594 Wisconsin Academy of Sciences, Arts, and Letters. 
when the angle between the two sources of light is 40° as it 
is when the angle is 15°. On this assumption, also, why 
should there not be a similar resultant reaction when the two 
stimuli are of unequal intensity? The lesser stimulus would 
be expected to produce a smaller effect, but why should it not 
exercise a proportional influence upon the reaction instead of 
being entirely neglected by the sporangiophore, as we have 
found is the case? It remains unexplained, too, on the as¬ 
sumption of a simultaneous influence of the two stimuli, why 
the sporanges which land in the median region usually lie at 
a lower level than do those which are aimed at one or the other 
of the two openings. 
The facts therefore suggest strongly that the sporangiophores 
which produced these particular sporanges were like the less 
sensitive Arena seedlings used by Xathansohn. They were 
prqbably weak and imperfect and lacked not only the turgor 
to give force to the discharge, but also the sensitiveness to per¬ 
ceive in two neighboring lights anything but one general source 
of illumination. 
What factors determine, when two lights are presented, 
which of the two lights shall prevail, is not always apparent. 
A slight difference in the relative intensity of the two lights 
may make a marked difference in the proportional numbers of 
sporanges found at the respective openings. This was well 
shown in the experiments in which the sporanges fired from 
the left half of the culture to the right opening and from the 
right half to the left opening were intercepted upon a vertical 
glass perpendicular to the culture surface. When the culture 
was ten centimeters from the openings, 14 per cent of the 
sporanges were caught upon this glass (Table XII). When 
the culture was 25 centimeters from the openings, 36% were 
caught upon it. The alteration in the relative intensity of the 
light received from the two sources was not great, but it made 
a difference in the result of 22 per cent of the total number of 
sporanges discharged. 
Again, the sporangiophores in a culture are not always per¬ 
pendicular to its surface when the experiment is set up. Con- 
