1925] Setchcll-Gardnrr: Melanophyceae 489 



1. Hecatonema Lawsonii S. and G. 



Plate 53, figs. 5-7 



Fronds formings microscopic cushions, 250-500/i. diam., irregular 

 in outline ; prostrate portion composed of very much contorted, pro- 

 fusely branched filaments with subterminal forking; erect filaments 

 forming a compact, palisade-like stratum interspersed with numerous 

 hair filaments, 0.75-1.5 mm. long; cells of creeping filaments dividing 

 radially as well as tangentially, forming a pseudoparenchymatous 

 tissue in the center of the thallus, irregular in shape, 4-5/a diam. ; cells 

 of the hair filaments quadrate at the base and surrounded by a sheath, 

 up to 25 times as long as the diameter above ; zoosporangia unknown ; 

 gametangia cylindrical to slightly fusiform, 25-80/* long, 5-6.5/* broad ; 

 many loculi formed by longitudinal and by oblique walls. 



Growing on the sporophylls of Nereocystis Luetkeana. IJyak Bay, 

 Alaska. 



Setchell and Gardner, Phyc. Cont. IV, 1922, p. 379, pi. 40, figs. 5-7. 

 Myrioncmo vnJgare Collins, Holden and Setchell, Phyc. Bor.-Amer. 

 (Exsicc), no. 924 (not of Thuret). Myrionema strangidans Setchell 

 and Gardner, Alg. N.W. Amer., 1903, p. 249 (not of Greville). 



This plant superficially resembles \evy closely Myrionema foecun- 

 ditm f. simpJicissimuni S. and G. Microscopic examination, however, 

 reveals several important differences. The creeping filaments are 

 unique. The branches are very numerous, come off at wide angles and 

 seem, for the mast part at least, to be subterminal, or if the terminal 

 cell splits, one of the dichotomy very frequently fails to develop till 

 much later. Many of the cells in the center, and even toward the 

 periphery, divide radially and perpendicular to the surface of the 

 host, forming a pseudoparenchymatous layer. Thus the species, 

 strictly speaking, cannot be said to be distromatic, since the distro- 

 matic condition of a frond is really brought about by radial divisions 

 of the cells of the filaments, but parallel to the surface of the host. 

 The character of the gametangia and of the creeping filaments will 

 not permit of its being placed with Myrionema vnlgare Thuret as 

 further described and figured by Sauvageau (1897, p. 186, et seq.). 

 On account of the radial divisions of the creeping filaments and the 

 prevailing biseriate condition of most of the gametangia, we have 

 placed this species with the genus Hecatonema. 



