1918] Gardner: New Pacific Coast Marine Algae II 445 



of the genus, having a single layer of cells for the base which gives rise 

 to perpendicular, parallel, unbranched filaments extending away from 

 the host and bearing each a single terminal gonidangium. The whole 

 cushion or colom*, is in reality a single plant resulting from the growth 

 of a single gonidium. 



The plants of tlii.s species start from a single cell, and by a few 

 divisions a small group of cells is formed. Around the margin the 

 cells soon begin to arrange themselves radially in rows and by divisions 

 in two planes, radial and tangential, a circular disk, one layer of cells 

 deep, is formed. The marginal cells enlarge tangentially and are 

 cut into two cells by a radial wall, the resulting "filaments" are of 

 equal growth, as a rule, and the forking thus becomes dichotomous 

 (pi. 37, fig. 16). Plants often become so closely associated as to form 

 a continuous layer over the surface of the host. Even under this 

 condition they have no effect upon the host, so far as death and dis- 

 integration of the cells is concerned, and hence the}^ are strictly 

 epiphytic. The erect filaments result from the horizontal division of 

 the prostrate or basal cells. They form very dense, compact masses, 

 their cell walls adhering firmly (pi. 37, fig. 15). The gonidangia are 

 numerous, spherical or sliglitly oval, being transformed terminal cells 

 of the erect filaments (pi. 37, fig. 14). 



Radaisia clavata Setchell et Gardner sp. nov. 

 Plate 37, figs. 17-18 



Pulvinis in superficie hostis, minutis, dense caeruleo-viridibus, ad 

 lOOfi diam., in sectione media, plus rainusve flabf^lliformibus : filamentis 

 dense compactis, 70-lOOfi longis, ad fines externos sparse ramosis ; 

 eellulis 4-5/x diam. infero, supra 7-8/a, 3-4/t longis, partitionibus 

 cellularum frequenter irregulariter obliquis; parietibus cellularum 

 tenuibus, hyalinis; contentu cellularum linmogeneo: gonidangiis 8-9/x 

 diam., terminalibus, hemisphoricis ; gonidiis angularibus, 1-1. 5/x diam., 

 partitione simultanea. 



Plants forming microscopic, deep blue-green cushions on the 

 surface of the host, up to 100/m diam., more or less fan-shaped in 

 median section ; filaments very closely compact, 70-lOOyu, long, sparsely 

 branched near the outer ends. Cells 4-5/a diam., at the base, 7-8/a 

 above, 3-4/i, long, cell divisions often irregularly oblique, cell walls 

 thin, hyaline; cell contents homogeneous; gonidangia 8-9/x diam., 

 terminal, hemispherical ; gonidia angular, l-1.5j(i diam., formed by 

 simultaneous division. 



Growing on Gyninogongrus linearis, in the lower littoral "belt. 

 Lands End, San Francisco, California. This is the only known 

 locality in wliich this plant grows. The host plant is common along 



