1920] HURD—FUCUS SPORES 47 



As regards Fucus, I found the situation very interesting and very similar 

 to that which exists in some of the lower animals. The egg shows a gradient 

 as soon as polarity is determined, but as you suggested, it might be the region 

 of highest susceptibility, which I believe to be the region of highest oxidation 

 rate, is at first at the rhizoidal end. This makes it look as if the effect of light 

 might be a differential inhibition rather than a differential stimulation. After 

 five or six days, however, the susceptibility of the rhizoid decreases, and at the 

 same time a new region of high susceptibility begins to appear at what is to 

 be the apical end of the thallus, and this becomes and remains the most 

 susceptible region of the plant, and from it a gradient of decreasing suscepti- 

 bility extends basally to the base of the thallus. It looks as if the out- 

 growth of the rhizoid represented a rather brief period of high rate of oxidative 

 activity, which soon slows down, and then the apical region of high rate 

 arises, just as a bud, previously inhibited, arises or begins to develop when 

 the activity of the growing tip which inhibited it is decreased. Of course these 

 are at present merely suggestions by way of interpretation of the observed facts. 



As for group orientation, it and the effect of light may have a 

 common physiological basis. In both phenomena the controlling 

 force may be gradients of increasing oxidation rates, but with 



different factors responsible. 



may 



energy speeding or retarding me 



where the wave lengths acting are such that they can be absorbed 

 by the active substances of the cell ; or in the case of group orienta- 

 tion, it may be available oxygen or other nutritive substance 

 varying in amount on two sides of the spore as the result of the 

 metabolic processes of adjacent spores. 



We must conclude, however, that the attempts at partial 

 explanation of these experiments and observations are far from 

 satisfactory. The application of the oxidation gradient theory 

 can only account for the later aspects of the polarity phenomena, 

 the determination of the apical and basal ends of the germinating 



spore. 



determ 



orientation of the spindle of the dividing nucleus is visible evidence 

 of forces existing within the cell, and the control of those forces 



energy 



there is no hint, and the mechanics of which must remain obscure 



for the present. 



Summary 



A convenient method for obtaining monochromatic lights 



1. 



of equal intensity is the use of the thermopile and galvanometer 



