434 



BOTANY 



PART II 



the first -named forms having been derived from the multinucleate condition. 

 Albugo tragopogonis occupies an intermediate position in that its oosphere is at 

 first multinucleate, but later contains only one female nucleus, the others having 

 degenerated. The superfluous nuclei in the oogonia and antheridia may be 

 regarded as the nuclei of gametes which have become functionless, and are com- 

 parable with the superfluous egg-nuclei of certain Fucaceae. The oospores either 

 produce a mycelium directly or give rise to swarm - spores. The nuclei in the 

 oospore are diploid ; their first division, which may occur before germination, is 

 the reduction division. 



Order 3. Zygomycetes 



1. The Mucorineae ( 57 ) comprise a number of the most common Mould Fungi. 

 They are terrestrial and saprophytic, and are found chiefly on decaying vegetable 

 and animal substances. Asexual reproduction is effected by non-motile, walled 

 spores, which either have the form of conidia or arise endogenously in sporangia. 



FIG. 377. 1, Mucor Miicedo. A sporangium in optical longitudinal section ; c, columella ; m, wall 

 of sporangium ; sp, spores. 2, Mucor mucilagineus. A sporangium shedding its spores ; the 

 wall (m) is ruptured, and the mucilaginous substance (2) between the spores is greatly swollen. 

 (1 x 225, 2 x 300, from v. TAVEL, Pilze. After BREFELD.) 



Sexual reproduction consists in the formation of zygospores, as a result of the con- 

 jugation of two equivalent coenogametes. 



One of the most widely distributed species is Mucor Mucedo, frequently found 

 forming white fur-like growths of mould on damp bread, preserved fruits, dung, 

 etc. Mucor stolonifer ( Rhizopus nigricans), with a brown mycelium, occurs on 

 similar substrata. The spherical sporangia are borne on the ends of thick, erect 

 branches of the mycelium (Fig. 376). From the apex of each sporangiophore a 

 single spherical sporangium is cut off by a transverse wall, which protrudes into 

 the cavity of the sporangium and forms a columella (Fig. 377). The contents 

 of the sporangium become divided by repeated cleavages into numerous spores. 

 These escape by the swelling of a substance which lies between the spores and the 

 bursting of the sporangial Avail. In Pilobolus, which occurs commonly on dung, 

 the sporangium is forcibly cast off from the turgid sporangiophore which bursts at 

 the columella (cf. pp. 349, 350). According to HARPER the spores of Pilobolus are 

 binucleate, while those of Sporodinia (Fig. 378) 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 coenogamete is cut off from each by a transverse wall (Fig. 379). 

 The two gametes thereupon coalesce, and fuse into a ZYGOSPORE, the outer wall of 



