ORDER CHTTRIDIALES 55 



basis of their position with reference to the substratum into family 

 Phlyctidiaceae, epibiotic and endobiotic, and family Rhizidiaceae, inter- 

 biotic, i.e., only the tips of the rhizoids penetrating into the substratum. 



Family Rhizidiaceae. As in the Olpidiaceae and Synchytriaceae, the 

 swarm spores in this family are provided with but one, posterior, flagellum. 

 Upon reaching the host they send each a haustorial process into the 

 matrix, the main body of the encysted swarm spore remaining outside. 

 The haustorium may be a short undivided peg-like structure or slender 

 and more or less branched, sometimes penetrating other host cells as well. 

 The external portion may enlarge directly to form the zoosporangium or 

 the latter may be formed above it, or as a swelling of the subjacent part 

 of the haustorium. The organism remains uninucleate until the spo- 

 rangium begins to develop when the nucleus divides many times to form 

 the nuclei of the swarm spores. The haustorium does not at any time 

 contain any nuclei and can hardly be considered as homologous to a 

 mycelium. Resting cells are produced in some species but their mode of 

 origin is unknown in most cases. Sexual reproduction by the union of two 

 well-developed cells has been observed in a few forms. Fusion of swarm 

 spores has been reported but whether it is a true sexual process or merely 

 a sort of rejuvenescence of weak or exhausted cells has not been deter- 

 mined. Petersen (1903), Sparrow (1936), and Karling (1945) have demon- 

 strated sexual reproduction in Siphonaria, and Sparrow (1937) also in 

 Rhizoclosmatium and in Asferophlyctis. A special rhizoid-like outgrowth 

 proceeds from one cell, presumably to be considered the male cell, to 

 another cell. This tube may be long or short or in some cases the two cells 

 are in direct contact. When this process is completed the female cell 

 enlarges and forms a thick-w^alled resting spore. After a variable length of 

 time this zygote serves as a prosporangium, a small pore being produced 

 and the contents emerging, enclosed by a thin cell wall, and forming an 

 external zoosporangium. The host organisms of the parasitic forms of this 

 family are mostly algae, pollen grains, or small aquatic animals, but 

 Rhizophydium graminis Ledingham is parasitic in the roots of Triticum 

 and Panicum. Many species are saprophytic. 



Phlyctochytrium grows on algae into which its much-branched haus- 

 torium penetrates. This arises from an apophysis just within the host 

 wall while the portion external to the wall enlarges to become the zoospo- 

 rangium. Only the presence of the apophysis distinguishes it from 

 Rhizophydium. (Fig. 14.) 



Rhizophydium occurs mostly in water on various substrata. It con- 

 sists, when mature, of an enlarged, more or less spherical, thin-walled 

 external sporangium, with a usually tufted haustorium within the host 

 cell. Upon the maturity of the numerous swarm spores they escape through 

 one or more inoperculate pores in the sporangium wall. Sometimes the 



