Allen and Jolivette—Light Reactions of Pilobolus. 593 
not be predicted. In a few cases in which Avena seedlings 
were used, no reaction occurred. On the other hand, with 
Brdssica , the most sensitive of the plants worked with, there 
was always a reaction in one direction or the other. 
Pilobolus also, as our experiments show, when subjected to 
two approximately equal beams of light, aims and discharges 
at one or the other of the two. In the majority of cases, the 
aim toward one source of light is as accurate as though the 
other were not in existence. Light rays from both sources 
reach the sensitive sporangiophore. Apparently there is noth¬ 
ing to prevent each set of light rays—or each individual light 
ray for that matter-—from setting up those changes in the proto¬ 
plasm which constitute the perception of a stimulus:, and 
nothing to prevent these simultaneous stimuli from acting to¬ 
gether to produce a resultant reaction. But this does not 
occur. The visible reaction of each sporangiophore is to one 
and one only of the two possible sources of stimulation. 
When the two openings which serve as sources of illumina¬ 
tion are close together, there are, to be sure, a small number 
of sporanges which land about midway between the openings. 
Attention has already been called to the fact that these particu¬ 
lar sporanges often fall considerably below the level of the rest 
of the sporanges on the glass. When the openings are so far 
apart that the angle between the two beams of light is more 
than 20°, the sporanges which fall markedly below their fel¬ 
lows are found, not under the middle of the field, but beneath 
one or the other of the two openings. 
It is possible that the sporanges which fall between and 
below the openings came from sporangiophores which per¬ 
ceived and reacted to both lights at once, thus aiming at a point 
between the two openings. But if this be the case, why should 
the resultant reaction to two simultaneous stimuli appear only 
when the openings are close together ? If two separate beams 
of light can be perceived at the same time by a single cell, and 
if each separate perception can produce its influence in the 
motor region, thus giving rise to a true resultant reaction, it 
is hard to see why the feat is not performed at least as often 
