634 AQUATIC PHYCOMYCETES 



isogamous gametes that, after emerging from the cyst, conjugate to 

 form zygotes which germinate to produce asexual plants; gametes 

 posteriorly uniflagellate, isogamous or anisogamous, if anisogamous 

 the smaller gamete always containing carotinoid pigment, fusing in 

 pairs to form a biflagellate planozygote, which, without a period of rest, 

 germinates to form an asexual plant. 



Three families are now recognized in the order. These are : 



1. The Coelomomycetaceae, in which the thalli lack demonstrable 

 walls and are without rhizoids. No reproductive structures save thick- 

 walled resting spores and their planonts are known. The family comprises 

 a highly specialized group of parasites primarily of mosquito larvae. 



2. The Catenariaceae, in which the tubular, walled thallus bears 

 rhizoids and forms cross walls which delimit reproductive structures or 

 sterile isthmuses. Zoosporangia and resting spores are produced and 

 Catenaria allomycis has a Cystogenes type of life cycle (p. 625). 



3. The Blastocladiaceae, members of which, so far as now recognized, 

 fall into two well-marked groups: the one contains the genera Blasto- 

 cladiella, Blastocladiopsis, and Allomyces; the other is coextensive with 

 the genus Blastocladia. 



The first group is typified ( 1 ) by having sporangia with several discharge 

 pores and (2) by sexual reproduction (where observed) accompanied 

 (except in Cystogenes types) by alternation of isomorphic generations. 

 In Blastocladiella hyphal branches are lacking; in Blastocladiopsis the 

 hyphae are without pseudosepta and limited in growth; and in Allomy- 

 ces, the hyphal branches of the often poorly defined basal cell are pseudo- 

 septate and unlimited in growth. 



The second group {Blastocladia) is characterized by the possession 

 of sporangia that develop a single papilla, often with an inwardly pro- 

 jecting plug and an apical pore. Branches of the hyphae, if any, lack 

 pseudosepta and are slight in extent. No sexual reproduction has yet 

 been observed in any of its members. 



Further investigations of the two groups of the Blastocladiaceae may 

 justify their segregation into separate families or may show that many 

 intermediate forms exist. 



