THALLOPHYTA. 



673 



Alliance XV. — Zygomycetes. 



Families: Mucoracece, Mortierellece, 



Are mould-like saprophytes with a much -branched, non-septate m5'celiuni, 

 sexual reproduction by conjugation; swarm-spores never met with. The com- 

 mon Mucor Mucedo (fig. 384) may be regarded as typical of this group. Its 

 mycelium establishes itself on the substratum, and develops long-stalked sporangia 

 at various points on its sur- 

 face (fig. 384'). In each 

 sporangium (fig. 384 ^) numer- 

 ous spores are contained, and 

 these can germinate, producing 

 new mycelia on a suitable 

 substratum. A conjugation of 

 branches of the mycelium 

 leading to the production of a 

 fruit or zygospore (fig. 384 ^) 

 occasionally takes place, but 

 the sexual method of repro- 

 duction is much commoner in 

 other members of the group 

 (c/. pp. 53, 54). The zygospore 

 (or zygote) is invested in a 

 strongly thickened membrane 

 and can remain dormant for 

 a considerable period. The 

 hyphas in many of the Mucors 

 can break up into continuous 

 chains of cells which disarticu- 

 late and propagate the plant; 

 these offshoots are known as 

 chlamydospores or gemmae. 



It often happens amongst the Mucors that although the conjugating branches are 

 produced, they do not conjugate but each produces a fruit parthenogenetically. 

 These, in contradistinction to zygospores, are called " azygospores ". Or, as in 

 Mucor tenuis, the "conjugating branches" no longer arise in pairs but isolated; 

 these also form azygospores. Thus in this group, as in the Saprolegniaceae of the 

 alliance Oomycetes, we note a tendency for sexuality to become obsolete (cf. p. 670). 

 A good deal of variety exists in the Mucoraceas in the arrangement of the sporangia. 

 In Thamnidimn the sporangial branch ends in a large sporangium, and in addition 

 bears laterally a number of tiny sporangia (sporangioles) containing four spores each, 

 whilst in Chcetocladium there is a further reduction, and the sporangioles contain 



Vol. II. 93 



Fig. Wi.—lIucoT. 



1 Mycelium with stallied sporangia; x40. 2 A single sporangium; x260. 

 8 A zygospore produced by coDJugation ; x 160. 



