IRRITABLE RESPONSE 219 



SECTION 3. THE PROCESSES OF ADJUSTMENT. 



Having considered the ways by which Plants maintain their 

 individual lives, and also by which they make increase, it remains 

 to study the third of the primal activities, — their adjustment to 

 their surroundings. Of this there are two phases which are 

 undoubtedly in some way connected, though by a bond at pres- 

 ent unknown, viz., first, adjustment of the individual through 

 definite responses to the forces of the environment acting as 

 stimuli, that is, Irritable Response, and, second, adjustment of 

 the race to its environment, fixed in heredity, that is, Adaptation. 



11. IRRITABLE RESPONSE. 



Already in this course the student has more than once come 

 close to contact with cases in which the plant responds adap- 

 tively to external conditions, and, indeed, the subject was defi- 

 nitely, though briefly, discussed when considering the influence 

 of external forces upon Protoplasm. We must now investigate 

 this subject more exactly, especially with reference to the nature 

 of the responses. The external forces have already been enu- 

 merated (page 65), and it will be well to begin with the one 

 in which it happens that the nature of the relation of stimulus 

 and response is most clearly exhibited; this is Gravitation, 

 the response to which introduces us to Geotropism. 



(a) Geotropism. 



The most familiar case of geotropism is doubtless the assump- 

 tion of the invariable up-and-down position of stems and roots 

 respectively as they grow from germinating seeds, regardless 

 of the position of the seeds, and our study may well begin by 

 exact observation of this phenomenon. 



What are the observable geotropic phenomena exhibited by the 



young parts of developing plants? 



Experiment. In a suitable moist-chamber, accessible to obser- 

 vation, fix a half dozen well-soaked large seeds, preferably of a kind 



