THE PILOBOLUS GUN AND ITS PROJECTILE 119 



on colourless protoplasm. Then it approached and travelled 

 on the reddish protoplasm 

 situated within and around 

 the base of the swelling ; and, 

 finally, about five minutes before 

 the end of the experiment, it 

 could be seen leaving the red- 

 dish protoplasm on the interior 

 of the swelling and passing on 

 to the very red protoplasm of 

 the protoplasmic septum at the 

 top of the stipe. Fifty-eight 

 minutes, or approximately one 

 hour, after the beginning of 

 the experiment when the spor- 

 angium and subsporangial swell- 

 ing had come to face the sun- 

 light head on, the spot of 

 light was seen resting sym- 

 metrically on the protoplasmic . 

 septum which, in consequence, 

 glowed with a reddish light (cf. 

 Fig. 46, p. 91). These obser- 

 vations show that, in the spor- 

 angiophore of Pilobolus, the 

 cessation of heliotropic move- 

 ment, or in other words the 

 establishment of photochemical 

 equilibrium, is correlated with 

 the passage of a spot of bright 

 light from one side of the 

 subsporangial swelling to a 

 radially symmetrical position 

 on the red protoplasm at the 

 base of the swelling and at 

 the top of the stipe ; and 



nn 



Fig. 57. — Pilobolus longipes. The fruit- 

 body shown in Fig. 56, drawn with 

 the aid of a camera lucida one hour 

 after the beginning of the heliotropic 

 experiment. The subsporangial 

 swelling s and the sporangium with 

 its black outer cell-wall c and its 

 bulging basal gelatinous band g, 

 through which the spores can be seen, 

 have been turned through a right 

 angle owing to the unilateral growth 

 of the motor region of the stipe, m, in 

 response to the heliotropic stimulus. 

 The light coming from the sun (c/. A 

 in Fig. 56) is now refracted through 

 the subsporangial swelling and is 

 thereby focussed on the red bicon- 

 cave perforate protoplasmic septum 

 situated at p. A few minutes after 

 this drawing was made the spor- 

 angium was shot away toward the 

 sun, and it struck and stuck to 

 the 2-mm.-wide window shown in 

 Fig. 56, A and C. The size of every 

 part is indicated by the scale. Mag- 

 nification, 37. 



they may be accepted as giving strong support to the explanation 



