BLASTOCLADIALES 679 



expanded lobes or short clavate branches or to cylindrical slightly taper- 

 ing dichotomously, subdichotomously, or sympodially arranged bran- 

 ches on which the reproductive organs are borne, septa only delimiting 

 the reproductive organs, the whole anchored to the substratum by a 

 system of branched holdfasts; zoosporangia sessile, with a single apical 

 discharge papilla and, often projecting downward from this, a refractive 

 peglike plug; zoospores posteriorly uniflagellate, with a hyaline often 

 conspicuous subtriangular nuclear cap, formed in the sporangium, after 

 the deliquescence of the papilla emerging individually or in an evanes- 

 cent vesicle, the papilla and peg sometimes persistent; resting spore with 

 a thick minutely punctate wall, persistent or deciduous, rounded or 

 beaked, with a truncate base, completely filling the thin-walled case in 

 which it is borne, formed on the same or on different plants from the 

 sporangia, upon germination producing planonts; gametophyte appar- 

 ently not formed. 



Saprophytes on decaying twigs and fruits, on which they form crisp 

 hemispherical pustules. Some species may occur alone, but usually they 

 are found growing with other water molds such as Rhipidiwn, Gonapod- 

 ya, and the like. One species, Blastocladiu pringsheimii, is undoubtedly 

 extremely common in all fresh-water habitats and it has been collected 

 from soils. 



No absolutely conclusive evidence for the occurrence or nonoccurrence 

 in this genus of sexuality or of a gametophyte phase has yet been 

 presented. This has been due largely to the difficulty in obtaining ger- 

 mination of the resting spores. Minden (1916) observed early stages of 

 germination, but evidently did not follow the fate of the swarmers. 

 Miss Blackwell (1937) secured abundant germination of the resting 

 spores in Blastocladia pringsheimii. The results of her investigations on 

 the further development of the swarmers (Blackwell, 1940) indicate that 

 there is no gametangial plant or alternation of generations in this species. 

 The zoospores from germinating resting spores formed small thalli on 

 which were produced ordinary sporangia and zoospores. The latter were 

 never observed to fuse, but, rather, gave rise to thalli similar to the par- 

 ent plant. The function, if any, of the sterile setae formed on some plants 

 has not been determined. 



As elsewhere noted, Emerson and Cantino (1948) and Cantino (1948, 



