1905] PEIRCE & RANDOLPH—IRRITABILITY IN ALGAE 341 
to the holdfast is ordinarily determined by light, which in nature falls 
from above, vertically or more less obliquely. The direction of 
growth of the rhizoid is also determined by the light, other -things 
being equal, and this frequently brings it into a more or less vertical 
position until it comes into contact with a solid object. 
Although under natural conditions the cell which is to form a 
thizoid, and the direction of growth of the rhizoid, appear to be deter- 
mined by light and by the direction from which it comes, the fact that 
spores will divide and form rhizoids both in the dark and under 
supposedly equal illumination from all sides successively, leaves us 
with a doubt whether, after all, the point of formation of the rhizoid 
may not depend on other factors also. Keeping the spores con- 
tinuously in the dark for forty-eight hours or more is extremely 
unnatural. The results of such procedure may also be unnatural; 
they may mislead us as to the natural course of events. Again, 
cultivating the spores in a dish on a clinostat is an unnatural proceed- 
ing, for in nature the illumination is not even approximately equal 
on all sides in one plane, but is unequal and in many planes suc- 
cessively. And finally, though darkness follows light and light 
follows darkness only gradually, on the clinostat as well as in nature, 
itis obvious that a spore in a dish on the clinostat receives the stimulus 
of light on one side, and as darkness comes on, there can be only 
weaker and weaker stimuli, and finally none at all on the other sides 
of the spore. The direction of division of the spore may or may not 
be determined by this last stimulus of light. According to WINKLER 
(20, P- 302), a three or four-hour exposure to one-sided illumination 
necessary to determine the direction of the first division wall in the 
CBgs of Cystoseira. He suggests that perhaps by changing the 
direction of illumination 90° or 180° about every three hours, we 
might Stop the germination altogether. This I have not tried with 
— but as the foregoing shows, such is not the case with 
“tyopteris spores, for they germinate in darkness, and also on the 
oni t, at once, although perhaps not as rapidly as under ordinary 
ties arse It may take three or four hours under ordinary condi- 
oes 9 ed the line of division of an egg or spore. It vie 
which of time, much less of a stimulus by light, to determine 
€ two cells thus formed shall send out the rhizoid. In 
