THE GREEN ALGAE OF NORTH AMERICA 221 



1848, p. 18, PI. I, figs. 10-16; Wolle, 1887, p. 106, PI. XCI, 

 figs. 25-27; P. B.-A., Nos. 1069, 1184. Frond of indefinite 

 form, rounded, lacerate, or plicate, without stipe, attached to 

 substratum by the edge of the frond or by fine fibrils ; usually 

 in dense masses ; fronds sometimes as much as 10 cm. high or 

 wide, but usually much smaller ; generally 13-16 /u. thick ; cells 

 squarish or rectangular, 8-13 /u. diam., in cross section square or 

 slightly higher than wide. Fig. 77. In moist and especially in 

 unclean places. Greenland, Newfoundland, Alaska, Cal. 



Europe. 



The description given above is of the normal adult frond ; 

 along with this are generally found very narrow forms, only two 

 or three cells wide, and also forms of a single series of cells ; 

 the latter agreeing in every way with what has been known as 

 Hormidium parietinum or H. murale ; these have been distri- 

 buted as P. B.-A., Nos. 969, 1274; but every gradation can be 

 found between these and the typical Prasiola fronds, and there 

 is little doubt that under certain conditions or in certain stages 

 the Prasiola develops the Hormidium forms. The areolate 

 structure is not so clear in P. crispa as in some other species, 

 but it shows distinctly enough in the younger plants, becoming 

 less distinct as the plant grows older.* 



3. GAYELLA Rosenvinge, 1893, p. 936. 



Frond filiform, simple or very slightly branched, at first of a 

 single series of cells, later dividing longitudinally into many 

 series, but always remaining filiform, not flat ; cell structure as 

 in Prasiola. Marine. 



G. POLYRHIZA Rosenvinge, 1893. p. 937, figs. 45 and 46; 

 P. B.-A., No. 914. Frond at first a simple filament of a single 

 series of disk-shaped' cells, 10-12 //. diam., attached to the sub- 

 stratum by a rhizoidal projection from the lower cell ; later 

 attached at various parts of the filament by rhizoidal growths, 

 one or two from a cell ; increasing in diameter by growth and 

 division of cells, up to 70 n diam.; terete or somewhat irregular 

 in surface, but not flattened ; cells with parietal chromatophore 

 and one pyrenoid ; in the mature plant showing superficially an 

 arrangement in longitudinal and transverse Hues ; in cross sec- 

 tion an arrangement by 2-4-8-16, etc., in somewhat Gloeocapsa- 

 like form ; asexual reproduction by aplanospores, arranged in 



*P. Gardneri Collins, P. B.-A., No. 1185. there placed with some doubt 

 in this genus, seems on further study to belong rather to the blue-green 

 algae. 



