ii6 RESEARCHES ON FUNGI 



sunlight to bend heliotropically through a right angle ; (2) to 

 determine the length of the latent period, i.e. the time which elapses 

 from the beginning of continuous heliotropic stimulation to the 

 beginning of heliotropic reaction as indicated by the curvature of 

 the sporangiophore ; and (3) to observe the spot of light formed 

 on the back wall of the subsporangial swelling move down the wall 

 of the swelling, as the stipe bends, and finally come to rest on the 

 perforate protoplasmic septum at the top of the stipe. 



The apparatus used in the experiment was a glass-ring chamber. 

 A glass ring 3 cm. in diameter was placed on a slide and covered with 

 a cover-glass in the manner shown in Fig. 56. To cut off undesirable 

 light, black paper was attached : to the outer and inner surfaces 

 of the glass ring, except for a window-slit 2 mm. wide, as shown at 

 A, B, and C ; to the upper and lateral sides of the slide, as shown 

 at B and C ; and to the upper surface of the cover-glass, except for 

 a peep-hole, as shown at B and C. 



One morning, a large well-developed fruit-body of Pilobolus 

 longipes which had been directed toward the source of strongest 

 light for some hours, together with a piece of horse dung on which 

 it was growing, was removed from a culture dish and was placed 

 in the experimental chamber in the manner shown in Fig. 56 ; and 

 two other pieces of horse dung were also placed in the chamber to 

 assist in keeping its air moist ; whereupon the ring was covered 

 with the special cover-glass which had been prepared for it. The 

 chamber was then set on the stage of a microscope and was attached 

 there by clips. The microscope was turned on its axis and its stage 

 tilted until the upper surface of the stage and of the glass slide were 

 parallel to the direct rays of the morning sun and a beam of sunlight 

 entered the window-slit of the chamber and impinged on the sub- 

 sporangial swelling, as indicated by the arrow in Fig. 56, A. Under 

 these conditions, the beam of sunlight struck the subsporangial 

 swelling transversely, and the sporangiophore was so directed that, 

 in order to make a complete heliotropic reaction, it was obliged to 

 bend through a right angle. After the beginning of the experiment, 

 in order to keep the beam of sunlight on the subsporangial swelling, 

 it was necessary (owing to the movement of the sun relatively to 

 the earth) to revolve the microscope slightly on its axis at frequent 



