Allen and Jolivette—Light Reactions of Pilobolus , 533 
A STUDY OF THE LIGHT REACTIONS OF PILOBOLUS. 
Ruth F. Allen and Hally D. M. Jolivette 
Introduction. 
The study of the reactions of plants to simultaneous stimuli, 
either of the same or of different kinds, affords perhaps one of 
the best means of approach to the fundamental questions of the 
physiology of stimulation of living organisms. The simpler 
organisms, devoid of specially differentiated parts for receiving 
and transmitting stimuli, are especially favorable for the study 
of such problems; and plants, because of their relative slowness 
of response, afford good opportunities for measuring and ana¬ 
lyzing the phases and elements of their reactions. 
As is well known, Pilobolus is strongly heliotropic and fires 
its sporanges toward an illuminated spot. By catching the 
sporanges on an interposed glass plate as they are discharged, 
we have a simple means of obtaining mathematical data as to 
the nature and accuracy of the reaction. Furthermore such 
difficulties of interpretation as are found in dealing with the re¬ 
actions of the complex and highly differentiated growing tis¬ 
sues of higher plants are eliminated here, for we are concerned 
with the reactions of practically a single cell. 
In the preliminary studies here described, we undertook to 
obtain data which might assist in answering the following ques¬ 
tions: (1) How accurately will Pilobolus fire its sporanges 
toward a source of light? (2) Hoes Pilobolus direct its spor¬ 
anges straight toward a source of light, or does it aim high ac¬ 
cording to the distance at which it is placed from the source of 
illumination? (3) In what direction will Pilobolus discharge 
