F. Børgesen: Rhodophyceæ of the Danish W. Indies. 15 



and the erect filaments of the Acrochætium have not much 

 difficulty in immerging from it. From this stage of endophytism, 

 if such it ought be called, there is an even transition to those 

 species (as Acrochætium occidentale and Aer. comptum) fixed to the 

 assimilating filaments of Liagora. The space between the fila- 

 ments of the species belonging to this genus is filled with mucilage 

 and also with more or less of a chalk incrustation. The basal 

 filaments of the above mentioned Acrochætium species creep 

 epiphytically upon the filaments of the host plant, but they are 

 on the other hand immersed in the mucilage. Quite immersed 

 in this mucilage is Acrochætium Liagoræ. 



In other species again the endophytic filaments are able to 

 penetrate more or less into the cellular tissue of the host plant. 

 Here also graditions may be observed. In some species e. g. 

 Acrochætium robustum, unipes and others only short endophytic 

 processes are present. In Acrochætium hormorhizum the basal part 

 is immersed in the thick wall of the host plant. Acrochætium repens 

 has endophytic filaments creeping extensively in the tissue of the 

 host plant sending up here and there erect free filaments. 



Further as pointed out by Kylin and later by Rosenvinge 

 the shape of the chromatophore is of great systematic importance. 

 Referring for details to Rosenvinge's description I will here only 

 mention that by far the greatest number of the West Indian 

 species have a parietal more or less lobed chromatophore with a 

 lateral excentric pyrenoid protruding more or less into the lumen 

 of the cell. In other species a stellate chromatophore with a 

 central pyrenoid is found. 



The following classification of the species is based essentially 

 upon the different development of the basal part, next upon the 

 shape of the chromatophore and other characters. 



Key to the West Indian species of Acrochætium. 



A. Epiphytes. 



a. A single undivided basal cell. 



1. With sex-organs, chromatophore parietal 



1. A. Sargassi. 



2. Without sex-organs, chromatophore stellate .... 



2. A. crassipes. 



b. The germinating spore is divided into two cells . . . 



3. A. pulchellum. 



c. Basal layer multicellular, composed of creeping fila- 

 ments fusing more or less together. 



