Allen and Jolivette—Light Reactions of Pilobolus. 595 
sequently, the light rays from the two openings do not hit a 
given sporangiophore at the same angle. We have not deter¬ 
mined for Pilobolus the angle at which the incident light rays 
produce maximum stimulation, hut it is very probable that the 
efficiency of a given light does vary with the angle at which 
it strikes the sporangiophore. This, too, may aid one set of 
light rays in gaining dominance. 
Besides these factors, there are doubtless conditions within 
the organism itself which we are at present unable to define, 
that may determine or aid in determining which light shall 
produce a visible effect. It is not impossible, for example, 
that different sides of the same sporangiophore may vary in 
sensitiveness, and that this difference in sensitiveness may der 
cide the course of the reaction. 
The experiments dealing with the response of Pilobolus in 
the presence of two differently colored lights involve much 
the same problems that we discussed in connection with the 
experiments in which two white lights were used simultan¬ 
eously. The data (Table XIY) showing that the presence 
of one light does not affect the aim of the sporangiophore to¬ 
ward a second light seem to corroborate the results obtained 
in the case in 'which the two white lights are used simultane¬ 
ously. For example, with the blue and white glasses placed 
at opposite ends of the box, there are found on the blue glass 
twenty-eight per cent more below than above the center of the 
opening when the culture is four centimeters distant, forty-nine 
and three-tenths per cent more at fourteen centimeters, and 
forty-five and six-tenths per cent more at twenty-six centi¬ 
meters. Tn the same way, the number below the opening on 
the white glass always exceeds that above. There is no ap¬ 
parent change of aim towards one light due to the presence of 
the second light, for this would result in a larger proportion 
of the sporanges striking above the center of one of both open¬ 
ings. The number of sporanges below the center of the open¬ 
ing always exceeded that above the center, just as was the case 
when a single light was used, and the proportional excess below , 
the center was about the same. 
