HISTORY OF PILOBOLUS 13 



view, according to Blakeslee/ cannot be accepted, because Thaxter 

 subsequently found the zygospores of P. Kleinii on sheep dung but 

 without the parasites. Morini ^ (1906) observed zygospores in a 

 species which he has named P. Borzianus. He found them a httle 

 below the surface in cultures several months old. From Blakeslee's 

 discussion^ (1904) of the conditions under which zygospores have 

 been observed in Pilaira anornala, P. crystallinus, and P. Kleinii, 

 it is clear that at that time we did not know whether these species 

 are homothallic or heterothallic and that exact experiment alone 

 could throw light on this problem. The thick- walled tuberculate 

 "spores durables" which van Tieghem* described as developing 

 terminally on short curved stalks from the mycelium of Pilobolus 

 nanus (Fig. 106, G, p. 212), are regarded by Fischer ^ as azygospores. 

 Recently (1931), Krafczyk ^ has solved the problem of the condi- 

 tions required for the formation of zygospores in Pilobolus crystal- 

 linus. He isolated single sporangia from goat dung and made pure 

 cultures from each one on a decoction of hay and dung solidified 

 with agar.'^ Then he paired mycelia obtained from different sporangia, 

 two by two, and soon found a pair which yielded zygospores along 

 the line where they met. Then, using the ( + ) and ( — ) strains as 

 testers, he was able to determine the sexual nature of a large number of 

 other mycelia derived from sporangia collected in various localities. 

 Thus he has shown conclusively that P. crystallinus is heterothallic.^ 



1 A. F. Blakeslee, " Sexual Reproduction in the Mucorineae," Proc. Amer. Acad. 

 Arts and Sci., Vol. XL, 1904, p. 238. 



2 F. Morini, " Materiali per una monografia delle Pilobolee," Memorie della R. 

 Accad. delle Sci. delVIst. di Bologna, Ser. VI, T. Ill, 1906, pp. 120-123. 



3 A. F. Blakeslee, loc. cit., pp. 238-239. 



* P. van Tieghem, " Troisieme memoire sur les Mucorinees," Ann. Sci. Nat., 

 6 ser., T. IV, 1876, pp. 312-398, PI. X-XIII. 



5 A. Fischer, in Rabenhorst's Kryptogamen Flora von Deutschland, Bd. I, Abt. IV, 

 p. 268. 



^ H. Krafczyk, " Die Zygosporenbildung bei Pilobolus cristallinus," Ber. d. D. 

 hot. Gesell, Bd. XLIX, 1931, pp. 141-146, Figs. 1 and 2. 



7 This medium was suggested by the work of E. Bersa (" Kultur und Ernah- 

 rungsphysiologie der Gattung Pilobolus," Sitzungsber. Akad. Wissensch. in Wien, 

 math.-nat. Kksse, Bd. CXXXIX, 1930, pp. 355-371) who cultivated Pilobolus 

 Kleinii and P. sphaerosporus on various media and found that they would grow very 

 easily on horse -dung agar. 



8 Krafczyk holds that, judging by his illustrations, Zopf did not find zygospores 

 in Pilobolus crystallinus, but in P. Kleinii. Loc. cit., p. 141, 



