18 



THE SIMPLE HOLOCARPIC BIFLAGELLATE PHYCOMYCETES 



crease in size and maturity the wall of the thallus 

 becomes well defined and conspicuous, while the pro- 

 toplasm takes on a greyish-granular appearance 

 like that of the Saprolegniaceae (figs. 1-2). Numer- 

 ous small vacuoles are usually present during the 

 early development stages (figs. 6, 7) but these 

 gradually fuse into a large central one with an ir- 

 regular outline (figs. 10, 19). As a result the re- 

 mainder of the protoplasm is displaced towards the 

 periphery of the sporangium where it forms a com- 

 paratively thin, irregular, parietal layer. Simultane- 

 ous with these changes and the transformation of the 

 thallus into a zoosporangium the valves of the dia- 

 tom cell are spread apart (fig. 18), and the exit tubes 

 begin the push out. These vary greatly in number, 

 diameter, and length, and in E. Licmophorae be- 

 come thick-walled and inflated at the base, whereby 

 they are able to separate the valves of the diatom. 

 As a result of this spreading apart of the diatom 

 shell and the growth of the exit tubes, the zoospo- 

 rangium may become partially extramatrical, and in 

 E. perforans and E. Licmophorae especially, it has 

 much the same appearance and relation to the host 

 as that of Eurychasma. 



The exit tubes and adjacent portions of the ma- 

 ture sporangium wall give a marked cellulose reac- 

 tion when treated with chloro-iodide of zinc, while 

 the remainder of the wall reacts only slightly or not 

 at all. Petersen (05) interpreted this difference as 

 a matter of age and activity. Scherffel found that 

 the empty zoospore cysts likewise give a marked 

 cellulose reaction. 



Cleavage and zoosporongenesis occur in the same 

 manner as in the Saprolegniaceae according to 

 Scherffel. Furrows progress centrifugally from the 

 border of the central vacuole (figs. 10, 19, 22) and 

 thus delimit the zoospore initials. As these furrows 

 progress numerous small refractive globules may 

 appear at the inner periphery of the zoospore rudi- 

 ments (fig. 23). When the cleavage furrows have 

 reached and cut through the plasma membrane, the 

 central vacuole disappears, and the whole sporan- 

 gium takes on a coarsely granular appearance and 

 the zoospore initials are no longer visible (fig. 11). 

 This is known as the homogeneous granular stage of 

 zoosporongenesis. It is doubtless the result of a re- 

 hydration and swelling of the zoospore anJagen, as 

 Harper ('99) described for Synchytrium, whereby 

 the lines of demarkation become very faint or in- 

 visible. After some time the outlines of the zoospores 

 become visible again (fig. 11), and shortly there- 

 after they begin to shake, wobble, and glide upon 

 eacli other. These are the so-called primary swarm- 

 ers which are oval to pyriform in shape and may be 

 aflagellate or have two equal rudimentary flagella 

 attached laterally (fig. 13), or at the anterior end 

 (figs. 26, 30, 31). These primary swarmers may 

 emerge and swim directly away as in Saprolegnia, 

 or glide out and encyst at the mouth of the exit tubes 

 as in Achhia (figs. 14,27,32). 



Scherffel regarded the method of cleavage and 

 zoosporongenesis described above as typical of the 



Saprolegniaceae and not of the Chytridiales, and 

 for this and other reasons he maintained that Ectro- 

 gella is a member of the former family. As the pres- 

 ent writer has pointed out previously ('37) cleavage 

 in this genus is not fundamentally different from 

 that described by Harper ('99, '14), Swingle ('03), 

 Sehwarze ('22) and others for the Mycetozoa, Chy- 

 tridiales, Ooomycetes and Zygomycetes in general. 

 The point which Scherffel tried to emphasize is not 



plate 5 



Ectrogella bacillariacearum 



(Figs. 2-5, 17, 18 after Zopf, '84; fig. 35 after Petersen, 

 '09; remainder after Scherffel, '25.) 



Fig. 1. Early infection of Synedra cell. 



Fig. 2. Synedra cell with 21 small parasites. 



Fig. 3. Synedra cell with one elongate tubular parasite. 



Fig. 4. Surface view of infected Synedra cell. 



Fig. 5. Mature sporangium with central vacuoles. 



Figs. 6-9. Stages of growth and maturation of spo- 

 rangium. Small vacuoles fusing to form central row of 

 larger vacuoles. 



Fig. 10. Centrifugal cleavage. 



Fig. 11. Contracted granular stage following cleavage. 



Fig. 12. Reappearance of outlines of cleavage segments. 



Fig. 13. Primary zoospores with rudimentary flagella. 



Fig. 14. Synedra cell with encysted Ectrogella zoospores 

 at mouth of exit papillae. 



Fig. 15. Empty cysts and secondary zoospores. 



Fig. lfi. Laterally biflagellate heterocont secondary 

 zoospores. 



Figs. 17, 18. Empty zoosporangia with several exit pa- 

 pillae. 



E. monostoma 



Fig. 19. Mature zoosporangia undergoing cleavage; exit 

 papilla at A. 



Fig. 20. Empty cysts and cystospores at mouth of exit 

 papilla. 



Fig. 21. Laterally biflagellate heterocont secondary zoo- 

 spores with a large vacuole. 



E. Gomphonematis 



Figs. 22-25. Cleavage stages in a zoosporangium. 



Fig. 26. Anteriorly biflagellate primary zoospore. 



Fig. 27. Empty sporangium with two exit papillae. Zoo- 

 spores encysted at one papillae, while those which emerged 

 from other papilla swam away. 



E. Licmophorae 



Fig. 28. Portion of a Licmophora cell showing a naked 

 amoeboid uninucleate thallus with pseudopods at left; 

 two uninucleate thalli with walls near host nucleus in 

 center; and a tetranucleate thallus at right. 



Fig. 29. Mature sporangium with cleavage segments. 



Figs. 30, 31. Anteriorly biflagellate primary zoospores. 



Fig. 32. Zoospores encysted at mouth of exit papillae. 



Fig. 33. Parthenogenetic resting spore (?) in rudimen- 

 tary oogonium (?). 



Fig. 34. Antheridium (?) and oogonium (?) with 

 oospore (?) connected by fusion canal (?). 



Fig. 35. Resting spore of E. perforans. 



