220 TUFTS COLLEGE STUDIES, VOL. II, No. 3 



The minute linear fronds with filiform stipe and truncate 

 apex, distinguish this species from others of the genus. 



2. P. FLUVIATILIS (Sommerf.) Areschoug in Lagerstedt, 

 1869, p. 28; Wittr. and Nordst., Alg. Exsicc., No. 1234; P. 

 Sauteri, Jessen, 1848, p. 15, PI. I, figs. 4-9. Fronds cuneate- 

 lanceolate to obovate, to 10 cm. high, 2 cm. wide, apex obtuse, 

 not truncate ; cells near the base in a few series, above more 

 numerous ; series of cells and intercellular spaces parallel be- 

 low, more areolate in the upper part ; cells 4-6 /j. wide ; frond 

 13-19 /A thick, cells 8-13 /x in cross section. Greenland, Alaska. 



Europe. 



A plant of cold mountain streams, iti general shape some\vhat 

 like P. calophylla, but larger, with rounded apex, and without 

 monosiphonous stipe except at very early stages. 



3. P. MEXICANA J. G. Agardh, 1847, p. 6 ; Jessen, 1848, p. 

 19, PI. I, figs. 17-20 ; P. B.-A., No. 1186. Frond up to 10 cm. 

 long and wide, of more or less rounded outline, attached by a 

 short, thickened stipe; cells 6-16 /* diam., rather closely set ; 

 asexual reproduction by division of frond, by akinetes more 

 rounded than the vegetative cells, and with slightly thicker 

 wall ; and by aplanospores formed 8-16 in a cell, in the upper 

 marginal portion of the frond, from subglobose to oblong, 6-8 ^ 

 wide, up to 14 /u. long. In cold mountain streams, sometimes at 

 very high altitudes. Wyo., Oregon, Colo., Mexico. 



So. America. 



In appearance quite like a small Ulva ; the areolate character 

 is not so conspicuous as in some other species ; at the formation 

 of the aplanospores the margin for a greater or less depth be- 

 comes softer and of lighter color, very much in the same way as 

 on the formation of the antheridia in Porphyra. 



4. P. BOREALIS Reed. 1902, p. 160, PI. XV. Fronds cune- 

 ate to obovate, stipitate or sessile, margin crenulate, crisped or 

 entire, soft membranaceous, 33-45 p- thick, 5-10 mm. high, in 

 tufts of several from one holdfast ; cells in distinct tetrads, areo- 

 lar arrangement manifest ; cells 4-9 p. diam., seen superficially ; 

 in cross section oblong or palisade-form, 11-14 P- high- On 

 rocks near high water mark. Alaska. 



The fronds of this species are very generally infested with a 

 fungus, Guignardia Alaskana Reed ; it has its counterpart in 

 antarctic regions in Prasiola tesselata Kiitz., infested by G^lig- 

 nardia Prasiolae (Winter) Reed. 



5. P. CRISPA (Lightf.) Meneghini, 1838, p. 36; Jessen, 



