25 



regards external characters, being mostly short, thick, spongy and cuneate, many of them 

 without marked differentiation into stipes and frond, but some of them distinctly stipitate ; and 

 the frond-filaments are coarse (up to nearly 70 u) lightly constricted, tapering down to 20 >j. at 

 the apices, but not tortuous nor zig-zag, nor more than about twice branched at the apices. 

 Nor are the fïlaments of a fulvous colour, but mostly hyaline, dotted with chromatophores, 

 and only here and there containing brown (rarely fulvous) contents. Those plants however on 

 account of their external character are probably referable to f. fulva. 



As regards the rest of the specimens of A. nigricans, they are so perplexing in their 

 variability, that it seems quite impossible to separate them into satisfactory groups. Some plants 

 are very large with wide thin flabellum, thick coarse rhizome, and frond fïlaments of 60 [j. diam. 

 and tapering down to 20 ij. at their divaricately ramified apices. These large plants appear to 

 be of common occurrence, ranging at least from St. Thomas to Grenada. Others again are 

 quite small and slender, and among these is Decaisne's type-plant. Decaisne's type is preserved 

 in the Paris Museum and bears his label " Avrainvillea nigra Dne. Hes des Saintes prés la 

 Guadeloupe. M. d'Avrainville. 1842". Thanks to Monsieur Hariot's kindness, we are able 

 to give a figure of it (fig. 78). It is a small blackish plant about 7 cm. in length including 

 the slender rhizome ; it is composed of moniliform fïlaments, the spherico-oval beads of which 

 have a diameter of about 45 u. tapering down to about 25 p. at the slightly branched apices 

 (hg. 79). The frond is faintly zonate. A descriptive note of the specimen has been published 

 by Dr. Howe (Buil. Torr. Bot. Club XXXII. 1905 p. 567). 



Decaisne's plant is quite unlike the above-mentioned large ungainly specimens with thick 

 •coarse rhizome and wide thin flabellum from St. Thomas (" Challenger" Expedition) and from 

 Grenada (Murray). 



In the British Museum is another specimen from Hes des Saintes (the type-locality), 

 viz. Mazé's n° 1126. This is a large plant (16 cm. high, stipes 7.5 cm. long, frond 10 cm. 

 broad) collected without a rhizome. It is in fact about as large as the "Challenger" specimens, 

 but is composed of rather narrower filaments (35 u. diam.), than those of the type, and only 

 half as thick as those of the ''Challenger' specimens. In fact it seems as if size of plant 

 and size of filament in this species vary quite irrespectively of one another. Of the big 

 plants some have large, some have small filaments. Even so with the small plants. 



Again, in some plants the filaments branch dichotomously 1 — 4 times in quick succession 

 at their apices; and this is accompanied by a marked tapering in some, but not in others. 

 Thus a filament with a diameter of 60 ij. rapidly divides into branchlets tapering down to 20 u.. 

 These ultimate ramuli, by interweaving with one another, form a stronger protective layer 

 on the surface of the frond. We do not however know whether the presence or absence of 

 this branching and tapering is dependent on seasonal changes, or upon the depth at which 

 the plants grow beneath the surface of the sea ; or whether it is proper to some plants and 

 not to others, and therefore is capable of being employed as a systematic character. We do 

 not know the length of life of these plants, how soon they reach maturity, whether they 

 ■endure for a few or many months, whether after producing their numerous but inconspicuous 

 sporangia they wilt away. These are questions which cannot be solved in a herbarium. 



S1BOGA-EXPEDITIE LXII. 4 



