1925] Setchell-Gardner: Melanophyceae 457 



Growing on the outer end of the young blade of Costaria costata 

 in the lower littoral and the upper sublittoral belts. Oregon (Coos 

 Bay) to central California. 



Setchell and Gardner, Phye. Cont. II, 1922, p. 334, pi. 34, fig. 12. 



We consider Myrionema primarium to be a typical representative 

 of a group whose members are the most primitive of the genus when 

 considered from the standpoint of differentiation. The prostrate 

 basal layer is composed of long, regularly radiating filaments with 

 apical growth. Radial divisions of apical cells occur just often enough 

 to completely occupy all of the space between one another as the plant 

 continues to increase in diameter and in circumference. The branch- 

 ing is always dichotomous, and is accompanied by widening of the 

 apical cell, and the establishment of two growing regions on opposite 

 corners, which are subsequently separated from the remainder of the 

 cell by a wall, thus establishing two equal branches. Beginning in the 

 center, each cell successively toward the periphery gives rise by hori- 

 zontal divisions to erect filaments of nearly equal length, the only 

 exception or modification being that some cells give rise to long 

 filaments, the so-called true hairs, with the meristem at the base, and 

 the outer cells long and colorless. All other erect filaments are 

 transformed into gametangia. Only the cells of the basal filaments 

 remain sterile, or probably a few short erect filaments at the periphery 

 of the frond may never come to maturity. Thus starting with a single 

 cell the maximum of reproductive cells arises in this group. Starting 

 with such forms the course of evolution seems to have been in the 

 direction of the sterilization of tissue. 



2. Myrionema primarium f. acuminatum S. and G. 



Fronds microscopic, growing among other small algae, erect fila- 

 ments in part sterile, 80-100/t long, cells slightly doliiform ; game- 

 tangia 4-5//. diam., acuminate, in part short-pedicellate. 



Growing on Macrocystis pyrifera. Carmel Bay, Monterey County, 

 California. 



Setchell and Gardner, Phyc. Cont. II, 1922, p. 335, pi. 32, fig. 9. 



There is but a slight difference between M. primarium and forma 

 acuminatum. It is worthy of note that many gametangia in the center 

 of the thallus are short-pedicellate, and that a few, about one in 

 twenty-five, of the erect filaments continue to grow, attaining a length 

 two to three times as long as the gametangia. 



