200 A TEXTBOOK OF THEORETICAL BOTANY 



akaryote stage and the production of metabolic monoploid nuclei in the 

 spores themsehes. 



A nuclear division similar to that in the Plasmodiophorales has been 

 described in certain of the Protozoa, which has led to the suggestion of a 

 common ancestry of the two groups. In the Myxomycetales, on the other 

 hand, the nuclear divisions in the plasmodium are mitotic. 



PHYCOMYCETES 



The Phycomycetes are Fungi in which the thallus is composed of single 

 cells or of filaments of cells forming hyphae. Frequently these hyphae are 

 coenocytic, transverse septa occurring either rarely, in the older hyphae, or 

 exclusively at the separation of the reproductive organs. The hyphae may 

 be multinucleate and the nuclei are frequently extremely small. 



Sexual reproduction is either by motile isogametes, in the lowest order, 

 or by means of an oogonium and antheridium. The oogonium may produce 

 from one to many oospheres. The antheridium rarely produces free anthero- 

 zoids, and fertilization is normally effected by direct contact through a 

 fertilization tube. The oospore is usually thick walled and functions as a 

 resting spore. It may germinate directly to form hyphae or by the develop- 

 ment of zoospores. In the IMucorales peculiar conditions obtain, sexual 

 reproduction being by isogametes or anisogametes which are invariably 

 non-motile. 



Asexual reproduction in the lower groups is by zoospores, which may 

 have either one or two flagella, a character to which some workers attach 

 considerable importance and use as a basis of classification. In the higher 

 members either sporangia are produced containing spores which germinate 

 directly into a hypha or aerially distributed conidiospores may be abstricted 

 in chains from the apices of special hyphae termed conidiophores. 



The species are predominantly aquatic and may live either as parasites 

 or saprophytes. Some occur in soil and others as endoparasites of higher 

 plants, and it is in these latter forms that we see the transition from an aquatic 

 to an aerial method of spore distribution being evolved. By virtue of their 

 aquatic habit and their marked similarity to certain Algae they were at one 

 time regarded as Algae which had lost their chlorophyll and were classified 

 after their supposedly corresponding algal groups. The name Phycomycetes 

 or " Algal Fungi " emphasizes this idea. This view now receives but little 

 support from mycologists. 



We shall consider examples from the following orders : — 



1. Chytridiales {e.g., Rhizophidium, Synchytrium). 



2. Saprolegniales {e.g., Saprolegnia). 



3. Monoblepharidales {e.g., Monoblepharis). 



4. Peronosporales {e.g., Pythium, Cystopus, Peroiiospora). 



5. Mucorales {e.g., Mucor). 



