iQ2o] i HURD—FUCUS SPORES 45 



cultures only the more isolated spores will be oriented by light, 

 the others all showing strongly the group orientation. The spores 

 show the greatest individual differences in their relative sensitive- 

 ness to light and to the group stimulus. Of two spores lying 

 within about 0.3 mm. of each other, one might be entirely oriented 

 by the adjacent spore, while the other, apparently like it, would 

 show only the action of the light stimulus. In many cases two 

 such spores would seem to show a resultant effect of the two stimuli, 

 so that both would be half turned toward each other, with both 

 rhizoidal cells showing a tendency to take a direction away from 

 the light at the same resultant angle (fig. 1). 



The substance or condition originating in the activity of 

 adjacent spores which has so powerful an effect in orienting the 

 first cleavage plane and in determining which cell shall become 

 the rhizoidal cell has no power to cause any chemotropism of the 

 rhizoids after they are started. No rhizoid has been found to have 

 its direction modified by the presence of other spores adjacent to 

 it. In the absence of any light stimulus the rhizoids continue in 

 the direction that they take originally from the spore. 



Discussion 



Child's (3, 4) metabolic gradient theory seems to offer the 

 most satisfactory explanation of the power of environmental 



germinating suores. He 



demonstrated in many marine plants and animals the existence of 

 "axial susceptibility gradients" which he considers due to a 

 decreasing rate in the metabolic processes from the apical to the 

 basal or posterior end. Such a gradient would be established by 

 differences in the rate at which oxidations and other reactions 

 proceed, and by this disturbance of the equilibrium of the physi- 

 ological mechanism would determine the basal and apical ends of 

 the organism. Child's explanation is as follows: 



Since extended experiment with the lower animals indicates that the 



cyanides 



metabolic 



activity or of certain fundamental metabolic processes, probably primarily 

 the oxidations, these axial differences in susceptibility in the algae are regarded 



a* inrKra tine* th& Avicfpnrp nf avinl mpfahnlir gradients In the final 



analysis such a gradient is not self-determined by some sort of organization, 



