THE GREEN ALGAE OF NORTH AMERICA 215 



A very common plant over the whole world, and extremely 

 variable in form, thickness and color. Two fairly marked 

 types can be distinguished in the species as found with us on 

 both Atlantic and Pacific coasts, connected by innumerable 

 forms. 



Var. RIGIDA (Ag.) Le Jolis, 1863, p. 38; Farlow, 1881, p. 

 42 ; P. B.-A., No. 407. Frond at first lanceolate or ovate- 

 lanceolate, firm and stiff, with a distinct stipe ; later somewhat 

 irregularly divided, and often with numerous perforations of 

 various sizes ; cells vertically elongate in cross section. 



This is a common form of exposed shores, but occurs also 

 sometimes in quieter waters. In its earlier stages it is dis- 

 tinctly lanceolate in outline, but this form is afterwards lost by 

 irregular growth. It is firm in texture, the color deepening as 

 the plant grows older, finally becoming brownish or blackish ; 

 the cells have their greatest length at right angles to the surface 

 of the frond, being sometimes three times as long as wide. 



Var. LATISSIMA (L.) DC., 1805, p. 9 ; P. B.-A., No. LXXVI. 

 Frond irregular in outline, soon becoming detached and passing 

 most of its life in a floating condition ; thinner than var. rigida, 

 lighter colored, and with cells nearer square in cross section. 



This is a common form of creeks and lagoons, where it forms 

 floating sheets, often of several square meters in extent. 



Var. MESENTERIFORMIS (Roth) Collins, 1900, p. 45. Frond 

 much contorted and bullate, forming crumpled masses, lying 

 loose on the bottom. 



This form is strikingly distinct in appearance, forming much 

 crisped and wrinkled masses, usually of a dark green color, 

 lying on the bottom in creeks and quiet bays. It is so twisted 

 and grown together that only by tearing can even a small piece 

 of it be spread out flat. In cross section the cells are nearer 

 square than those of the type. It is common in marshy pools 

 near Bridgeport, Conn., and will probably be found in similar 

 places. Known elsewhere only in the Baltic. 



2. U. CALIFORNICA Wille, P. B.-A., No. 611. Frond 1.5 

 to 2 cm. long, up to 1.5 cm. wide, triangular or reniform with 

 wavy edge, sometimes with proliferations of a few cells each ; 

 passing quickly into a flattened, tapering stipe. The cells of 

 the stipe, which on the inner side form rhizoidal prolongations, 

 are in cross section about quadrate ; the cells in the upper part 



