76 



THE SIMPLE HOLOCARPIC BIFLAGELLATE PHYCOMYCETES 



and encyst (fig. 43), later germinating and leaving 

 a hyaline shell behind as in Achlya, Ectogella, etc. 

 Diplanetism has also been shown by Scherffel and 

 Couch to occur in L. enecans and Lagenidium sp. 

 (figs. 52, 53). It is thus obvious that species of 

 Lagenidium may exhibit a wide range of variation 

 in place and time of sporogenesis and zoospore be- 

 havior. 



The zoospores are generally described as bean- 

 shaped, but in species which have been critically 

 studied this appears to be the initial shape only. As 

 they grow older they become more pyriform and 

 reniform with a pointed anterior and a rounded 

 posterior end and a ventral groove. The flagella are 

 inserted laterally in this depression, and in most 

 species they have been figured as equal in length. 

 The secondary swarmers in L. enecans and L. Oedo- 

 gonii (figs. 81, 32, 41, 42), however, are heterocont 

 with the shorter flagellum extending forward, ac- 

 cording to Scherffel ('25), but in Lagenidium sp., 

 Couch illustrated them as isocont (fig. 53). Their 

 method of swimming is rather smooth and regular 

 in contrast with the darting movement of the chytrid 

 swarmspores, and in this respect they are very simi- 

 lar to those of Achlya, Saprolegnia, etc. 



The antheridia and oogonia may occur in the same 

 thallus and among the sporangia (fig. 16) or in sepa- 

 rate thalli. The elongate multicellular thalli more 

 often bear both gametangia. In the small unicellular 

 species the thallus is usually divided by a transverse 

 septum into two cells at maturity, which become 

 the so-called antheridia and oogonia, respectively, 

 while in other reduced species conjugation may 

 occur between separate unicellular thalli. Species 

 in which both gametangia are borne on the same 

 tli alius have been generally referred to as monoe- 

 cious, while those in which the respective gametangia 

 are borne on separate thalli are regarded as dioe- 

 cious. It is not known and has never been demon- 

 strated, however, whether the so-called dioecious 

 thalli have arisen from zoospores of the same or 

 different sporangia and thus represent distinctly 

 male and female strains. Until monozoospore stud- 

 ies have been made and it has been shown that such 

 thalli possess the potentialities of only maleness or 

 femaleness or both, it is premature to describe some 

 species of Lagenidium as heterothallic or dioecious. 



The so-called oogonium may be elongate, spindle- 

 shaped, oval, ellipsoidal, almost spherical, locally 

 paunchy, and irregular, with or without conspicu- 

 ous protuberances. The antheridium is usually 

 smaller, more elongate and cylindrical, but varies 

 somewhat in the different species. In L. rabenhorstii, 

 L. marchalianum and L. enecans it is relatively 

 slender and cylindrical like the vegetative filaments. 

 In L. Cyclotellae and L. Oedogonii it is usually 

 slightly smaller than the oogonium and not particu- 

 larly different in shape. In most species it forms a 

 conspicuous perforation or conjugation tube — i.e., 

 L. rabenhorstii (fig. 14), L. marchalianum (fig. 37), 

 L. enecans (fig. 33), etc, while in the smaller species, 

 L. Oedogonii and L. Cycloiellae, the tube is not very 



evident, according to Scherffel's drawings (figs. 44, 

 51). 



Zopf's description of fertilization in L. raben- 

 horstii is the only careful and detailed account of 

 the process for the whole genus. After the conjuga- 

 tion tube has pierced the oogonium wall, the granu- 

 lar refractive protoplasm of the antheridium con- 

 tracts into a globular mass and accumulates at the 

 side adjacent to the oogonium (fig. 12). Shortly 

 thereafter it begins to flow into the latter, and after 

 approximately three hours the process is completed 

 (figs. 13, 14). While this is going on the ooplasm be- 

 gins to contract toward the tip of the conjugation 

 tube, and at the same time the granules, bodies, and 

 discrete elements of the protoplasm undergo visi- 

 ble movements. As the contraction continues the 

 ooplasm becomes more and more coarsely granular, 

 and by the time the antheridium is empty, the gran- 

 ules have coalesced into two large refringent glob- 

 ules (fig. 13), which later usually fuse into a single 

 larger one (fig. 14). Fusion is then complete, and 

 eventually the zygote becomes invested with a thick 

 wall. 



It is to be particularly noted from Zopf's account 

 that no egg cell and periplasm are differentiated 

 prior to fusion as in the Saprolegniales and Perono- 

 sporales, respectively. The contraction of the 

 ooplasm during plasmogamy, however, may perhaps 

 foreshadow this development. Cook ('35), on the 

 other hand, described the formation of an egg cell 

 with a definite wall prior to fusion in L. rabenhorstii, 

 and in his treatment of the whole genus he fre- 

 quently referred to the presence of an oosphere. His 

 figures, however, are not very clear and convincing, 

 and in the writer's opinion the presence of a well- 

 differentiated egg cell remains to be shown. 



In parthenogenetic species the antheridium is 

 lacking, and the ooplasm contracts, rounds up and 

 becomes invested with a thick wall. Germination of 

 the oospores has been reported only in one species. 

 In L. rabenhorstii this occurs within 24 hours after 

 fertilization, according to Cook. "The wall of the 

 oospore breaks down and a single zoospore is lib- 

 erated, which .... is almost spherical in shape, 8 /i 

 in diameter and is provided with two large flagella" 

 (fig. 17). He described this motile cell as a sexual 

 zoospore in contrast to the asexual zoospores pro- 

 duced in sporangia. Inasmuch as nothing is known 

 about the time and place of meiosis as well as sex 

 determination in Lagenidium, Cook's use of the term 

 sexual in this relation is obviously premature and 

 unwarranted. 



So far no cytological study of fixed and stained 

 material has been made of the genus. Accordingly 

 nothing is known about mitosis and the details of 

 cytokinesis. It also remains to be seen whether the 

 gametes are uni- or multinucleate at the time of fer- 

 tilization, whether karyogamy immediately follows 

 plasmogamy, and at what stage meiosis occurs. Such 

 problems must first be solved before the relationship 

 of Lagenidium with the higher Oomyeetes becomes 

 clear. 



