24 THE LOWER FUNGI— PHYCOMYCETES 



the cytoplasm maintain a constant and intimate association 

 with individual nuclei. 



The thallus of the Phycomycetes is usually provided with a 

 membrane or wall, but in some of the lower groups (Plasmodi- 

 ophoraceae, Woroninaceae) it is naked throughout all or the 

 major portion of its existence. Although the wall in the majority 

 of the fungi is composed of fungus cellulose (a substance resem- 

 bling chitin), in certain members of the Phycomycetes it gives 

 the reaction for pure cellulose. 



In some of the lower families the entire thallus is transformed 

 at maturity into a single reproductive organ (Olpidiaceae) or 

 group (sorus) of them (Synchytriaceae). In such cases the 

 organism is said to be holocarpic. In the higher famihes, only a 

 portion of the thallus is transformed for purposes of reproduction, 

 and the remainder continues to function vegetatively. Such 

 organisms are termed eucarpic. In species developing abundant 

 mycelium the reproductive bodies are relatively less conspicuous 

 than the thallus. Prominent compound fructifications, compara- 

 ble to those of the Ascomycetes and Basidiomycetes, are almost 

 wholly absent. A tendency toward the formation of such struc- 

 tures is present in the INIucorales in Endogone and to a lesser 

 degree in Mortierella. 



Asexual Reproduction.— In the Phycomycetes asexual repro- 

 duction occurs in most cases by means of spores (sporangio- 

 spores) borne in unicellular sacs called sporangia. In the 

 lower orders, the sporangiospores are usually naked and motile 

 and are termed zoospores. In the higher, they are non-motile, 

 have definite membranes, and may be designated as aplanospores. 

 Though in a few of the lowest forms the zoospores arc amoeboid, 

 they are usually ciliate and may then be called swarmspores. 

 In most of the famihes of the lowest order (Chytridiales) they 

 are unicihate, but in the single family Woroninaceae of that order 

 and in almost all cases in higher groups they are biciUate. When 

 biciliate, the two ciha arise from the same point on the swarm- 

 spore. In some genera, one cilium is markedly longer than the 

 other. The swarmspore is usually either pyriform (pear-shaped) 

 or reniform (bean-shaped), the ciha being attached at the nar- 

 rowed end in the former case and at the lateral depression (hilum) 

 in the latter. Zoospores are characteristic of the more primitive 

 genera, and are indicative of an aquatic or semi-aquatic habit. 

 When present in terrestrial forms they swim in soil water or on 



