LAGENIDIALES 957 



spores ellipsoidal, laterally biflagellate ; resting spore not observed. 

 In spiny and smooth resting spores of Sphaerita endogena, Dangeard 

 (loc. cit.), France. 



PETERSENIA Sparrow 



Dansk Bot. Ark., 8 (6): 13. 1934 



(Figs. 75 H, p. 939; 76 C-I, p. 960) 



Pleotrachelus sectio lobati H. E. Petersen, Oversigt Kgl. Danske Vidensk. 

 Selskabs. Forhandl., 1905:460. 



Thallus appearing at first somewhat plasmodial, at maturity lobed, 

 contorted, or tubular, rarely ellipsoidal and unlobed, holocarpic, with- 

 out a specialized vegetative system; sporangium with from two to 

 several discharge tubes (rarely one); zoospores laterally biflagellate, 

 formed within the sporangium, where they assume motility, escaping 

 individually through pores at the apex of the discharge tubes; resting 

 spore not observed. 



Primarily parasitic or saprophytic in marine algae; one species 

 (Petersenia irregulare) parasitic in the hyphae of fresh-water aquatic 

 Phycomycetes. 



In the method of formation of the zoospores, as well as in their shape 

 and flagellation, Petersenia strongly resembles Olpidiopsis. The irregular 

 lobed or tubular character of the mature thallus, however, and, to a 

 lesser degree, the formation typically of more than one discharge tube 

 distinguish it. 



As understood here, Petersenia includes only forms with irregular 

 tubular or lobed thalli. The species with more regular spherical or 

 ellipsoidal sporangia also found in marine algae {Pleotrachelus sectio 

 integri H. E. Petersen, op. cit., p. 448) are considered too closely allied 

 to Olpidiopsis to be segregated generically. They differ so far as is 

 known from members of Olpidiopsis only in their marine habitat and in 

 being generally provided with more than one discharge tube. Further 

 work may reveal that they have distinguishing characters of more 

 significant taxonomic worth. 



In both marine species the thalli are predominantly lobed under 

 optimum conditions for development. When the plant is crowded or 



