Download PDF

1925] Setchell-Gardner: Melanophyceae 489
1. Hecatonema Lawsonii S. and G.
Plate 53, figs. 5-7
Fronds forming microscopic cushions, 250-500/* 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/*, diam. ; cells
of the hair filaments quadrate at the base and surrounded by a sheath,
zoosporangia unknown
up to 25 times as long as the diameter above ;
gametangia cylindrical to slightly fusiform, 25-30/* long, 5-6.5/x broad
many loculi formed by longitudinal and by oblique walls.
Growing on the sporophylls of Nereocystis Luetkeana. Uyak Bay,
Alaska.
Setchell and Gardner, Phyc. Cont. IV. 1922, p. 379, pi. 40, figs. 5-7.
Myrionema vvlgare Collins, Holden and Setchell, Phyc. Bor.-Amer.
(Exsicc), no. 924 (not of Thuret). Myrionema strangulans Setchell
and Gardner. Alg. N.W. Amer., 1903, p. 249 (not of Greville).
This plant superficially resembles very closely Myrionema foecundum
f. simplirissimum 8. 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 most 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-
matie 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 vulgare 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.
;