52 PLANT-ANIMALS [CH. 



higher animals, are named choice and volition are 

 illustrated in our plant-animals by the simplest of 

 working models. If this view is accepted, it would 

 seem to follow that the intricacy and ni} 7 stery of 

 complex habits and instincts are begotten of the 

 ever-increasing complexity of conditioning or ac- 

 cessory stimuli which have first attached them- 

 selves to and then replaced the original series of 

 stimuli. 



Some further facts with respect to the photo- 

 tropic responses of C. roscoffensis are worthy of a 

 passing word. 



In the first place, just-hatched animals, though 

 they respond to the directive stimulus of gravity, do 

 not respond to that of light. After a few hours of 

 free existence, they acquire the faculty of responding 

 tropistically to unilateral light, which henceforth be- 

 comes a masterful lactor in determining their habits. 



In the second place, the rays of light to which 

 C. roscoffensis responds tropistically are not those 

 which induce phototropic curvatures in plants. As 

 is well known, a plant exposed, unilaterally, to rays 

 of the less refrangible part of the spectrum the red 

 for example shows no phototropism ; whereas a plant 

 subjected on one side to blue-violet light reacts as 

 readily as to white light. Convoluta roscoffensis, on 

 the other hand, responds not to blue or violet light, 

 but to green light. The diagram (Fig. 12) represents 



