INTRODUCTION 13 



flagellum of the secondary spore, is flattened and finlike in life with an 

 internal framework of fine hairs. If this is true, it would, along with the 

 greater length, explain the more powerful swimming action of the sec- 

 ondary zoospore as compared with the primary one. An essentially 

 similar type of flagellar structure on the secondary zoospore of Phy- 

 tophthora has been reported by Ferris (1954). 



The cysts formed by the biflagellate primary zoospores have curious 

 double-headed processes, which strongly resemble miniature boat hooks, 

 scattered over the surface (Manton, et ah, 1951). Meier and Webster 

 (1954) found that the cysts formed by the motile primary spores of 

 Saprolegnia and Isoachlya are essentially smooth-walled, although some 

 bear tufts of radiating hairs that are believed by them to be the remains 

 of flagella. Those of the secondary swarmers, however, have the stalked 

 double-headed hooks. In Protoachlya, Achlya, and Brevilegnia neither 

 type of cyst bore hooks and in Dictyuchus sterile the cysts of the second- 

 ary spores bore large spiny projections. The latter two investigators 

 believe that more extensive study of cysts may show them to be of value 

 in determining relationships within the family. Incidentally, their find- 

 ings confirm Rothert's (1894) as to the fate of the flagella of the second- 

 ary zoospore; namely, they are shed. The flagella of the primary zoo- 

 spore are, however, resorbed save for the tufts of radiating hairs pre- 

 viously alluded to. These are, in fact, the Flimmer-like hairs of the 

 flagellum. 



The movements of the various types of zoospores were extensively 

 studied by Couch (1941) with the aid of dark-field illumination. He 

 found that those of the posteriorly uniflagellate type are propelled by 

 the transmission of waves through the flagellum in one plane and in 

 one direction with respect to the spore. A spore may swim (1) in a wide 

 circular orbit; (2) in a straight line, either rotating or not rotating on 

 its axis; or (3) in a spiral path and rotating on its axis. If the spore ro- 

 tates on its axis the flagellum presents an alternating single and double 

 image, the single image when it undulates in a plane vertical to the ob- 

 server, the double when it undulates in a plane which is horizontal or 

 diagonal to him. Further information on the movement of the chytrid 

 swarmer is given on page 60. 



With respect to the anteriorly uniflagellate type of zoospore that 



