RESPONSE TO CERTAIN STIMULI 37 



practically speaking, scarce, and must be sought for ; and, 

 lastly, this seeking for food involves dangers which must 

 be avoided, dangers which do not exist for the plant. 



That this latter feature, the activity of animals in 

 seeking food, necessitated those special organs, is borne 

 out by the fact that in such animals as lead a sedentary 

 Me, especially in those whose food supply is abundant, 

 there is almost a total absence of specialised sense- 

 organs. 



CHAPTER VII 



RESPONSE TO CERTAIN STIMULI IN PLANTS 

 AND ANIMALS 



LET us examine a little more closely the responses to 

 various stimuli, e.g. gravity, light, touch, chemical sub- 

 stances, and the electric current. 



Geotropism, or the response to the force of gravity, 

 seems to be more clearly manifested among plants than 

 among animals. Among the lower forms of plants and 

 animals the response seems to be either very vague or 

 undetermined, but among the higher forms of plant life 

 the response is a general tendency to maintain a line 

 of growth at right angles to the earth's surface, and this, 

 too, irrespective of the source of heat or light. 



Knight found that if germinating seeds were fastened 

 on to a wheel rapidly revolving in a vertical direction, 

 by means of which the force of gravity was overcome, 

 the direction of growth obeyed the laws of centrifugal 

 force, and the shoots grew towards the centre of the 

 wheel, and the roots away. If the wheel was stopped 



