STIMULATION AND ITS KESULTS 369 



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 looked upon as the 



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