910 AQUATIC PHYCOMYCETES 



1953), though resembling to a degree that of Sirolpidium, is more 

 irregularly cylindrical and usually forms short branches (Fig. 78 A, p. 

 969). At maturity thin cross walls are laid down which delimit irregular, 

 constricted segments. Rarely, what appears to be a rudimentary type 

 of fragmentation may be found. The typical mature thallus consists of 

 a series of somewhat variously shaped connected segments. Occasionally 

 it may be an unsegmented cylindrical structure which curves around 

 the periphery of the utricle of the host cell (Fig. 78 B). The contents of 

 the thallus, though varying in aspect from time to time, generally 

 contain during the period of vegetative activity a large number of 

 regularly placed vacuoles. In more mature plants the protoplasm con- 

 sists of a mass of closely packed refractive granules. 



Each of the segments of the thallus is ultimately transformed into a 

 sporangium having a single discharge tube (Fig. 78 E). Tubes not orient- 

 ed so as to secure egress from the host cell may ramify extensively 

 within the substratum, frequently simulating to an astonishing degree 

 the hyphae of filamentous Phycomycetes. Unusual powers of growth 

 have also been observed in the discharge tubes of Sirolpidium (Fig. 77 F, 

 p. 967) (Sparrow, 1934c, 1936b). 



The development and morphology of the thallus of aquatic genera of 

 the Lagenidiaceae are well known from the excellent researches of Zopf 

 (1884), Scherffel( 1925a), and others. In both Lagenidium and Myzocy- 

 tium the zoospore accomplishes infection of the host in the same manner 

 as in the Olpidiopsidaceae. Zopf noted, however, that the cyst and the 

 infection tube of L. rabenhorstii persisted throughout the life of the 

 thallus, whereas those of Myzocytium soon disintegrated. A similar 

 situation has been found by Scherffel (op. cit.) in several other species 

 of Lagenidium. In L. rabenhorstii the minute sphere of protoplasm, 

 once established within the alga, elongates to form a relatively thick 

 tube. As this increases in length, irregular swellings, which originate as 

 papilla-like or clavate protrusions from the main body, are formed on 

 it. These outgrowths arise at varying intervals and may develop into 

 long, sometimes curved, branches which curl around the inner face of 

 the host-cell wall. After from twenty-four to seventy-six hours of purely 

 vegetative growth this one-celled, usually branched, tube, which ordi- 

 narily never penetrates into adjoining host cells, becomes segmented by 



