162 Frank W. Bancroft 



stimulation takes place at anode or cathode and dependent upon 

 a certain definite ratio of the calcium ions to those of sodium, 

 potassium, lithium, ammonium and other metal ions. The cur- 

 rent can vary this ratio, as described by Loeb ('05), since many 

 organic ions precipitate calcium. Lack of material prevented an 

 investigation of the effect of chemical substances on the galvano- 

 tropism of volvox. But it was found possible to change the pole 

 at which the galvanic current stimulates by means of pressure. 

 In all of my first experiments, which were made on the anodal 

 colonies the coverglass was rested on but one thickness of filter 

 paper so that the colonies were slightly pressed, and were thus pre- 

 vented from moving. In all of these colonies it was uniformly 

 seen that the cathodal flagella stopped contracting during the flow 

 of the current. As the plants swam to the anode it was difficult 

 to see how a stopping of the cathodal cilia could bring about this 

 result, and much time was lost before it was discovered that the 

 pressure just like the bright sunlight had changed the pole at which 

 the current stimulates. What the pressure and the sunlight have 

 in common is hard to see, unless it is that they both produce an 

 intense stimulation and possibly a slight injury. 



DISCUSSION OF RESULTS 



It has been shown that the galvanotropic orientation 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 bring- 

 ing 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 cause 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. 



