32 I'nivdsity of Californkt Public-atiotis in Botany [Vol.8 



Iceland plant differs from the Greenland plant onlj' in color, but for- 

 bears naming it on that character alone. The Iceland plant seems, 

 therefore, certainly to be a Pleurocapsa. Our plant divides vegeta- 

 tively in but two planes, and, accepting Thuret's understanding of his 

 genus Xenococcus, it belongs to that genus rather than to Pleurocapsa. 

 A brief discussion of the structure and relation of these two genera 

 is given in Gardner's New Pacific Coast Marine Algae II, 1918, p. 436. 



Xenococci(s acervatiis differs from P. amethystea in the number of 

 planes of vegetative cell divisions, in the shape and size of the cells 

 and in their color, the color of the latter being "sordide violacea, " 

 and the former pale blue-green. None of the seven illustrations of 

 Rosenvinge (loc. cit.) resembles very clearly any phases of the develop- 

 ment of our plant except A, the surface view of a group of vegetative 

 cells. 



At times the cells as viewed in the median plane of the host plant 

 are piled up several cells deep, as though they had arisen by horizontal 

 divisions. If this were the case, our plant would properly belong to 

 the genus Pleurocapsa. This docs not seem to be the case, however, 

 as the cells above the surface layer are very generally spherical, appar- 

 ently independent, and very variable in size. They appear rather 

 to be gonidia in various stages of development, that have come to rest 

 on the surface layer, or in some cases, seem to have grown in position 

 where they were formed. The cell walls are decidedly gelatinous, 

 which is conducive to holding the colonies together. 



2. Xenococcus Gilkeyae S. and G. 

 Plate 5, fig. 11 



Cells solitary or aggregated into small colonies, spherical when 

 solitary, angular and more or less elongated in colonies, 4—7 /x, rarely 

 9/A diam. ; cell wall inconspicuous, hyaline ; protoplast light blue-green ; 

 gonidangia of the same shape and size as the cells; gonidia 0.8-1/x 

 diam., formed by successive divisions of the protoplast. 



Growing on the filaments of Elachistea sp. which is epiphj^tic on 

 Fucus sp. Lower littoral belt. Sitka, Alaska. 



Setchell and Gardner, in Gardner, New Pac. Coast Alg. Ill, 1918a, 

 p. 462, pi. 39, fig. 11. 



Having vegetative cell divisions in but two planes perpendicular 

 to the substratum, Xenococcus Gilkeyae is a typical member of the 

 genus. It is an exceedingly delicate species but the type material 



