456 RESEARCHES ON FUNGI 



discussion of the factors which permit of the successful operation of the 

 Pilobolus gun. 



The sporangium-wall dehisces by splitting horizontally into two parts, 

 a lower narrow band which remains attached to the base of the columella 

 and an upper, much larger, cap-like portion which covers the spores. 

 The escape of the spores from a sporangium which has just dehisced is 

 prevented by a ring of jelly. This jelly, after the sporangium has been 

 discharged, serves to stick the sporangium to the surface of a grass-leaf, 

 etc. 



The number of "spores in a sporangium of P. Kleinii was estimated to 

 be 30,000-90,000, varying with the sporangium's size. 



The level of abscission in the sporangiophore when the sporangium is 

 shot away is at the top of the subsporangial swelling just beneath the 

 junction of the sporangium- wall with the wall of the columella. 



The structure of a discharged sporangium has been described and 

 illustrated in detail. As a discharged sporangium dries, the sporangium- 

 wall becomes locally depressed above and circularly tucked-in beneath 

 the spores below, while the spores contract in size and become polygonal. 



A sporangium-wall, before the sporangium has been shot away, is 

 always smooth and rounded. The more or less regular pattern of dim- 

 ples, hexagonal areas, etc., which can often be seen on a dried discharged 

 sporangium is formed as the sporangium dries up and contracts. The 

 pattern on a dried discharged sporangium of P. longipes differs from that 

 on a dried discharged sporangium of P. Kleinii ; and, in general, the 

 patterns are an aid in distinguishing these two species of Pilobolus from 

 one another. 



The drops excreted by the sporangium and subsporangial swelhng of a 

 Pilobolus fruit-body contain a colloidal mucilaginous substance, for they 

 dry up with an irregular surface. The drops on the sporangium usually 

 dry up before the sporangium is discharged ; and, as they do so, they 

 become darkly pigmented like the sporangium- wall. The drops on the 

 upper part of the subsporangial wall tend to dry up sooner than those on 

 the lower part, and their interference with the light rays which cause 

 heliotropic response is thus diminished. 



The wall of a Pilobolus fruit-body bears numerous very minute crystals 

 of calcium oxalate. In P. lo7igipes these crystals are arranged : most 

 densely on the sporangium-wall, less densely on the wall of the subspor- 

 angial swelling, and far less densely on the wall of the stipe. The un- 

 wettability of the sporangium-wall may be due to the numerous close-set 

 crystals imprisoning air and thus preventing water from coming into 

 contact with the wall's surface. 



The subsporangial swelling of the Pilobolus gun functions in two 

 different ways : ( I ) as an ocellus which receives the heliotropic stimulus 

 which causes the stipe to direct the free end of the gun toward the source 

 of brightest light ; and (2) as part of a squirting apparatus which, 



