794 AQUATIC PHYCOMYCETES 



The uninucleate swarmers are cleaved out within the sporangium and 

 discharged after a highly characteristic and distinctive series of cytoplas- 

 mic changes (see Biisgen, 1882; Hartog, 1887; Rothert, 1888; Hum- 

 phrey, 1893; Schwartze, 1922; Couch, 1924). Briefly, these changes as 

 noted by Hartog and, particularly, by Humphrey are as follows : 



1. The protoplasm in the young sporangium at first surrounds a 

 central axial vacuole. 



2. Irregular lines of cleavage extend outward from the central vacuole. 

 These increase in number, connect with one another, and delimit the 

 spore initials as irregularly polygonal uninucleate masses. At this time, 

 if at all, the discharge papilla makes its appearance. 



3. A homogeneous stage develops abruptly, accompanied by a loss 

 of turgidity of the sporangium. The cleavage lines or extensions of the 

 central vacuole now suddenly become invisible because of the increase 

 in size of the zoospore initials. Small vacuoles appear and disappear in 

 these initials, the contents of which become less granular. Fluid, possibly 

 of vacuolar origin, is probably expelled at this time. 



4. The small vacuoles vanish, the cytoplasm again becomes granular, 

 and the zoospore initials contract and separate into rounded individuals. 



The structure of the zoospores is discussed in the "Introduction," 

 p. 9. Their behavior differs depending on which genus is involved. 

 In the Saprolegniaceae, the method of liberation and subsequent activity 

 after escape are relatively constant for a given fungus and are of great 

 diagnostic importance in distinguishing genera. Diplanetism (dimor- 

 phism *) is present in Isoachlya, Saprolegnia, Leptolegnia, and in certain 

 species of Ectrogella. Vestiges of it, according to most interpretations, 



1 Bessey's (1950: 106) statements concerning terminology are pertinent. He says: 

 "It is customary to refer to the characteristic of forming primary zoospores only, 

 as monoplanetic and of the formation of two successive types of zoospores as dipla- 

 netic. More correctly these two terms should be monomorphic and dimorphic (italics 

 ours). Properly speaking monoplanetic means wandering once, or with only one 

 swimming stage, while diplanetic means with two swimming stages. Since in Pythiop- 

 sis the primary type of zoospores may swim and encyst several times and in Achlya, 

 Dictyuchus, and other genera, the secondary type of zoospore may also do the same 

 thing the customary terms are not used in their correct etymological sense." 



Unfortunately, Bessey has not given us a proper term to apply to an almost 

 universal condition found in higher aquatic Phycomycetes, formation of the second- 

 ary type of zoospore only. 



