisation of the store of energy 
which each cell possesses, and to be perhaps primarily 
concerned in the transformation of that energy from the 
potential to the kinetic form. The oxygen appears to be 
necessary mainly for the purpose of exciting those decom- 
positions of the protoplasm which are so dependent upon 
its instability. It is not, however, certain that this is the 
only part it plays. It is possible that some of the products 
of the protoplasmic disruption are oxidisable substances, 
and that to a certain extent a direct oxidation of them takes 
place. There is undoubtedly some evidence pointing in 
that direction. 
We have, besides the true respiratory processes, a second 
series of chemical decompositions going on in plants, pro- 
bably in many cases closely allied to those of the first, if 
not inseparable from them, but differing in that the auto- 
decomposition of protoplasm is not necessarily mvolved. 
We have seen already that many processes of oxidation and 
reduction are probably always taking place among the sub- 
stances which are in solution in the water with which the 
cytoplasm is saturated. Besides these, other changes take 
place in which no oxidation is involved, and this whether 
oxygen is present or not. If the access of oxygen to a 
protoplast is interfered with, its normal respiration soon 
ceases, but very frequently other changes supervene, involv- 
ing decompositions of a different character, which yield, at 
any. rate for a time, the energy required for life, 
