958 AQUATIC PHYCOMYCETES 



when nutriment is not readily available lobation may be less pronounced 

 or even absent, in which case an "olpidioid" type of thallus and spo- 

 rangium may be formed. 



KEY TO THE SPECIES OF PETERSENIA 



In marine algae; thallus elongate and lobed or consisting of a com- 

 plex of lobulations 



Thallus predominantly elongate, strongly lobed P. lobata, p. 95» 



Thallus consisting of a complex of strongly inflated broadly pynform 



closely interlocked lobes P- pollagaster, p. 959 



In hyphae of fresh-water filamentous Phycomycetes; thallus tubular, 



branched P. irregulare, p. 961 



Petersenia lobata (H. E. Petersen) Sparrow 



Dansk Bot. Ark., 8 (6): 13, pi. 2, figs. I-N. 1934 



(Fig. 76 D-I, p. 960) 



Pleotrachelus lobatus H. E. Petersen, Oversigt Kgl. Danske Vidensk. Sels- 

 kabs. Forhandl., 1905: 460, fig. V, 1-7. 



Sporangium elongate, somewhat cylindrical, with lobes of various 

 sizes, particularly near the extremities, sometimes subspherical, ellip- 

 soidal, and lobed or unlobed, 30-192 p long by 15-30 p in width, at 

 times extending through several cells of the alga, with from one to 

 three discharge tubes of varying length; zoospores pyriform, 4.5 [x 

 long by 3 \l in width, with a refractive anterior globule and two 

 anteriorly attached oppositely directed flagella; resting spore not ob- 

 served. 



Weakly parasitic or saprophytic in Callithamnion corymbosum, coll. 

 K. Rosenvinge, C. hookeri, Spermothamnion turneri Petersen (he. cit.), 

 S repens var. turneri, Sparrow (he. cit.), Denmark; Callithamnion 

 roseum, Sparrow (1936b: 245, pi. 2, figs. 1-2), United States; Seiro- 

 spora interrupta, J.andG. Feldmann (1940:72, fig. 1), Callithamnion 

 sp., Aglaothamnion sp., Herposiphonia tenella, Seirospora interrupta, 

 probably also on tetraspores of Ceramium, Aleem (1950d: 427, fig. 

 10), Spermothamnion repens, Seirospora griffithsiana, i. Feldmann 

 (1954:134), France; Ceramium rubrum, C. pedicellatum, C. cortical- 



