CHAPTER VI 

 CLASS PHYCOMYCETES 



The Phycomycetes owe their name to the fact that they were long 

 considered as degenerate algae. Because of their similarity to the 

 Siphonales, they are sometimes called Siphonomycetes. 



The thallus, in contrast to that of the Archimycetes, is surrounded by 

 a membrane. In the simpler forms, it is a uninucleate cell; in the higher 

 forms a true mycelium with aseptate multinucleate hyphae; in the high- 

 est forms, the hyphae are abjointed into uni- or multinucleate cells. In 

 several families the hyphae may fragment and become a sprout mycelium. 



Sporangia and conidia are fructifications of the haplont. In the 

 simplest forms, sporangia arise by a change of the thallus; in the forms 

 with a better developed thallus, they are eucarpic, with special organs 

 which occupy only a part of the thallus. In time, such a small part of 

 the thallus is used for the formation of sporangia that these may be 

 repeatedly formed on nearly all the hyphae of the plant; thus the repro- 

 ductivity of an individual is increased many times. 



At maturity the sporangia are generally coenocytic. By the individ- 

 ualization of single energids, they produce motile zoospores, or by the 

 cleavage of the protoplasm into multinucleate portions, non-motile 

 sporangiospores. By the combined effect of several factors, especially 

 by the transition from submersed to terrestrial and finally to parasitic 

 habits, the development is gradually inhibited before the daughter cells 

 have individualized : the sporangia germinate as a whole with a coenocytic 

 germ tube. Subsequently they assume the task of propagation instead 

 of their daughter cells, and become successively spores and conidia. 



By this degeneration of sporangia to conidia, the number of propagative 

 organs formed by an individual is greatly reduced. This numerical dis- 

 advantage is offset by a considerable increase of branching of the conid- 

 iophores, whereby the number of sporangia formed by an individual 

 increases in geometric progression. Aplanes Braunii and Pythium 

 Indigoferae, however, are on the point of giving up their degenerate asexual 

 reproduction in favor of the sexual, and in many Entomophthoraceae and 

 Endogonaceae, only the sexual reproduction is known. The preference 

 for sexual reproduction at the cost of the degenerate asexual, will be met 

 later in a greater degree in the Ascomycetes and Basidiomycetes. 



As sexual organs, gametangia and gametes are formed. In the lower 

 forms, the gametangia arise like the sporangia by the transformation of 



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