Allen and Jolivette—Light Reactions of Pilobolus. 591 
ranges falling upon the opening decreases with an increase in 
the distance of the culture from the source of light (as shown 
by Table I), because of the effect produced by gravity upon 
the path of the dying sporange. 
The arrangement of the sporangiophores in the culture is 
also to be taken into account, since, as we have pointed out 
and as Figures 8 and 9 plainly show, this factor may be a 
source of irregularity in the final distribution of the sporanges 
upon the glass plate-—since it of course affects the position of 
the individual sporangiophore relatively to the source of il¬ 
lumination. 
It was found that the proportion of sporanges which reached 
the opening varied somewhat with the size of the latter, the 
proportion being greater with the larger opening. Although 
no extensive series of experiments was performed with refer¬ 
ence to this point, the results obtained were sufficiently decisive 
to show that the size of the opening is a factor which affects 
the accuracy of aim of the sporangiophores. 
There are doubtless other factors which influence the final 
distribution of the sporanges, but upon which our experiments 
have as yet thrown no particular light. Tight intensity is cer¬ 
tainly one such factor, and another, to be discussed below, is 
the wave length of the light which serves as a stimulus. But 
when all of the factors are taken into consideration which may 
influence the path taken by the sporanges and their ultimate 
position, it becomes evident from our observations as well as 
from those of previous investigators that the sporangiophores 
of Pilobolus aim with remarkable accuracy toward a source 
of white light. 
Toward a blue light the sporangiophore is aimed, on the 
whole, just about as accurately as toward a white light. In 
some cases, indeed, the reaction is even more accurate; for 
example, when the culture was twenty centimeters from a 
four-centimeter opening, ninety-seven per cent of the sporanges 
fell upon the opening. 
Toward a yellow light, on the other hand, we find that the 
aim is much less accurate than toward either the blue or the 
