1922] Setchell-Gardner : Phycological Coninhutions 335 



We are considering Myrionema primarium to be a typical repre- 

 sentative of a group whose members are the most primitive of the 

 genus as considered from the standpoint of differentiation. The 

 prostrate basal layer is composed of long, regularly radiating filaments 

 with apical growth. Kadial divisions of apical cells occur just often 

 enough to completely occupy all of the space between each other as 

 the plant continues to increase in diameter and in circumference. The 

 branching 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 

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

 exception or modification being that some cells give rise to long fila- 

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

 outer cells long and colorless. All other erect filaments are trans- 

 formed 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. 



Myrionema primarium f. acuminatum forma nov. 



Plate 32, figure 9 



Frondibus microscopicis, inter algas parvas alteras crescentibus ; 

 filamentis erectis pro parte sterilibus. SO-lOO/x longis, cellulis leviter 

 doliif ormibus ; gametangiis 4r-5jx diam., acuminatis, pro parte brevi- 

 pedicellatis. 



Growing on Macrocystis pyrifcra. Carmel Bay, Monterey County, 

 California. Type, Gardner, no. 3110& (Herb. Univ. Calif., no. 207014), 

 December. 



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 (plate 32, fig. 9), and that a few, 

 about one in twenty-five, of the erect filaments continue to grow, attain- 

 ing a length of two to three times as long as the gametangia. 



