F. Børgesen: Rhodophyceæ of the Danish W. Indies. 141 



thallus may tear horizontally after having acquired a thickness 

 of 11 cells or more. The inferior 4 — 5 cells are the largest, the 

 cells higher up are smaller in diameter, and the peripheral cells 

 are the shortest of all. 



Between the inferior large cells and the succeeding ones, 

 which are somewhat smaller, the membrane thickens. This thicke- 

 ning of the membrane takes place in horizontal direction, and 

 may spread over some distance. In this thick membrane ap- 

 pears at first a slight opening, this enlarges, tears the membrane 

 horizontally and divides the perithallus into a superior and inferior 

 part; these can remain jointed but very often they tear asunder. 



Fig. 146. Peyssonnelia Nordstedtii nov. spec. Section through thallus in 



transverse direction in the moment that the upper part tears itself from 



the partially decaying inferior part. (210 : 1). 



In either case the upper part will increase in thickness and its 

 inferior cells increase in size, and then the membrane will again 

 thicken at the same place as formerly, above the four or five 

 layers of large cells. If the perithallus is not torn, we see the 

 succeeding layers forming one continuous mass. 



But often the perithallus splits into two layers; if this hap- 

 pens we observe first a little opening in the thickened membrane, 

 this enlarges quickly and divides the perithallus into two parts 

 (Fig. 146). 



The upper part transforms its basal cells, that were formerly 

 in the middle of the ascending filament, into a young hypothallus, 

 with uni- and pluricellular rhizines that penetrate in the inferior 

 part of the perithallus. 



