Sporangial discharge in Pilobolus. A. H.R. Buller. 63 
and 0-16 mm. respectively. A simple calculation based on these 
data shows that the sporangium, when head on to a beam of 
parallel light rays, casts a shadow which has an area 7-2 times 
that of a cross-section of the stipe. If there were no subsporangial 
swelling, the shadow of the sporangium would cut off the light 
from the top of the stipe before the ortho-heliotropic position 
had been completely attained. This would prevent the gun 
from being accurately directed toward the source of light. 
Evidently the difficulty of supplying the stipe with its required 
delicate heliotropic stimulus has been surmounted in the course 
of evolution by the intercalation between the sporangium and 
stipe of the large light-collecting subsporangial swelling. That 
part of the swelling which bulges out beyond the black spor- 
angium receives the light and concentrates it by refraction upon 
the base of the swelling, the asymmetrical position of the spot 
of light so produced providing the stimulus to which the motor 
region of the stipe can react. 
A model for illustrating the Pilobolus gun in its relations with 
light can be made for demonstrations to an audience as follows. 
Take a Florence flask, fill it with water, stuff a smooth plug 
of cotton wool down the neck as far as the base of the neck 
and then close the mouth of the neck with a cork. To represent 
the opaque sporangium, stick a plano-convex mass of moulding 
clay covered with black tissue paper over the flat surface of the 
flask’s base so as just to cover it. As a source of light use direct 
sunlight or a beam from the arc of a projection lantern. 
(1) To imitate the condition of the Pilobolus gun when in 
a state of physiological equilibrium. Hold the flask in the beam 
of light with its long axis parallel to the direction of the incident 
rays and its black base facing the rays. It will now be found 
that the rays of light falling on that part of the flask’s bulb 
which bulges out beyond the black base are refracted so that 
they converge within the bulb and brilliantly illuminate the 
cotton wool plug at the junction of the flask’s bulb and neck. 
(2) To imitate the condition of the Pilobolus gun when not 
in a state of physiological equilibrium. Hold the flask in the 
beam of light with its long axis making a considerable angle 
with the direction of the incident rays and so that the rays fall 
obliquely upon the flask’s black base. It will now be found 
that the cotton wool plug is no longer brilliantly illuminated 
but that a spot of light is formed by the refracted light rays upon 
the side of the bulb. 
(3) To imitate the movement of the spot of light down the 
side of the subsporangial swelling of the Pilobolus gun when 
the stipe, responding to a heliotropic stimulus, is turning the 
subsporangial swelling and sporangium through an angle. Hold 
the flask as just described, so that the spot of light is upon the 
