F. Borgesen: Rhodophyceæ of the Danish W. Indies. 



21 



Fig.ll. Acrochætium crassipes 

 var. ty pica. A and B, two 

 plants growing on Cen- 

 troceras ; B, with a hair 

 (300 : 1). C, piece of a branch 



mersed with the basal part in the wall 



of the host plant, in agreement with 



M. A. Howe's description of Acrochætium 



catenulatum'^). 



The basal cell bears as a rule one 



or two erect filaments which most often 



are more or less curved and decumbent 



and graduallly taper towards their 



summits, the basal cell being the 



thickest of all. 



Hairs seldom occur ; most of the plants 



are quite destitute of them. Where they with monosp'orangia (300 : 1) 



are present they are always to be found fj'^'p^^ ZTfliS^t 



at the tips of the filaments (Fig. 11 B). 



The chromatophore is stellate with a central pyrenoid. In 



most of the cells plenty of starch is present and the contents of 



the cells then have a quite homogenous appearance ; but when 



boiled and coloured e. g. by means of hæm- 

 alun the stellate chromatophore is easily seen. 



var. longiseta nov. var. 



Upon specimens of Chætomorpha antennina 

 from the harbour of St. Thomas and on 

 Chætomorpha brachygona from Christianssted 

 was found abundantly a small Acrochætium 

 which shows so much likeness to Acrochætium 

 crassipes that I have no doubt in referring it 

 to this species from which it essentially differs 

 by the usual presence of long hairs. 



The basal cell originating from the ger- 

 minating spore is, as is the case in var. typica, 

 the largest of the whole plant (Fig. 12), being 

 8 — 10 ^y. in diameter. It is fixed to the host 

 plant by means of a ring of cementing sub- 

 stance and, so far as I have been able to see, 

 it is also often somewhat immersed in the 

 wall of the host (Fig. 12, 13 e). From the 

 basal cell arise a single or 2 — 3 suberect 

 branches, the cells of which grow gradually 

 thinner and at the same time longer towards 



Fig. 12. Acrochætium 

 crassipes Børgs. var. 

 longiseta nov. var. 

 Plant with a single 

 sporangium and hairs, 

 in the cells the stel- 

 late chromatophore 

 with the central pyre- 

 noid. (about 800 : 1). 



^) M. A. Howe, The marine Algæ of Peru ("Memoirs of the Torrev Bo- 

 tanical Club", vol. XV, 1914, p. 84). 



