LAGENIDIALES 909 



remaining vacuoles become regular in outline, and the nuclei themselves, 

 regularly spaced in the cytoplasm, go into the resting state (Fig. 74 G). 

 No evidences of amoeboid movement or of the fusion of thalli to form 

 a Plasmodium were observed by Barrett or McLarty. 



The thallus of the Sirolpidiaceae (Petersen, 1905; Sparrow, 1934c, 

 1936b; Aleem, 1953; Kobayasi and Ookubo, 1953) is tubular, septate, 

 and branched or unbranched. In Sirolpidium two main types of vege- 

 tative structures are formed. The first is somewhat spherical or elliptical, 

 distinctly olpidioid, and occurs singly or in dense linearly arranged 

 groups (Fig. 77 B, D, p. 967). These bodies are the thalli which were 

 considered by de Bruyne(1890) to be typical of Sirolpidium (Olpidium) 

 bryopsidis. At maturity each is provided with a single discharge tube, of 

 varying length, which usually penetrates the wall of the host. The 

 precise method of formation of such groups has not been ascertained. 

 It has been suggested by Petersen (1905) that they have been produced 

 from zoospores discharged inside the alga. The second type of thallus 

 formed in Sirolpidium is an exceedingly interesting one. In very early 

 stages the body of the fungus is a somewhat irregular curved cylindrical 

 tube, often lying near the periphery of the central cavity of the host cell 

 (Fig. 77 A). The thallus, after a period of growth, characterized mainly 

 by elongation, becomes separated by constrictions into a linear series of 

 spherical or cylindrical segments of varying size, each of which at 

 maturity is disarticulated and becomes an independent, walled unit 

 (Fig. 77 C, E, G). These fragments then for a time undergo individual 

 growth. Each one soon loses its original orientation with respect to the 

 parent thallus and ultimately becomes a single sporangium equipped 

 with a discharge tube. 



The thallus of Pontisma (Petersen, 1905; Sparrow, 1934c; Aleem, 



nuclei in division. E. Young sporangium, showing peripheral arrangement 

 of cytoplasm. F. Portion of developing sporangium in which large vacuoles 

 are forming and nuclei are undergoing final division. G. Portion of sporangium 

 after segmentation of zoospores has occurred. H. Vacuolate receptive (large) 

 and contributing (small) thalli containing irregularly distributed dividing 

 nuclei. /. Connection between the two thalli established, nuclei of smaller 

 thallus dividing, those of larger, resting. 

 (Barrett, 1912b) 



