231 



The specimen from an exposed place which I am going to 

 describe was growing upon Amphiroa jragilissima originating from 

 a coral reef at the south coast of St. Croix. 



The specimens found here are scarcely 1 cm. high. Their 

 colour is more or less bluish green, while specimens from more 

 protected places and from deep water have a red brown colour. 



The thallus consists of a central ramified filament composed 

 of large cylindrical cells near the upper end of which a whirl of 

 4 di- or trichotomously ramified branchlets are issued (Fig. 219). 



The basal part of the fila- 

 ments is decumbent and fastened 

 to the host plant by means of 

 rhizoids. In the most vigorously 

 developed rhizoids the stalk is 

 moniliform owing to the oval 

 shape of the thickwalled cells, in 

 the less vigorous cylindrical ; the 

 stalk ends in short, irregularly 

 ramified filaments often forming 

 a small disc. These rhizoids grow 

 out from the basal cell of the 

 branchlets (Fig. 220). 



The cells in the central fila- 

 ments are cylindrical with thick 

 walls ; in vigorous filaments they 

 reach a length of about ISO// 

 and a breadth of 120 p. They are 

 not corticated. The branchlets are 

 repeatedly di- or trichotomously 

 ramified ; the cells in the branch- 

 lets are broadest and shortest 

 near the base, tapering gradually 

 upwards growing at the same time longer. The uppermost ones, 

 in any case in the lower part of the thallus, are often long cylin- 

 drical growing up between the filaments of the branchlets above, 

 and because of this, covering in most cases the main stem quite 

 densely (Fig. 219). 



In the cells of the branchlets we find well developed chro- 

 matophores. These consist of a parietal campanulate plate, down- 

 wards with large openings or with irregularly shaped prolongations. 

 In the cells of the central filaments the chromatophores, on the 



Fig. 219. Crouania attenuata 



(Bonnem.) J. Ag. Part of a main 



filament with branchlets. 



(About 240:1). 



