ORIENTATION IN COLONIAL FORMS 145 



They become negative in high intensity and appear to be 

 more active in some intensities than in others, but since it 

 is practically impossible to subject the different parts of the 

 colonies to constant intensity owing to the shadow of one 

 part on another and the movement of the organism, it is 

 impossible to say whether or not they would become nega- 

 tive or more active if there were no such change of light 

 intensity. This subject was dealt with more in detail under 

 Euglena. 



Bancroft (1907, p. 163) intimates that orientation of 

 Volvox in light takes place in the same way as it does in a 

 constant electric current; that it is not due to changes of 

 intensity but to constant intensity; and that it is therefore 

 a tropic reaction in accord with Loeb's definition. He says 

 (p. 162): ** It has been shown that the galvanotropic orien- 

 tation of Volvox is brought about by a cessation or great 

 diminution in the stroke of the flagella at one pole of the 

 organism. This diminution in activity of the flagella 

 appears to be the only way in which Volvox is capable of 

 responding to stimuli. Nothing in the nature of a motor 

 reflex has ever been observed in this organism so far as I 

 know. The flagella always strike most strongly backward. 

 We have then the simplest possible kind of a mechanism 

 for bringing about galvanotropic orientation. The current 

 diminishes the activity of the flagella at one pole of the 

 colony and consequently the activity of the flagella at the 

 other pole causes the organism to turn in that direction. 

 We have here a tropism reduced to its lowest terms. There 

 is nothing of the nature of trial and error present at all." 



We have clearly demonstrated above that orientation of 

 Volvox in light is not due to a " cessation or great diminu- 

 tion of the stroke of the flagella at one pole of the organ- 

 ism " as it is in a constant electric current. It is due to 

 an acceleration in the backward stroke of the flagella on 

 the shaded side. This is caused by a response to a decrease 

 in the light intensity on the zooids as a whole together with 

 a decrease in intensity on certain structures within the 



