10 AQUATIC PHYCOMYCETES 



logues are formed (sometimes by a single fungus), is remarkable. 



4. The anteriorly biflagellate "primary" type of saprolegniaceous 

 zoospore (Fig. 1 E-F) has a somewhat narrowly pyriform body and its 

 two flagella are apically or slightly subapically attached. Although these 

 flagella have usually been said to be anteriorly directed in motion and 

 of about equal length, they are, at least in some cases, disposed oppo- 

 sitely and are slightly unequal, the shorter one being anteriorly directed. 



5. The secondary zoospore in the Saprolegniales (Fig. 1 F, first seven) 

 has been characterized as "bean-shaped," "kidney-shaped," "grape- 

 seed-like," and so on. The flagella, which arise from a shallow groove 

 on the body, are about equal in length and are oppositely disposed. 

 Such a type of spore is also to be found in the Leptomitales, the Lage- 

 nidiales (at least in the filamentous types), and the Peronosporales. Other 

 but somewhat differently shaped biflagellate spores have been figured, 

 particularly in holocarpic forms (Fig. 1 G and the two above). Owing 

 to their small size, however, they are frequently difficult to delineate. 



In recent years considerable work has been done with the aid of 

 modern microscopic techniques and equipment on the structure of these 

 various types of zoospores, particularly on their flagella and on their 

 movement. The finer structure of the flagella has been carefully worked 

 out by Couch (1941). Using Vlk's (1939) procedure, dark-field illu- 

 mination, and special stains, he determined that the posteriorly flagel- 

 lurn of the Chytridiales, Blastocladiales, and Monoblepharidales (Fig. 

 2 A-G, p. 12), with few exceptions, consists of a long, relatively thick 

 rod which narrows abruptly at its tip to a delicate prolongation of 

 variable length. In the living state this tailpiece, or whiplash, is ordi- 

 narily invisible in many forms, but in others it may be seen when the 

 zoospore is quiescent or comes to a temporary stop. Some spores, how- 

 ever, appear to lack it altogether. There is evidence that the tailpiece 

 may be an extension of the more fluid endoplasmic core of the flagellum. 



Manton, Clarke, Greenwood, and Flint (1952) made a study with 

 an electron microscope of the posteriorly uniflagellate zoospores of 

 Allomyces (Blastocladiales) and Olpidium (Chytridiales) which further 

 explains flagellar structure of the whiplash type. In Allomyces, under 

 magnifications of 10,000, a wide translucent sheath may be seen 

 surrounding a central axis. Faint longitudinal striations are detected 



