THE PILOBOLUS GUN AND ITS PROJECTILE 6i 



coloured protoplasm ; but, subsequently, as its wall matures and 

 becomes pigmented, it turns black (Figs. 12, c, and 23, C-E). The 

 conical columella, which separates the sporangium from the stipe, 

 is developed early in the evening ; but the spores are not formed 

 until after midnight. Shortly after midnight, the top of the stipe 

 immediately under the sporangium begins to become transformed 

 into a subsporangial swelling. The development of the swelling 

 and the ripening of the spores take place simultaneously between 

 12 midnight and 6 a.m., i.e. while in winter it is still dark (Figs. 23, 

 C-E, and 24, D-F). In the morning, after daylight has appeared, 

 the top of the stipe just beneath the subsporangial swelling bends 

 heliotropically, so that the sporangium comes to face the strongest 

 incident rays of light with great precision (c/. Figs. 13, p. 37, and 



Fig. 24. — Pilobolvs longipes. Rhythmic development and heUotropism of the fruit- 

 bodies, represented diagrammatically. In tlie right-top comer of a drawing, 

 arrows indicate daylight and the direction of the strongest incident rays, and 

 black triangles indicate night and the absence of effective heliotropic radiation. 

 A-H, successive stages in the development of three fruit-bodies of the first 

 diurnal generation ; I-K, successive stages in the early development of six 

 fruit-bodies of the second diurnal generation. The fruit-bodies are supposed 

 to be developing on a horse-dung ball in a field. The times given are those 

 which were observed in a laboratory culture (fresh unsterilised horse dung) late 

 in November. A, at 3 p.m. ; three red tubers (trophocysts) have appeared on 

 the surface of the dung-ball. B, at 4 p.m. ; each tuber, now to be regarded as 

 a basal swelling of a fruit-body, has given rise to a stipe which is elongating 

 apically and is positively heliotropic. C, at 6 p.m. ; darkness has set in ; the 

 stipes are still elongating ; in the absence of light they are growing straight 

 forward in the direction they took up in the afternoon. D, at 10 p.m. ; the 

 stipes temporarily have ceased to elongate ; each of them has given rise to a 

 terminal sporangium. E, at 12.30 a.m. (midnight) ; each fruit-body has now 

 developed a subsporangial swelling ; the stipes are about to resume their 

 growth in length. F, at 4 a.m. (still dark) ; the stipes have elongated con- 

 siderably. G, at 12 A.M. (noon) ; the direction of the strongest incident rays 

 of light, as indicated by the arrows, has changed since the previous afternoon ; 

 as daylight dawned, the fruit-bodies, with the help of their subsporangial 

 swellings, readjusted themselves heliotropically and so their stipes are now 

 curved. H, at 1 p.m. ; the climax of development has arrived ; two of the 

 fruit-bodies have discharged their sporangia in the directions indicated by the 

 long curved arrows ; the discharged sporangia have struck and stuck to near-by 

 grass ; the third fruit-body has just exploded ; the jet of sap squirted out from 

 the mouth of the subsporangial swelling has broken up into a series of drops of 

 which the largest is attached to the wettable gelatinous under side of the 

 sporangium ; the projectile (sporangium and sap-drop) may be rotating in its 

 flight ; the two sporangiophores which have discharged their projectiles are 

 lying on the dung-ball ; owing to action and reaction being equal and opposite, 

 they were of necessity forced backwards as the projectiles were forced forwards. 

 I, at 3 P.M. ; the three discharged fruit-bodies of the first diurnal generation 

 can be seen lying on the surface of the dung-ball where they are rapidly dis- 

 integrating ; six new tubers (trophocysts) of the second diurnal generation 

 have now appeared (c/. A). J, at 4 p.m., and K, at 6 p.m., show stages in the 

 development of the second diurnal generation of fruit-bodies resembling stages 

 B and C for the first diurnal generation. Magnification, 1 • 5. 



