198 THE LOWER FUNGI— PHYCOMYCETES 



sporangia which discharge through other branches. The spo- 

 rangia are thus terminal or intercalary, and ultimately may form 

 throughout the entire mycelium. Here, as in the remainder of 

 the genus, the swarmspores pass out incompletely differentiated 

 into a spherical, thin-walled vesicle which later bursts allowing 

 their escape. Germination of the sporangium by a germ tube 

 has not been observed, i.e., so called conidia are lacking. The 

 antheridia and oogonia are cut off by septa except in a single 

 species, P. tenue Gobi, where the septum is absent at least in 

 the antheridial branch. As treated by Butler the subgenus 

 includes, P. teiiue, P. gracile Schenk, P. monospertmim Prings., 

 P. indigoferae Butler, and P. didyospermum Racib. The species 

 are chiefly parasites in aquatic algae. More recently other 

 species have been described. The fungus Rheosporangium 

 aphanidermatum Edson (1915), cause of a disease of radish known 

 as black-root and damping-off of beet seedlings has been shown 

 to belong here (Fitzpatrick, 1923). It is stated by Carpenter 

 (1921) to be the same as P. butleri Subramaniam (1919) which 

 causes a disease of ginger, tobacco, papaya, and sugar cane. Also 

 P. afertile Kanouse & Humphrey (1928) falls in this group. 



The subgenus Sphaerosporangium contains a larger number of 

 species. Butler recognizes thirteen, and others have been 

 described since, including P. graminicolum Subramaniam (1928), 

 P. torulosum Coker & Patterson (1927), P. irregular e Buisman 

 (1927), P. arrhenomanes Drechsler (1928), P. mamillatum Meurs 

 (1928), and others. No paper has yet appeared giving a separa- 

 tion of all the described species. 



The sporangium in Sphaerosporangium is usually spherical to 

 ovoidal, but may have the obpyriform shape more typical of 

 species of Phijtophthora. It is cut off from the fertile hypha by 

 a septum. Definite sporangiophores are lacking, the sporangia 

 arising throughout the mycelium from undifferentiated hyphae. 

 Though usually terminal the sporangia are sometimes intercalary, 

 and may exist in chains. When developed in the air they may 

 be deciduous, but when submerged usually remain attached. 

 Germination takes place either by swarmspores or by germ tube, 

 and in some species it is possible to predict one type or the other 

 in cultures under controlled environmental conditions (Fig. 

 70). The" conidia" of Butler and other writers are merely 

 sporangia undergoing direct germination. 



In swarmspore formation, as in Aphragmium, a thin-walled 

 vesicle is extruded, and the protoplasm passes out into it before 



