F. Børgesen: Rhodophyceæ of the Danish W. Indies. 131 



two and three remnants of old thalii underlying a young one. It is 

 curious that the old thalii decay in this species, whereas in P. 

 (C.) dura — the type of Heydrich's Polystrata — all the thalh 

 are preserved. This depends, as I believe, on the fact that P. 

 (C.) dura has a calcareous frond and that P. (C.) Nordstedtii, as 

 we will presently see, is mostly of soft texture with only little 

 calcareous matter distributed throughout the frond. 



The anatomical structure of the Peyssonneliæ is much alike; 

 it consists, as we know, of creeping fdaments — the hypothallus 

 — and each cell of these filaments gives ofT, seen on longitu- 

 dinal sections, an obliquely ascending filament, these constitute 

 the perithallus. The cells of the perithallus differ in heigth and 

 breadth in different species, but still these characters are to be 

 used very carefully, for the size of the cells will vary in the same 

 specimen according to the spot where the section is made and 

 according to the age of the plants. On the other hand, when 

 studying many species of this genus, we find different species 

 with cells of almost the same size. Searching for a character 

 that might help to distinguish the puzzling species, I was struck 

 with the fact that the topcell or apical cell divided difTerently 

 in some species, though these divisions corresponded essentially 

 with the divisions of P. (Eup.) squamaria. 



The anatomical structure of this alga has been, as we well 

 know, the object of careful research by Naegeli^), who has also 

 given good figures to show, how the filaments of the hypothallus, 

 seen from above, divide dichotomously to increase their number, 

 according to the fan-shaped growth of the frond. 



The longitudinal growth of the filament that interests us for 

 the present, depends upon the partition of the apical cell by a 

 more or less oblique membrane on its longitudinal axis. In 

 P. (Eup.) squamaria and — as far as I can judge — in most 

 species, this apical cell is relatively high (in vertical direction) 

 and horizontally short. After division, the outermost segment 

 grows on, until it has reached its former size and then divides 

 again. The inner segment increases also in size, it may divide 

 again by a vertical cellwall, though this is rarely the case; as a 

 rule it divides, after having attained a given size, by a hori- 

 zontal wall into two unequal portions of which the uppermost is 

 destined to become the mothercell of the ascending filament and 

 the inferior constitutes part of the hypothallus. This inferior cell 

 may be less high than the superior one {P. (Eup.) squamaria) or 



1) Nakgeli, Die neueren Algensysteme, 1847, p. 248. 



