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BOTANY: A. M. HURD 
behind the filter screens. The illumination of the cultures was continued six 
to eight hours, this time having been found more than enough to cause the 
first cleavage plane to be permanently oriented regardless of subsequent 
illumination. 
In the experiments for which the naked arc was the source of light, the 
heating effect was so great that the spores were killed very quickly. The mer- 
cury vapor lamp was next used to obtain wave lengths of the blue end of the 
spectrum and a 1000 watt nitrogen filled Tungsten globe for the red. But 
as in the case of the electric arc, the Tungsten light killed the spores by the 
high temperature produced at the distances where it was necessary to place 
the cultures for a sufficiently intense illumination. With the mercury vapor 
lamp, however, positive results were secured. The wave lengths which were 
found to produce the orientation of the first cleavage plane such that all the 
first cross walls formed perpendicular to the direction of the incident rays, 
are those between 4000 and 5200 Angstrom units. Behind the two other 
filter screens used with the mercury vapor lamp and transmitting wave 
lengths of 5200 to 5900 Angstrom units, the spores germinated as if in darkness 
with the orientations of the first cleavage planes following no rule, and the 
rhizoids extending in all directions. However, the intensities of the lights 
behind these color screens were not equal when the mercury lamp was used 
because the shorter blue wave lengths predominated to so great a degree and 
hence produced greater intensities. 
With regard to the phototropism of the young rhizoids, it was found that 
very weak white light, too weak to orient the cleavage planes, would 
cause the growing tips to turn sharply away from the source of light. With 
the intensity of illumination behind all the color screens equal to 1800 meter 
candles, only the blue and violet lights produced the phototropism. The 
other wave lengths at this intensity had no effect, the young rhizoids con- 
tinuing in the direction in which they had started just as did those of the 
control in darkness. However when a more intense illumination was secured 
by placing the boxes in direct sunlight, the rhizoids behind the green filter, 
in addition to those behind the blue and violet ones, showed the same nega- 
tive phototropism. This and subsequent experiments would lead us to be- 
lieve that both quantity and quality, or intensity and wave length, are deter- 
mining factors in the power of light stimuH to produce phototropisms. 
In every culture of Fucus inflatus whether germinated in darkness or in 
strong unilateral light a most striking orientation of the first cross-wall with 
reference to adjacent spores appears. Wherever a group of spores are lying 
within about 0.2 mm. of each other, the first cleavage plane is perpendicular 
to the direction of the center of the group. The cell toward the interior in- 
variably becomes the rhizoidal cell. This phenomenon was reported by 
Rosen vinge^ in other species of Fucus and in Ascophyllum, For want of a 
better term I have called it group orientation. A study of the phenomenon 
was made to determine the strength of this stimulus, compared to that of 
