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- 



