368 VEGETABLE PHYSIOLOGY 
Many changes take place in protoplasm which escape 
our observation, originating perhaps in the condition of 
the protoplasm itself, or being due to disturbances in the 
interior of the plant. The normal course of metabolism 
may undergo a marked change in consequence of varia- 
tion in the amount of some particular constituent of the 
food or of an alteration of the distribution or direction 
of the translocatory stream. Injury to the body of the 
plant may involve redistribution of energy or of material 
within its interior, which may have far-reaching effects 
upon the course of the vital processes. Variations in the 
supply of food, which may range between absolute starva- 
tion and over-engorgement, may produce very great changes 
not only in the outer life of the plant, but in the substances 
it produces in its metabolism and the energy which it 
liberates. The lack of oxygen may provoke an almost 
entirely new metabolism in connection with the produc- 
tion of such energy. These internal changes have been 
already discussed, and the effect of various factors at work 
in the organism have been examined, so that it is not 
necessary in the present connection to do more than 
emphasise the fact that we have in such matters evidence 
of stimulation and the. response it provokes -— evidence 
which points to the sensitiveness or irritability of proto- 
plasm, as much as do the results of those changes in the 
environment which are purely external. The internal 
stimuli just noticed are largely chemical in character, and 
though chemical changes in the protoplasm are continu- 
ously occurring, many of them are directly instigated by 
such stimuli. Whether the ‘automatic changes in organs 
and cells which we have already studied are due to stimu- 
lation is perhaps a little doubtful, but at any rate the 
nature of any stimulus provoking them has so far eluded 
investigation, and to all appearance they are not initiated 
in that way, but are independent of all stimulation. 
Stimulation which is directly due to the physical con- 
ditions of the environment may be lonked upon as the 
