62 Transactions British Mycological Soctety. 
When the sunlight strikes upon one side of the swelling, the 
light rays are refracted through it and converge so as to form 
a spot of light on the other side. When the incident rays of 
light strike the sporangium head on and are exactly parallel to 
the long axis of the swelling, the spot of light which is formed 
by the rays entering that part of the swelling which bulges out 
beyond and around the sporangium, is symmetrically placed at 
the base of the swelling. Under these conditions there is physio- 
logical equilibrium and no heliotropic reaction takes place. 
When, however, the incident rays of light strike a swelling 
obliquely, the spot of light is placed on one side of the wall of 
the swelling in a manner which is asymmetrical for the swelling 
as a whole. Under these conditions, the protoplasm which is 
lighted by the spot of light, sends a heliotropic stimulus down 
to the protoplasm at the top of the stipe just beneath the base 
of the swelling. The top of the stipe then reacts by growing 
in length most rapidly on the side nearest to the spot of light 
and thus bending as a whole. As a result of this reaction, the 
swelling is moved about its base through an angle and the spot 
of light gradually passes downwards on the wall of the swelling 
until it comes to be symmetrically placed at the base of the 
swelling. As soon as the spot of light reaches this symmetrical 
position, a physiological state of equilibrium becomes estab- 
lished in the sporangiophore and the heliotropic reaction ceases. 
At the end of the turning movement the gun is directed toward 
the source of the brightest light. 
In bright sunlight directed perpendicularly to the long axis 
of the swelling, the stipe of Pzlobolus longipes was observed to 
turn the swelling and the sporangium through an angle of go°, 
and thus complete its heliotropic reaction, in about one hour. 
In Pilobolus Kleinit the protoplasm in the lower part of the 
subsporangial swelling and at the top of the stipe contains a: 
red pigment; and, at the top of the stipe just above the stipe’s 
motor region, the protoplasm is often heaped up in such a way 
as to form a strongly bi-concave, very red, centrally perforated 
septum. As shown by direct observations and by theoretical 
diagrams, this protoplasmic septum is admirably shaped and 
situated for receiving the light rays converging upon it when 
the gun is in a position of complete or almost complete physio- 
logical equilibrium. Its concave upper surface, its position with 
respect to the subsporangial lens, and its strong pigmentation 
which is especially marked on its upper surface, suggest a com- 
omens in function with the retina of the eyes of certain Mol- 
usca. 
The diameters of the sporangium, the subsporangial swelling, 
and the motor region of the stipe below the swelling in a well- 
grown Pilobolus Kleinit were observed to be 0-43 mm., 0-76 mm., 
