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1. A convenient method for obtaining monochromatic lights of equal intensity is the use of the thermopile and galvanometer 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 thermopile 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 orientation 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 Ångstrom 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'' is suggested for the phenomenon 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 rhizoidal cell. 5. This group orientation reported in other species is a conspicuous 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 0 2 mm. and often more, light stimuli as a rule fail to overcome it. 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 chemotropism of the rhizoids.

Effect of Unilateral Monochromatic Light and Group Orientation on the Polarity of Germinating Fucus Spores