Allen and Jolivette—Light Reactions of Pilobolus. 589 
reflected backwards inside the bulb. It may be, however, that 
the bulb itself is sensitive and transmits the stimulus to the 
motor zone below. In either case the perfect symmetry of the 
bulb probably facilitates exact aiming, for a one-sided bulb 
would be unequally illuminated on its different sides. 
Sensitiveness to light, as our experiments show, is maintained 
throughout the later stages of development ; and in spite of the 
changing turgor within the bulb and the sporangiophore imme¬ 
diately preliminary to the discharge of the sporange, even at 
this late stage an accurate heliotropic reaction can be carried 
out. The two sets of processes, leading respectively to the dis¬ 
charge of the sporange and to a change of aim on the part of 
the sporangiophore, proceed independently of each other, 
neither inhibiting the other. If Hoiks theory that curvature is 
due to an unsqual change in the elasticity of the walls of a 
cell, be extended to apply to the present case, it must be as¬ 
sumed that there is a very nice adjustment of the amount of 
change in elasticity as the turgor of the bulb increases. With 
one degree of turgor, a given increase in elasticity of the wall 
on one side of a sporangiophore would result in a stretching of 
the wall on that side and a consequent bending of the cell as 
a whole. With a greater degree of turgor, an equal change in 
elasticity would result in a greater stretching and so in a 
greater curvature. Yet. we find that reactions are essayed 
and successfully controlled during the period of the final turgor 
changes within bulb and sporangiaphore. jit would be inter¬ 
esting to know whether reactions are performed more rapidly 
wfien the turgor is greater, but this is a question which we 
have not yet investigated. 
Whatever the mechanism in Pilobolus for the perception of 
light may be, it is certainly efficient. TW example, in the 
white light, ninety-five per cent of the sporanges struck a four- 
centimeter opening when the culture was twenty centimeters 
distant from the light; and with one or two exceptions, the re¬ 
maining five per cent struck within one or two centimeters of 
the opening. It is plain that the aiming has been done with 
remarkable precision. 
