THE GREEN ALGAE OF NORTH AMERICA 219 



ened stipe ; cells with stellate chromatophore and one pyrenoid, 

 dividing to form groups of fours, these groups forming similar 

 larger groups, the spaces between the groups of various orders 

 constituting narrower or wider spaces, running in definite direc- 

 tions through the frond. Asexual reproduction, ist, by the 

 breaking off of small portions of the frond, which attach them- 

 selves and grow independently ; 2d, by akinetes formed from 

 individual cells assuming thick walls : these akinetes may 

 develop either directly into a filament or membrane, or indi- 

 rectly by aplanospores, several in each akinete ; 3d, by aplano- 

 spores, formed 4-8 in a cell, by walls in 2 or 3 directions ; sexual 

 reproduction unknown. 



The species of this genus resemble small forms of Ulva or 

 Monostroma ; most of them are so minute as to be barely notice- 

 able, but some may reach a length of 10 cm. Some species 

 grow in cold mountain streams, but most of them are found in 

 moist places, not immersed ; some inhabit by preference quite 

 uncleanly stations ; a few species are marine. While there 

 have been reports of the production of zoospores, it is now gen- 

 erally supposed that these reports were based on misapprehen- 

 sions, and that the reproduction is asexual and non-motile. In 

 the character of the frond, and in the formation of spores, there 

 are quite interesting similiarities between Prasiola and Porphyra. 



KEY TO THE SPECIES OF PRASIOLA. 



i. Marine. 4- P- borealis. 



i. Fresh water or terrestrial. 2. 



2. Frond with long, stipe-like base. 3. 



2. Stipe short or wanting. 4. 



3. Frond near base of a single series of cells. i. P. calophylla. 



3. Frond always of several series of cells. 2. P.fluviatilis. 



4. Terrestrial. 5. P. crispa. 



4. In cold mountain streams. 3. P. mexicana. 



i. P. CALOPHYLLA (Spreiig.) Meneghini, 1838, p. 36; Jes- 

 sen, 1848, p. 14, PI. I, figs. 1-3; P. B.-A., No. 1273. Fronds 

 linear to narrowly cuneate, with truncate apex, many from 

 the same holdfast, seldom over i cm. long, i mm. wide ; cells 

 near the base in a single series, about 10 ^ long by 3-5 ^ broad ; 

 farther up in two rows, the number increasing towards the upper 

 part of the frond or as the frond grows older ; the series of cells 

 and the intercellular lines nearly parallel throughout ; cells near 

 the apex of the frond about 3-5 ^ square ; thickness of frond 

 about 15 /A; cells S-io /j. high in cross section. Wash., Cal. 



Europe. 



