

34 BOTAMCAL GAZETTE [july 



with Equisetum, but with puzzling exceptions. Where the spores 

 were in groups, the cell toward the interior of the group became 

 the rhizoidal cell; and in the lower part of hanging drops the 

 rhizoids appeared on the upper side of the spore regardless of the 

 light direction. He concluded, therefore, that not only light but 

 a difference in the concentration of oxygen on the two sides of the 

 spores could determine their polarity. He says that as a result 

 of their respiration the water in the center of the groups of spores 

 is less rich in oxygen, with the result that the rhizoids are formed 

 on that side. In support of this theory is the fact that although 

 light can determine the polarity of all the species studied except 

 Fucus serratus, namely, Ascophyllum nodosum, Fucus vesciculosns, 

 F. spiralis, and Pelvetia canaliculata, their sensitivity to light differs, 

 and the oxygen factor or internal causes produce frequent exceptions 

 in all but Pelvetia. The rhizoids of the latter species are always 

 formed on the darker side of the spore, and this is the one species 

 in which the egg is surrounded by an oogonial wall which might 

 prevent any of the effects of varying oxygen concentration that can 

 act more potently than light on the spores of the other species. 

 Rosenvixge quotes Kny as finding that neither light, gravity, nor 

 contact can influence the point of origin of the pollen tube from 

 pollen grains, but that in the neighborhood of other grains the tube 

 will be sent out from the side away from them, on which side the 

 supply of oxygen or nutritive elements would be greater. 



Farmer and Williams (6) state that if Fucus spores are illumi- 

 nated on all sides they tend to remain spherical instead of pro- 

 ducing a rhizoid by the elongation of one of the two cells. Again 

 (7) they experimented with one-sided illumination, with the usual 

 result that most of the rhizoids originated on the shaded side 

 of the spore and the others were turned that way. The fact that 

 some grew out at an angle to the incident light was attributed to 

 "the character of the egg itself." 



Winkler (21) found the same orienting effect of light on the 

 spores of Cystoseira barbata, but failed to find any effect of a dif- 

 ference in the oxygen content of the water. He also said that 

 gravity and contact are not factors in the establishment of the 

 polarity of the sporelings. He found that it is determined during 





