F. Børgesen: Rhodophyceæ of the Danish W. Indies. 17 



2. A multicellular vertical basal layer 20. A. phacelorhizum. 

 D. The whole plant immersed in the host plant . 21. A. Liagoræ. 



In addition is described below A.ernothrix the base of which 

 I have not succeeded in seeing clearly. 



1. Acrochsetium Sargassi nov. spec. 



Thallus usque ad 700;/ altus. E cellula basali, discum par- 

 vum formante, filum erectum, a basi ramosum, egreditur. 



Rami sparsi aut secundati aut oppositi, simplices aut ramosi, 

 ad apicem versus attenuati, in pseudopila sæpe producti, ex cellu- 

 lis in parte inferiori thalh 5,5// latis, 9—18;/ longis, in superiori 

 parte 2—3// latis, 30-40/« longis composite 



Ghromatophorum parietale, pyrenoide laterali instructum. 



Sporangia sparsa aut pauca secundata, in ramulis sæpe bina 

 præsentia, sessiha aut pedicellata, obovata, 10/« longa, 7// lata. 

 Antheridia in ramulis opposita aut plus minus irregulariter 

 aggregata, globularia, ca. 2// lata; carpogonia lageniformia, 

 sessilia. 



This plant reaches a length of up to 600 — 700 //. The 

 base (Figs. 7 and 8 a) consists of a flat disc (about 20// diameter) 

 formed by the original spore and apparently in a similar 

 way to that found in Acrochsetium {Chantransid) microscopicum 

 Nægl. var. collopoda Rosenvinge, described and figured by Ro- 

 SENViNGE in "Deuxieme Memoire sur les Algues mar. du Groen- 

 land" (Meddelelser om Grønland, XX, p. 412). By means of this 

 small disc the plant is fixed firmly to the host plant (old leaves 

 of Sargassum vulgare). 



From this disc is given off a single filament which immedia- 

 tely begins to branch (Fig. 7). The cells in the lowermost part 

 of the filaments are short and have thick walls ; they are about 

 9// long and 5,5 /« broad, in vigorous plants up to 8// broad. 

 Higher up the cells grow longer, up to about 18 /i, tapering at the 

 same time, and the filaments end with long, thin nearly colour- 

 less, hairlike prolongations which soon die away; in these the 

 cells are only 2—3// broad while their length is about 30—40« 

 or more. Rosenvinge also found such discoloured prolongations 

 in several species and compared them with the hair-Hke organs 

 in the Phæophyceæ. 



Dansk Botanisk Arkiv, Bd. 3. Nr. 1 (1915). 2 



