236 University of California Puhlioations in Botany [Vol. 8 



Key to the Species 



1. Frond saccate until late, then rupturing 2 



1. Frond expanded very early 3 



2. Membrane 15-20m thick, dehcate in texture 1. M. Grevillei (p. 236) 



2. Membrane 25-45m thick, tougher 2. M. arcticuin (p. 238) 



3. Frond light green on drying, adhering well to paper 4 



3. Frond darkening on drying, not adhering well to paper 8. M. fuscum (p. 242) 



4. Membrane 40-50m thick 7. M. orbiculatum (p. 242) 



4. Membrane less than 30m thick 5 



5. Frond 7-10m thick 6 



5. Frond 20-25m thick 7 



6. Membrane distinctly areolate, cells distinctly grouped 



4. M. areolatum (p. 240) 



6. Membrane not areolate, cells slightly, if at all, grouped 



3. M. zostericolu (p. 238) 



7. Cells arranged in distinct groups of 4-6, rounded 5. M. quatemarium (p. 240) 



7. Cells not in distinct groups, angular 6. M. latissimum (p. 241) 



1. Monostroma Grevillei (Thur.) Wittr. 



Frond attached, at first saccate, then opening at the top, and 

 ultimately splitting to the base; soft and delicate, pale green; mem- 

 brane 15-20ju, thick, cells quadrate with rounded angles, closely set, 

 horizontally oval in cross section, 12-14jLt high ; sporiferous cells 

 enlarged, vertically elongate in cross section ; cell wall dissolving 

 after emission of spores. 



On stones in the upper sublittoral and lower littoral belts, Alaska. 



Wittrock, Monostr., 1866, p. 57,* pi. 4, f . 14 ; Collins, Green Alg. 

 N. A., 1909, p. 209; Setchell and Gardner, Alg. N.W. Amer., 1903, 

 p. 208. Enteromorpha Grevillei Thuret, Note sur la Syn. Ulv., 1854, 

 p. 25. 



Certain of the species of Monostroma retain the saccate habit until 

 late, or rather until the sack has reached considerable size, before 

 splitting. Two species, at least, on our coast do this; one is Mono- 

 stroma Grevillei and the other is M. arcticum. In M. Grevillei, 

 especially if obtained in position, the saccate habit is usually easily 

 observed or inferred, even when the plant is split to the base into 

 segments. When detached fragments, especially of some size, are 

 collected, the saccate habit may not be in evidence. Monostrovia 

 Grevillei is of delicate consistency and lubricous, differing in both 

 these characters from any of the forms of M. arcticum. It is also 

 decidedly thinner than M. arcticum. Otherwise the two species are 

 much alike. Rosenvinge (1893, p. 949 and 1894, p. 152) has united 

 them as varieties of one species. We are inclined, however, to follow 



