28 THE LOWER FUNGI— PHYCOMYCETES 



Chytridiales, especially those having resting spores of unknown 

 origin. This type of sexual process probably originated very 

 early in the copulation of swarm cells in species in which such 

 cells had functioned previously only asexually as swarmspores. 

 In the relatively few present-day Phycomycetes in which such 

 swarm cells fuse as gametes, there occur in the life cycle other 

 cells of identical aspect which still function as swarmspores. The 

 rather meager data gained from observations indicate that the 

 three sorts* of swarm cells (swarmspores, male gametes, and 

 female gametes) are delimited in different parent cells. The cell 

 in which the swarmspores are borne is called the sporangium. 

 Those in which the gametes are formed are termed respectively 

 the male and female gametangia. The three sorts of parent 

 cells are morphologically indistinguishable. Considerable evi- 

 dence indicates that during the period of active growth while 

 the food supply is abundant and other conditions favorable the 

 fungus forms several successive generations of swarmspores, and 

 that when conditions become unfavorable and the rest period is 

 approached gametes are produced instead. In parasitic species, 

 infection of the host is accomplished by either swarmspores or 

 zygotes. After the gametes fuse the zygote (planozygote) is 

 motile for a time, but finally comes to rest, rounds up, increases 

 considerably in size, and at maturity develops into the resting 

 spore. As the resting spore in germination frees swarmspores, 

 it may be termed also the resting sporangium. The term 

 zygospore, used for similar structures in certain algae, is avoided 

 here since it has become fixed in its application in a different sense 

 in the group of higher Phycomycetes called the Zygomycetes. 



As has been indicated above the species representative of this 

 type of sexuality are termed isogamic, because the fusing gametes 

 are alike. They may be called also isogametangic in that 

 their gametangia are indistinguishable. Though no isogamic 

 species known at present are heterogametangic it is possible 

 that future investigations will bring to light forms of that type 

 also. 



In those Phycomycetes in which planogamic copulation of like 

 gametes occurs, the fusing cells and the resultant zygote are 

 usually both ciliate. However, in a single imperfectly known 

 species, described by Griggs (p. 75) under the name Mono- 

 chrjtrium stevensianum the gametes are non-ciliate and fuse in 

 the amoeboid condition. Moreover, in two species of Olpidium, 



