THE ANALYSIS OF BEHAVIOUR 



135 



latter contracts, lifting the weight against the resistance of 

 gravity. Do this again and again, and the same effect follows. 

 By-and-by the muscle, or the nerve terminations in it, will 

 become fatigued, but until this happens an electric stimulus will 

 always elicit the same muscular 

 contraction or response. Here, 

 also, there is, perhaps, strict 

 determinism. 



Tropisms in 



When a seed 



Green Plants. — 



germinates, two 



structures grow out from it. One 



of these is the original root and 



the other is the original shoot. 



The former always grows down 



into the soil, and the latter grows 



upwards into the air. When a 



green plant is placed in front of 



a window, the leaves tend to ^^* 



turn towards the source of the light. The leaves of a tree 



tend always to place themselves so that their flat surfaces 



are perpendicular to the principal direction from which the 



light comes. All these effects are called " tropisms," or directed 



growth movements of fixed organisms. The tissues of the 



plant have the general power of growth, but the stimulus of 



light falling on them is a directive agency, which causes growth 



to take place in one direction rather than any others. Such 



directed growth movements in response to the stimulus of light 



are called heliotrofisms. They are invariable and subject to 



determinism, just as are the deviation of the compass needle 



or the contraction of the isolated frog's muscle when the nerve 



supplying it is stimulated. 



Taxis in the Lower Animals. — Chip off some barnacles 

 (Balanus) from the stones on the beach between tide-marks 

 during the last week of March or the first ones of April, and place 

 them in a soup-plate containing clean sea-water. In a short time 

 the embryos contained in the reproductive organs will hatch out 

 and swim about in the water. Examine the latter in a feeble, 

 diffused light, and it will be seen that the larvse swim about at 

 random and in no particular direction; but place a lamp close 

 to one side of the vessel, and it will be seen that they swim 



