60 THE PHYCOMYCETES 



repeated growtn through the base of the old empty sporangium 

 the fertile hvphae may give rise to a chain of sporangia, as occurs 

 in Fythiimi prolijerinn or in Fythiovwrpha gonapodioides. If the 

 axis does not elongate, the successively formed sporangia remain 

 one within the other in a nest-like arrangement. In another type 

 of proliferation, exhibited by Achlya raceviosa and Phytophthora 

 injestans, the new branches arise laterally immediately below the 

 sporangium and near the tip of the fertile hypha, and a sympo- 

 dially arranged series of sporangia is thereby produced. 



Sporangiospores may also proHferate, giving rise to series of 

 successively smaller spores, sporangia, or sporangiophores. Repe- 

 titional development of this type has been recorded for Achlya 

 racemosa, Fythhmi prolifenmi, P. diacarpinn, Phytophthora 

 phase oH, P. cactorimi, P. mfestaiis, and Dictyiichiis sp. by vari- 

 ous workers and for Peronospora tabacina by Wolf and McLean 

 (1940). 



Another manifestation* of what appears to be the same phe- 

 nomenon is exhibited by the Saprolegniales and is known as 

 diplanetism. In certain genera of this order two motile or 

 planetic stages are a more or less fixed character. The zoospores 

 are pyriform in the first motile stage and, after encystment, 

 emerge a second time and are reniform. 



Sexual spores. Diversity in the sexual process is as great 

 among Phycomycetes as in the asexual processes. The thallus 

 arising from a single spore may produce both kinds of gametes, 

 that is, may be homothallic. Instead thalli from two spores, each 

 producing only one kind of gamete, may be required for zygote 

 formation; that is, the species is heterothallic. The gametangia 

 are differentiated in most genera, but in a few instances they are 

 undifferentiated cells. Two morphologically similar gametes 

 may fuse (isogamy) or the gametes may be quite dissimilar 

 (heterogamy). This feature is the basis for separating the Phyco- 

 mycetes into the subclasses Zygomycetes (isogamous) and 

 Oomycetes (heterogamous). Some species regarded as Oomy- 

 cetes, however, are isogamous, as are Synchytrmm endobioti- 

 cum and Olpidhnn viciae, and similarly some species regarded as 

 Zygomycetes, such as Zy gorhynchiis heterogavms, have unlike 

 gametes. Both gametes may be flagellated, as in Alloviyces ar- 

 biiscida and A. javaniciis; the male gamete only may be flagel- 



