38 



THE SIMPLE HOLOCARPIC BIFLAGELLATE PHYCOMYCETES 



"about" equal length. Whether or not the heterocont 

 species should he segregated in a separate group on 

 this basis is of questionable diagnostic value until the 

 zoospores of all species have been intensively stud- 

 ied. According to Couch ('41) one of the flagella of 

 0. Saprolegniae has a long distinct tail piece, while 

 the other one possesses nine to eighteen lateral "tin- 

 sel" cilia, 1.5 /a long, which may occur on one or both 

 sides or follow a spiral course. The same type of 

 flagella will probably be found to occur in all species 

 of Olpidiopsis. 



During growth and development of the thalli the 

 nuclei undergo simultaneous and synchronous divi- 

 sion (figs. 34, 35, 37), according to Barrett and Mc- 

 Larty. Nuclear division is typically indirect and not 

 of the so-called promitotic type reported for the 

 Plasmodiophorales, although the resting nuclei pos- 

 sess a very large nucleole and little chromatin in the 

 form of granules on a reticulum (fig. 39). Succes- 

 sive division stages from thalli of 0. Achlyae are 

 shown in figures 39 to 53. The fact that most of the 

 stainable material is in the form of a globular nu- 

 cleole which often appears to be vacuolate or made 

 up of differentially stainable regions (figs. 41, 42) 

 suggested to McLarty that the nucleole functions as 

 a storehouse of chromatin. This is further substan- 

 tiated, in his opinion, by the radial orientation of the 

 chromatin network on the nucleole (figs. 40, 41) and 

 the latter's gradual disappearance as the prophase 

 changes progress. Barrett, on the other hand, found 

 a well-defined dense chromatin reticulum in addition 

 to a large nucleole in resting nuclei of the female 

 thalli (figs. 58 to 60). McLarty interpreted the 

 emergence of radially oriented chromatin threads 

 as the beginning of the prophases (figs. 40, 41), and 

 as these stages progress the chromatin becomes more 

 evident as globular (fig. 43) or rod-shaped densely 

 stained bodies (fig. 44). By this time the nucleole 

 has usually disappeared. Concurrent with the emer- 

 gence of chromosomes a deeply stainable centro- 

 some-like body appears at one of the nuclear poles in 

 O. Achlyae (fig. 43) and undergoes division (figs. 

 45, 46). The daughter bodies thus formed then 

 gradually migrate around the nuclear membrane 

 (fig. 47) to the opposite poles of the nucleus (figs. 

 48, 49, 50). Barrett, however, found no evidence of 

 centrosomes in 0. vexans. No conspicuous astral 

 rays have been found in association with these bodies 

 in O. Achlyae, nor does the division spindle appear 

 to arise from them as far as is now known. Accord- 

 ing to Barrett, the latter originates from the chro- 

 matin mass in the equator of the nucleus and gradu- 

 ally grows towards the poles, but his figures of the 

 process are not definite and clear. In profile views 

 of the equatorial plate stages the chromosomes are 

 often arranged in a broken ring (fig. 50) around the 

 margin of the spindle, but usually they are closely 

 crowded together and appear as a dark band. In 

 polar views of this stage (fig. 51) McLarty found 

 six chromosomes in O. Achlyae which is the same 

 number reported by Barrett for O. vexans. No evi- 

 dence of meiosis has been found in these divisions in 



the thalli and zoosporangia. The halves of the chro- 

 mosomes separate in the metaphases and migrate 

 (fig. 52) to the poles as two compact deeply stained 

 masses (fig. 53), which may often be connected by 

 a slender chromatin filament (figs. 53, 76). The nu- 

 cleus and spindle elongate considerably in the late 

 prophases and metaphases, and by the time of the 

 late anaphases the nuclear membrane has usually 

 disappeared entirely. The formation of the daughter 

 nuclear membranes and the telophasic reconstruc- 

 tion stages of the nuclei are not well known, al- 

 though figure 76 suggests that the daughter chromo- 

 some groups become surrounded by clear spaces, the 

 boundaries of which later become the nuclear mem- 

 branes. Nuclear division in the so-called antheridia 

 and oogonia is also mitotic (figs. 68 to 76). In O. 

 I'exans, however, Barrett found prophase stages in 

 which the chromatin was aggregated in synaptic- 

 like masses at one side of the nucleus (fig. 72), but 

 he did not believe that they relate to prophases of 

 meiosis. 



Resting Spore Development and Sex 

 Differentiation 



As has been noted in the generic diagnosis above 

 the resting spores of Olpidiopsis may develop par- 

 thenogenetically without sexual fusion or as the re- 

 sult of fusion of a large female thallus with one to 

 several smaller male thalli. Some species, i.e., 0. 

 gracile and O. Pythii, appear at present to be wholly 

 parthenogenetic, while others are only partially so 

 or entirely sexual. The small male and larger female 

 thalli are generally referred to in the literature as 

 antheridia and oogonia, respectively, and the fusion 

 of these cells has been regarded as a primitive or 

 rudimentary type of heterogamous and oomycetous 

 sexual reproduction. Petersen and Scherffel re- 

 garded the resting spores of Olpidiopsis as an 

 oospore in an oogonium which lacks periplasm, and 

 on these grounds the latter worker in particular 

 postulated the origin of a Pythium-Peronosporaceae 

 series from simple species through Olpidiopsis. Use 

 of the terms antheridia and oogonia for the male and 

 female thalli respectively in this genus is obviously 

 open to serious question since these terms in their 

 proper sense relate to gametangia which produce 

 more or less differentiated gametes. Furthermore, in 

 some species, i.e., O. andreei, the thalli which fuse 

 may be equal in size, so that sexual reproduction is 

 occasionally isogamous. Nevertheless, the develop- 

 ment and evolution of antheridia and oogonia and 

 the oomycetous type of sexual reproduction is 

 clearly foreshadowed in Olpidiopsis. In the discus- 

 sion which follows the non-committal terms male 

 and female will be used for the thalli which fuse. 



In the early developmental stages in living ma- 

 terial, the female thalli are identical in appearance 

 to those which are to develop into zoosporangia, and 

 it is not until thev have attained considerable size 



