376 VEGETABLE PHYSIOLOGY 
to observe and perhaps measure by even the most delicate 
instruments at our disposal. 
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 under- 
go a marked change in consequence of variation in the 
amount of some particular constituent of the food, or of an 
alteration of the distribution or direction of the transloca- 
tory 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 starvation and over-engorge- 
ment, 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 production 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 connec- 
tion to do more than emphasize 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 protoplasm, 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 continuously occurring, many of them are directly in- 
stigated by such stimuli. Whether the automatic changes in 
organs and cells which we have already studied are due to 
stimulation is perhaps a little doubtful; but at any rate the 
nature of any stimulus provoking them has so far eluded 
investigation, and to all appearances they are not initiated 
in that way, but are independent of all stimulation. 
Stimulation which is directly due to the physical condi- 
