48 BOTANICAL GAZETTE [july 





to obtain the relative intensity of the light transmitted by accurate 

 color screens, and the adjustment of the distances of these screens 

 from the light source such that the deflections of the indicator on 

 the galvanometer scale are equal for each exposure of the thermo- 

 pile screened by the light filters in turn. 



2. The effective wave lengths in the establishment of the 

 polarity of Fucus spores, the result of whose use for unilateral 

 illumination is the same as that produced by white light (the orien- 

 tation of the first cleavage plane perpendicular to the direction 

 of the incident light with the cell on the darker side of the spore 

 becoming the rhizoidal cell) are, with the intensity of strong 

 diffused daylight, the shorter rays of the blue end of the spectrum 

 of approximately 4000-5600 Angstrom units. There is some 

 evidence that ultra violet light can produce the same effect. 



3. The negative phototropism of the rhizoids in monochromatic 

 light is also primarily a function of the quality of the light, since, 

 with equal intensity of illumination, the wave lengths of the red 

 end of the spectrum are without effect, while those of 4000-5200 

 Angstrom units produce the same phototropism produced by white 



light. 



4. The term "group orientation' y is suggested for the phenom- 

 enon of the orientation of the first cleavage plane of a dividing spore 

 with reference to the position of adjacent spores, such that it is 

 perpendicular to the direction of the center of a group or of a single 

 spore within the effective radius, with the subsequent development 

 of the cell on the side toward the source of stimulus as the rhizoi- 

 dal cell. 



5. This group orientation reported in other species is a con- 

 spicuous phenomenon in every culture of Fucus inflatus, the stimulus 

 acting in such orientations being so strong that when spores are 

 separated by as much as o . 2 mm. and often more, light stimuli 



rule 



6. The chemical stimulus which orients the direction of the 

 first cleavage plane and determines which cell shall become the 

 rhizoidal cell in group orientations has no power to cause a chemo- 

 trooism of the rhizoids. 





