1963 Volume 76 Issue 904 Pages 381-387
In Monostroma fuscum var. splendens the sexual plants and the asexual plants are isomorphic. The sexual plants are dioecious and the sexual reproduction is isogamous. The swarmers are liberated through round pores opening on the same side of the thallus. The gametes are elongated pear-shaped or fusiform, having two flagella at the anterior end. The zoospores are elongated pear-shaped, having four flagella at the anterior end. Both gametes and zoospores have one chloroplast with an eye-spot and a pyrenoid in the posterior part; they are positively phototactic. The gametes conjugate generally side by side to form the zygote. Both zygotes and zoospores germinate by pushing out a protuberance after a resting period. The cell divides with a transverse wall into two parts, one of which elongates and forms the primary rhizoid. By successive cell-divisions the germlings (derived from zygotes or zoospores) develop into a simple filament. Then longitudinal cell-divisions take place and the germlings have a cylindrical structure. After two months the germlings attain about 3mm. in height and develop into a tubular structure with a cavity in cross section. In such tubular structures a split later takes place resulting in an expanded membranous frond of a single layer of cells. After about ten months the plant derived from the zygotes produces 4-flagellated zoospores; the plant derived from the zoospores produces biflagellated gametes.
The type of the life history of M. fuscum var. splendens is quite different from those of other Monostroma, which have so far been studied, and it is identical with those of Ulva and Enteromorpha.