34 THE LOWER FUNGI— PHYCOMYCETES 



the oospore in being a resting spore. After a period of quiescence 

 it germinates by a germ tube. 



It is evident that the zygomycetous type of reproduction 

 differs from the oomycetous essentially in that the oosphere is 

 lacking, and the sexual identity of the interacting gametangia 

 is in doubt. It differs also in that true gametangial copulation 

 occurs. As there is no oosphere the fertilization tube is unneces- 

 sary and is lacking, and the resting spore tends to fill the cell 

 instead of lying free as in the Oomycetes. 



These various features of dissimilarity considered together 

 indicate clearly that the Oomycetes and Zygomycetes are not 

 closely related. Though presumably at a remote period they 

 had a common ancestry, it is evident that they represent two 

 series of genera which have arisen in parallel from still lower 

 forms. The probable line of development of the Oomycetes 

 from primitive aquatic forms with planogamic copulation has 

 been indicated above. The origin of the Zygomycete line is 

 somewhat more obscure, though forms possessing one or more 

 undoubted zygomycetous characters exist among the Ancylistales 

 and Chytridiales. 



In certain of these lower Phycomycetes primitive types of 

 sexuality exist in which oomycetous and zygomycetous charac- 

 ters are found associated in the same species. These cases have 

 been inadequately studied, and knowledge of them is too frag- 

 mentary to justify conclusions as to their relationships. It 

 has long been known that in some of the Chytridiales (e.g., 

 Olpidiopsis and Pseudolpidiopsis) the resting spore is accom- 

 panied at maturity by one or more small empty appendages. 

 Early investigators, ignorant of the real nature of these struc- 

 tures, called them companion cells. Now they are known to 

 be emptied male cells. Though few Chytridiales having com- 

 panion cells have been studied critically they are apparently all 

 endophytic parasites, and their life history is believed to be 

 essentially as follows. Ciliated, free-swimming swarm cells 

 exist in the water outside the host, and in all cases function as 

 swarmspores. There 'is no basis for an assumption that any 

 of them ever act as gametes, as in Olpidium viciae. All are 

 apparently capable of accompHshing infection. On coming in 

 contact with the host the swarmspore rounds up, effects pene- 

 tration, and comes to lie within the host cell, where, for a brief 

 period at least, it exists as a naked protoplast. It gradually 



