THE PII.OBOLUS GUN AND ITS PROJECTILE 91 



Fig. 46.- — A median longitudinal 

 section through the upper part 

 of a fruit-body of Pilobolus 

 Kleinii, just before the dis- 

 charge of the projectile. The 

 gun is shown directed toward 

 the source of the brightest 

 light, in which position it is, 

 heliotropically (or photo- 

 chemically), in a condition of 

 physiological equilibrium. 



The sporangium is filled 

 with spores and covered with 

 an outer intensely black wall o 

 which is now broken below, 

 thus allowing a thick gelati- 

 nous inner ring, g, which is 

 only present around the base of 

 the sporangium, to bulge out- 

 wards. The subsporangial 

 swelling, which is pear-shaped, 

 has a thin elastic wall lined by 

 a layer of protoplasm which is 

 very thin everywhere except 

 at r, where it is slightly 

 thickened, and at p, where it 

 bulges inwards so as to form a 

 large biconcave septuni which 

 is perforated in the centre. The 

 protoplasm at r is reddish and 

 at p, as indicated by the 

 shading, very red, especially 

 on its upper side where it 

 receives the light rays. The 

 subsporangial swelling is con- 

 tinued above into a conical 

 columella c, and below into the 

 upper part of the cylindrical 

 stipe or shaft sh. The proto- 

 plasm of the stipe, swelling, and 

 columella contains one large 

 continuous vacuole filled with 

 clear cell-sap. The broken line 

 a passes through the plane of 

 abscission and indicates where 

 the Pilobolus projectile, con- 

 sisting of the sporangium and 

 the columella, separates from 

 its attachment when the Pilo- 

 bolus gun is discharged. 



Twenty-one parallel rays 

 of light are shown diagram - 



A.aR.B. 



matically, by means of arrows, striking the fruit-body head-on in a direction 

 parallel to the longitudinal axis of the subsporangial swelling. Nine of these rays 

 strike the black cell-wall of the sporangium, which they cannot penetrate. The 

 other twelve rays strike the vipper surface of the subsporangial swelling and are 

 there refracted through the wall into the interior of the swelling and, as shown 

 by the arrows, converge upon the red perforated protoplasmic septum. The 

 septum, like the retina of an ocellus, comes to be lit up with a spot of light, 

 in consequence of which it gives out a red glow which in living fruit-bodies can 

 be seen with the naked eye. s, s, s are parts of the subsporangial swelling 

 which are not pierced by any direct rays of light and are therefore in the shade. 

 Some of the light of all of the rays Nos. 1-6 is reflected at the surface of the wall 

 of the subsporangial swelling ; and, in passing in succession from ray No. 1 to 

 No. 6, more and more light is reflected and less and less refracted. The size of 

 every part is shown by the scale. Magnification, (59. 



