384 



BOTANY 



I'AKT II 



the cavity of the sporangium aiid forms a columella (Fig. 323, 1, c). The contents 

 of the sporangium become divided into numerous spores. These escape by the 

 swelling of a substance which lies between the spores and the bursting of the 

 sporangial wall. In Pilobolus, which occurs commonly on dung, the sporangium 

 is forcibly cast off from the turgid sporangiophore which bursts at the columella. 



Fio. 323. 1, Mucor Mwxdo. A sporangium in optical longitudinal section : <. ccilumella ; m. wall 

 of sporangium ; sp, spores. 2, Mwxtr mucilagineuf. A sporangium shedding its spores ; the 

 wall (i) is ruptured, and the mucilaginous substance between the spores (<) is greatly swollen. 

 (After BREFELD, 1 x 225, 2 x 300, from v. TAVKI.. /;/:,-.) 



According to HARPER the spores of Pilobolus are binucleate, while those of Sporo- 

 dinia (Fig. 324) are multinucleate. 



Under certain conditions, instead of asexual sporangia, organs of sexual repro- 

 duction are produced. The hyphae of the mycelium then give rise to lateral, club- 

 shaped branches. When the tips of two such branches come into contact, a 



conjugating cell or ccenogamete is cut off 

 from each by a transverse wall (Fig. 325). 

 The two gametes thereupon coalesce, their 

 nuclei conjugating in pairs, and fuse into 

 a ZYGOSPORE, the outer wall of which is 

 covered with warty protuberances. After 

 a period of rest the zygospore germinates, 

 developing a germ-tube, which may at 

 once bear a sporangium (Fig. 325, 5). 



BLAKESLEE'S ( x ) demonstration of the 

 dioecious nature of the mycelium of Mucor 

 stolonifer is of great interest. The formation 

 of zygospores only takes place when male 

 an( j female myce li a come j u contact. In 



other Mucorineae the two conjugating 

 gametes may arise on the same mycelium. 



Within the group of the Zygomycetes a reduction of sexuality is perceptible. 

 Thus, in the case of certain -Mucoriueae, although the conjugating hyplite meet in 

 pairs, no fusion takes place, and their terminal cells become converted directly 

 into spores, which are termed AZYGOSPORES. In other forms again, hyplm- 

 producing azygospores are developed, but remain solitary, and do not, as in the 

 preceding case, come into contact with similar hyphse. There arc also many 

 species in which the formation of zygospores is infrequent. 



Both the size and number of spores produced in the sporangia of M-ttfnr 



Fi<;. 324. Sporwlinia grandif. Median section 

 of a ripe sporangium. The spores are 

 multinncleate. (After HARPER, x 4-25.) 



