30 STRUCTURE AND CLASSIFICATION OF FUNGI 



of the cells of the oogonium. These fertilized cells then become the 

 oospores, by enlarging and developing a thick membrane. They are 

 capable of remaining dormant for considerable periods of time. 



Thus in Saprolegnia two types of spores are formed, the endog- 

 enous motile asexual zoospores and the sexual oospores, which are 

 heterogamous and homothallic in origin. In some of the terrestrial ^ 

 forms of Oomycetes parasitic on plants, such as Cystopus, the spores 

 germinate directly by a germ tube and simulate conidia. But these 

 show their relationship by first developing flagella when they lodge 

 in a drop of water and later developing mycelium. 



Life Cycle of Mucor. In most of the Zygomycetes, the third sub- 

 class of Phycomycetes, the multinucleate character is maintained 

 throughout the life cycle. Mucor may be taken as an example. The 

 aerial mycelium gives rise to non-sexual spores in a sac or membrane, 

 the sporangium. The terminal portion of the filament becomes 

 greatly enlarged, vacuoles appear within the protoplasm, and by en- 

 larging and coalescing these gradually separate the protoplasm into 

 individual cells. Each of the spores so formed contains several 

 nuclei. They mature by developing a spore wall. The inflated tip 

 of the sporangiophore, which projects into the sporangium, is the 

 columella. When the spores are mature the sporangial membrane 

 dissolves away, liberating the spores, which are distributed by the 

 wind. A portion of the sporangial wall remains attached to the base 

 of the columella. (See Fig. 34.) 



The sexual spores of the Zygomycetes are called zygospores. They 

 are usually isogamous and heterothallic in origin. When two hyphae 

 capable of conjugating come together, their terminal portions become 

 separated by crosswalls. Their cell walls dissolve where they touch, 

 and the two multinucleated masses of protoplasm run together, the 

 nuclei proceeding to unite two by two. The cell develops a thick 

 wall with generally warty or spiny projections from its surface. 

 Various stages in the formation of a zygospore are illustrated in 

 Fig. 25. 



Zygospores, like oospores, generally remain dormant for consider- 

 able periods. When they revive, they may germinate by putting 

 forth a single filament which at once forms a sporangium and spo- 

 rangiospores or they may produce a more or less extensive mycelium. 

 Further discussion of morphology of Zygomycetes is given in Chap- 

 ter IV. 



Although there is no morphologically distinguishable differentia- 

 tion into two sexes in most of the Zygomycetes, it can be shown with 

 the heterothallic species that there is a physiological differentiation. 



