BOTANY: A. M. HURD 
205 
light, in its power to establish the orientation of the plant. It was at once 
very evident that for most spores the former stimulation is stronger when the 
spores are within a short distance of each other — 0.2 mm. or often more — 
but beyond this distance, the chemical stimulus becomes too weak and only 
the light is able to determine the polarity of the plant. Only the compara- 
tively isolated spores therefore show the orientation to light with the sources 
of illumination used here. 
The phenomenon is very conspicuous in groups of 2, 3, or 4 eggs as well as 
in masses of 50 or 100. In these large groups it is made evident by the in- 
variable rule that no rhizoid ever extends outward from a group. When two 
spores are within the distance through which the stimulus is effective, the first 
cleavage planes of the two are parallel and the rhizoids grow towards each 
other and often meet tip to tip. The groups of 5 or 6 often make symmetrical 
star-like designs when the rhizoids have grown and project beyond the group. 
The spores are more rarely affected in this way when the distance between 
them is over 0.3 mm. but the phenomenon is sometimes observed in spores 
as much as 0.5 mm. apart. Within a distance of 0.2 mm. there are practically 
no exceptions. 
The relative sensitiveness of a spore towards light and towards this chemical 
(?) stimulus varies greatly for different spores. When cultures were placed in 
the window to get as strong a light stimulus as possible in order to determine 
at what distance from each other the eggs had to be not to show a greater 
sensitiveness towards the chemical stimulus than towards the light, it was 
found that this distance followed no rule, the spores showing the greatest 
individual differences. Of two spores lying within 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 towards each other with both rhizoidal cells showing 
a tendency to take a direction away from the light at the same resultant angle. 
Rosenvinge ascribes this group orientation to a difference in the concen- 
tration of oxygen or of nutritive substances on the two sides of the spore. He 
thinks the rhizoid forms on the side toward the center of a group or towards 
another egg because the water on that side is less rich in the active substance 
than on the outer side as a result of their metabolism. Winkler^^ working 
with Cystoseira harhata found that a difference in oxygen concentration has 
no such effect. Apparently the phenomenon does not occur naturally in this 
species since his figure shows nothing but the effect of light. I have never 
seen a culture of Fiicus inflatus with spores germinating so near each other, 
which showed only hght orientation and not the group orientation. Almost 
invariably when the spores of this species germinate in such close proximity, 
light appears to have no power to estabhsh the polarity of the plant. 
The possibility that the group orientation is due to a polarity established 
by the position of the egg in the oogonium is suggested by finding many 
