CHYTRIDIALES 51 



Group II 



Type 4. The zoosporangium develops from an enlargement of the germ 



tube as in Entophlyctis. 

 Type 5. The zoosporangium develops from a prosporangium which 



originates as an enlargement of the germ tube as in Diplophlyctis. 



Whether Whiffen's types accomplish some degree of natural grouping 

 is problematical, particularly in view of the many variations in types 

 of development recorded in recent years (see, for example, Antikajian, 

 1949; Karling, 1949d, f; Haskins, 1948; and the discussion under 

 Diplophlyctis, p. 384). Data derived from pure cultures in artificial 

 media are beginning to accumulate and they may or may not throw 

 light on just what is fundamental here. Until further evidence is forth- 

 coming it seems wiser not to make any final decisions. In the meantime, 

 to continue to be of some practical assistance to those who have material 

 from the field before them and who are not concerned as yet with theo- 

 retical matters, the admittedly somewhat artificial system of the first 

 edition of the Aquatic Phycomycetes is maintained. 



Structure of the Thallus 



The thallus of the chytrid consists of at least one center of repro- 

 ductive activity and, with the exception of the Olpidiaceae, Achlyo- 

 getonaceae, and Synchytriaceae, of a more or less well-developed nu- 

 trient-gathering system (Fig. 3 B, p. 48). In the three families of holo- 

 carpic chytrids just mentioned the absorption of materials doubtless 

 occurs over the entire surface of the thallus, which is then converted 

 as a whole into a reproductive organ (Fig. 3 A). The remaining eucarpic 

 chytrids are for the most part "rhizoidal," that is, the purely vegetative 

 part of the thallus consists of unbranched or branched, distinctly 

 tapering threads or "rhizoids," which are frequently of great delicacy 

 (Fig. 27 A, p. 428). By repeated branching the surface over which, 

 presumably, active absorption of materials occurs is enormously en- 

 larged. In species which form an apophysis (Fig. 19 H-I, p. 318) this 

 surface is still further increased. There is no direct evidence, however, 

 to show that all the endobiotic parts of a chytrid are active in the ab- 

 sorption of materials from the substratum. Sometimes the vegetative 

 system, instead of being rhizoidal, either is cylindrical and blunt-tipped 



