PHYCOMYCETES 27 



and it is possible to use the terms sporangium and conidium 

 together there with less difficulty. 



The transition from the typical persistent sporangium of the 

 lower Phy corny cetes to the deciduous conidium of the higher 

 has been accompHshed in various ways in different groups. 

 While in the Peronosporales the transition has been marked 

 by a rather abrupt change from the swarmspore type of germina- 

 tion to the germ tube type, in the Mucorales gradual and pro- 

 nounced changes in morphology have occurred. In the latter 

 order the sporangium has tended to decrease in size, to become 

 few-spored, and to fall away. The small few-spored sporangia 

 thus resulting have been termed sporangiola. In some cases 

 they are monosporic, the wall of the spore being clearly discerni- 

 ble within that of the sporangiolum. In their highest develop- 

 ment, the spore wall fails to form, and the whole cell is then 

 termed a conidium. 



In various genera of the Phycomycetes {e.g., Saproleg?iia, 

 Miicor) an additional asexual spore form is present, which cannot 

 be clearly homologized with the sporangium. These spores 

 have been termed chlamydospores or gemmae. They occur 

 chiefly on old thalli, and are essentially merely vegetative cells 

 with thickened walls. Similar spores are not uncommon among 

 the higher fungi. They function as resting spores and germinate 

 by germ tube. They are more often intercalary than terminal, 

 and usually are not deciduous. 



Sexual Reproduction. — A comprehensive general discussion 

 of the phenomena of sexuality in the Phycomycetes must of 

 necessity be somewhat detailed. Broad general statements are 

 impossible on account of the great variation within the group. 

 Several distinct types of sexuahty occur, and a consideration of 

 their relationships is important in connection with questions of 

 phylogeny and classification. 



The simplest form of sexuality represented is that in which 

 swarmspore-like cells, called gametes, fuse in pairs to form 

 motile zygotes. Because the fusing cells are motile the process is 

 termed planogamic copulation. As they are morphologically 

 indistinguishable it may be called also isogamy. This is probably 

 the ancestral type of sexuahty in the group. It persists today in 

 a few known cases, e.g., Olpidium viciae (p. 75) and Syncluj- 

 trium endobioticum (p. 82), and perhaps is present in a consider- 

 able number of other incompletely investigated members of the 



