CHAP. VI. | TROPISMS. 53 



Chapter VI. 

 T ROPI SMS. 



"The word tropism means the tendenc) to react in a definite manner I 



external stimuli." ,,, „ _ , . 



Willev — Convergence m Evolution. 



IX the last chapter it was seen that the actions of insects are 

 onlj controlled by intelligent reasoning in very rare cases, if at all. 

 In practically all cases their actions under any given conditions are 

 rigidly guided by what are known as tropisms. Here it may be 

 useful to give a few definitions. A habit may be defined as the 

 behaviour of an organism under natural conditions, a reaction as 

 its behaviour in the laboratory or under more or less natural condi- 

 tions of observation in the open. A tropism is its reaction or res- 

 ponse to a definite external stimulus. For example, an organism 

 may be attracted by light, in which case it is said to be positive^ 

 phototropic (/.<.. turning towards light) ; or it may be repelled by 

 light, in which case it is said to be negatively phototropic ; or light 

 may not affect it at all. in which case it is said to be passively 

 phototropic. All these tropisms are to be observed in eveij 

 organism, either positively or negatively or passively, and often, at 

 different times, both positively and negatively in the same 

 individual. 



Tropic responses have been divided into the following classes: — 

 (i) Phototropism or response to stimulus of Light. 



(2) Heliotropism .. „ Sunlight. 



(3) Chemotropism ., .. Chemicals. 



(4) Thermotropism ., .. Temperature. 

 (3) Thigmotropism .. .. Contact with 



solids. 



(6) Geotropism ,, ,. Gravity. 



(7) Rheotropism ., ., Water currents. 



(8) 1 1\ drotropism ., ,, Water and 



moisture. 



(9) Anemotropism „ ., Air currents. 

 These are shown grapically in figure 27, but it may be remarked 



that the territories of the stimuli may sometimes overlap, and that 

 a different grouping of these tropisms may be expected as our 

 knowledge increases. 



Phototropism, sometimes known also as phototaxis, covers all 

 responses due to the stimulus of Light, but it is perhaps better to 

 restrict the term to cases in which direct sunlight is not employed 



