THE PILOBOLUS GUN AND ITS PROJECTILE 



99 



retina, while the pear-shaped body of the subsporangial sweUing is 

 comparable with the lens, of the eye of certain Mollusca. In this 

 connexion one may compare Fig. 46 which shows a median vertical 

 section through a subsporangial swelling and stipe of Pilobolus 

 Kleinii with Fig. 49 which shows a median vertical section through 

 the ocellus of a Snail, Helix pomatia. Since the subsporangial 

 swelling is so like the ocellus of a Mollusc and 

 is used as an organ for detecting the direction 

 of the incident rays of light, there seems no 

 reason why we should not regard it as a very 

 simple eye which functions like an ocellus. 



The fact that the photochemically sensi- 

 tive protoplasm in the basal part of the 

 subsporangial swelling and at the top of the 

 stipe is so rich in carotin and that this pig- 

 ment is so densely aggregated in the upper 

 surface layer of the perforate protoplasmic 

 septum (Fig. 46, p) suggests that the carotin 

 absorbs light and thereby plays an important 

 part in the heliotropic reaction of the spor- 

 angiophore. Zopf came to the conclusion 

 that the carotin of Pilobolus is simply a 

 reserve food material, but this explanation 

 does not account for the aggregation of the 

 carotin particles around the base of the sub- 

 sporangial swelling and at the top of the 

 stipe nor for the fact that the pigment 



persists in this position until the Pilobolus gun is discharged. 

 When discharge of the gun takes place in water on a slide under 

 a cover-glass, the ring-like mass of orange-red protoplasm at 

 the top of the stipe is often shot out through the mouth of the 

 subsporangial swelling, and I have several times identified it as it 

 lay some distance from the mouth of the sporangiophore not merely 

 by its form but by its high content of carotin (Fig. 65, D-F, p. 137). 



The diameters of the sporangium, the subsporangial swelling, 

 and the motor region of the stipe below the swelling in a typical 

 large fruit-body of Pilobolus Kleinii (the one shown in Fig. 46) were 



Fig. 49.^The eye of a 

 snail, Helix pomatia, 

 retracted : ep, epider- 

 mis ; c, cornea ; I, 

 lens ; r, retina ; op. 

 n., optic nerve. For 

 comparison with the 

 subsporangial swell- 

 ing of Pilobolus. From 

 Vol. Ill of the Cam- 

 bridge Natural 

 History. By courtesy 

 of Macmillan and Co. 



