572 TRANSACTIONS OF SECTION K. 



9. The Life-Cycle and Affinities of the Plasmodiophoracese. 

 By T. G. B. Osborn, M.Sc. 



During the last few years there has been a considerable increase in our know- 

 ledge of the Plasmodiophoraceaj, largely owing to the work of Maire and Tison. 



The life-cycle, as far ae it has been described for the following genera, 

 Plasmodiophora, Sorosphcera, Ligniera, and possibly Tetramyxa, consists of the 

 following stages :— 



(1) A eehizont stage in which the nuclei divide in a distinctive manner. 



(2) An akaryote or chromidial condition after the close of the vegetative phase. 



(3) The nuclei are subsequently reconstructed, and pass through stages of 



synapsis, diakinesis (in the first two genera at least), and a double 

 mitosis, regarded as reduction divisions, after which uninucleated masses 

 of protoplasm become encysted. Prowazek's observation of a pseodogamy 

 in the sporocysts of Plasmodiophora lacks confirmation and support. 



Spongospora subterranea has recently been shown to have just such a similar 

 life-cycle as that outlined above, with one important addition. Following the 

 akaryote condition, the nuclei arrange themselves in pairs and then unite. A con- 

 dition resembling synapsis succeeds the karyogamy, and is followed by two 

 karyokinetic divisions. 



An examination has been made of Plasmodiophora brassicce, with the result 

 that appearances suggestive of fusion in all its stages have been observed. The 

 evidence is supported by a reduction in the number of nuclei and their increase in 

 size before the first karyokinetic division. 



By the kindness of Dr. Blomfield and Mr. Schwartz I also have been able 

 to make a comparison with Sorosphcera veronica, and here similar phases have 

 been observed. 



Maire and Tison have seen a pairing of the nuclei in Sorosphcera suggestive of 

 karyogamy, but for them the pairing is merely the telophase of division or else 

 accidental juxtaposition. Against the appearances here described being the telo- 

 phase of division is the fact that the nuclei are those of the sporont and not the 

 schizont. Against accidental juxtaposition, in addition to the appearances directly 

 suggestive of fusion, there is the improbability that more than half the nuclei 

 in a Plasmodium would be paired, and also the fact that the nuclei are reduced 

 in number while they are increased in size previous to karyokinesis. 



These observations tend to strengthen the suggested relationship of the 

 Plasmodiophoracese to the Myxomycetes, since their sexual life-cycle has so many 

 features in common. The connection of the Plasmodiophoraceae, through Ligniera 

 and Rhizomyxa, with the Chytridinefe put forward by Maire and Tison on mor- 

 phological grounds, has received valuable support from the cytological point of 

 view by the observations of Nemec on Sorolpidium. While further critical work 

 is needed on these organisms, it may be pointed out that the suggested affinities 

 are not mutually incompatible. 



10. Somatic Nuclear Division in Spongospora Solani (Brunch.). 

 By A. S. Horne, B.Sc, F.G.S. 



During the early stages of the life-history of this parasite (vegetative phase) 

 the nuclei divide by a peculiar form of karyokinesis. During prophase a definite 

 spireme is formed. Ultimately, four loop-shaped chromosomes appear. These, 

 during metaphase, join end to end to form an equatorial ring about the per- 

 sistent nucleus. This ring divides into two daughter rings. Each of the latter, 

 during anaphase, breaks up into four chromosomes. 



The nucleolus constricts, during anaphase, to form two daughter nucleoli. The 

 divisions appear to take place within the nuclear membrane. Spindle-fibres, few 

 in number, have been distinguished. No centrosomes have been observed. 



After a time the nuclei cease dividing by this form of karyokinesis. A period 

 of chromidial activity ensues, which affects the parasite whether in the form of 

 myxamceba, ccenocyte, or Plasmodium. 



The spindle-figures which are formed in these divisions vary considerably in 

 size. Centrosomes are present. The nucleolus may persist; if so, it is often 



