36 THE LOWER FUNGI— PHYCOMYCETES 



interest, nevertheless, to note that the resting spore in Poly^ 

 phagus is formed in the tube rather than in the so called female 

 individual. 



A consideration of these variations in the sexual process as 

 exhibited in the lower Phycomycetes indicates clearly that 

 various primitive types of sexuality exist here which are, strictly 

 speaking, neither oomycetous nor zygomycetous. Some of these 

 probably represent lateral lines of development or terminations 

 of series having little if any connection with the two main lines 

 ending in the higher Oomycetes and Zygomycetes. Until these 

 lower forms have been studied much more thoroughly the solu- 

 tion of the problem of their interrelationships cannot be reached. 



In numerous genera of the Phycomycetes, species have been 

 described in which a sexual stage is not as yet known to occur. 

 In primitive members of the group, such as the lower Chytrid- 

 iales, it is possible that some of these are forms in which sex- 

 uality has not yet arisen. In the higher Phycomycetes absence 

 of a known sexual stage may be due to the existence in the species 

 of a heterothallic condition, i.e., segregation of sex in such a 

 fashion- that constantly some thalli are wholly male while others 

 are wholly female. In such heterothallic species the presence 

 of both sorts of thalli, and in most cases their actual contact, is 

 necessary before a sexual union can be effected. It is possible 

 that in some species one of the two sorts of thalli has died out 

 completely, leaving the species unisexual. When both sexes 

 occur in the same thallus the species is termed homothallic. 

 Failure of a homothallic species to form sexual spores is usually 

 attributed to unfavorable features of nutrition or other environ- 

 mental factors. These factors are as yet only imperfectly 

 understood. 



The gametangia of the Phycomycetes are usually functional. 

 However, in several families (e.g., Saprolegniaceae, Mucoraceae, 

 Entomophthoraceae) sexual organs of normal aspect may fail to 

 react sexually. In these cases spores morphologically equiva- 

 lent to the sexual spores are formed parthenogenetically. In 

 the Zygomycetes such spores have long been termed azygospores. 

 In the Oomycetes they may well be called aboospores. They are 

 capable of germination and the production of normal thalli. 

 Though the chromosome condition in such cases has not been 

 compared with that in normally sexual species it is probably 

 unchanged throughout the entire hfe cycle. Since the fusion 



