PERONOSPORALES 1043 



calary on the ordinary elements of the mycelium or on richly branched 

 lateral hyphae, spherical or more commonly unsymmetrical and bur- 

 siform, the long axis often nearly at right angles to that of the concom- 

 itant hypha, forming a more or less prolonged slender evacuation tube, 

 internally proliferous; zoospores somewhat renifoim, each containing a 

 single vacuole and bearing two oppositely directed flagella of approx- 

 imately equal length emerging from a shallow longitudinal groove, ex- 

 pelled from the sporangium as an undifferentiated irregularly tubular 

 mass surrounded by an elongate quickly evanescent vesicle, undergoing 

 their maturation free in the water, capable of repeated emergence; 

 hyphae bearing the sex organs often helically involved; oogonium ter- 

 minal or intercalary, spherical or polygonal, antheridium usually single, 

 terminal or with a short appendage, arising (always?) from the same 

 hypha as the oogonium; oospore spherical, filling the oogonium, with 

 an enormously thickened refractive often concentrically layered wall 

 and a large globule, upon germination converted (always?) into a zoo- 

 sporangium after resorption of the greater part of the wall. 



Saprophytic on vegetable debris in fresh water. 



Because of the lack of essential data in Minden's descriptions and of 

 the variations in shape and position of the zoosporangia his species are 

 difficult to distinguish on the basis of the sporangial stage alone. A study 

 of Minden's illustrations of his species, which are presumed to represent 

 typical material, reveals the following qualitative differences between 

 them. The sporangia of Pythiogeton utriforme and P. ramosum are pre- 

 dominantly terminal. Those of P. transversum are usually (but not al- 

 ways) intercalary, the distal part of the hypha often appearing as a 

 slender appendage near the orifice of discharged sporangia. The spo- 

 rangia of P. utriforme are somewhat more broadly pouchlikc, their dis- 

 charge tubes are more nearly in line with the mycelial axis, and they 

 are borne on a more sparingly branched mycelium than are the narrowly 

 bursiform ones of P. ramosum. 



Sexual stages have been associated with Pythiogeton utriforme and P. 

 transversum. 



Species of the genus are as yet poorly defined in the literature, and 

 further work will be necessary before taxonomic criteria of significant 

 import can be established. Evaluation of the shape of the sporangium 



