5. Chorda. "Fructification a mucous fluid in the hollow part of a cylindrical frond, with naked seeds affixed inward ly." The principal example of this genus is F. filum.
6. Codium. Fructification invisible to the naked eye ; frond roundish ; soft and spongy when wet ; velvety when dry." This embraces only the sponge-like foci, particular ly F. tomentosus, a fine downy or spongy sea-weed found on the south-west shores of England, (Turn. t. 135.) Stackhouse expresses his conviction, that his genera Focus and Sphxrococcus will soon fall to be further divi ded into several new genera ; and he mentions some spa des having anomalous fructification, particularly F. ligula tus, (Ft. Scot. t. 29.) and lycopodiodcs, (Turn. t. 12.) to which he had not been able to give a place in his arrange ment.
Dr Roth's Method Dr Roth, in his Catalecta Botanica, and in his Remarks on the study of Cryptogamic Water Plants, proposes the following genera : Focus, Ceramium, Batrachospermum, Conferva, Mertensia, Hydrodictyon, Ulva, Rivularia, Line kia, and Tremella. The plants usually denominated sea weeds, are contained in four of these genera, viz. Focus, Ceramium, Conferva, and Ulva.
Funs is characterised thus : " Vesicles (receptacles) ag gregate, imbedded in the• substance of the frond, and fur nished with mucifluous pores." Ceramium thus: Plant filiform, substance membrana ceo-cartilaginous, with adnate granuliferous capsules." Of this genus there are two divisions : 1st, With uniform ca pillary fronds, containing some of the more slender fuel, and of the unjointed capsuliferous confervx ; 2d, With the fronds irregularly jointed, comprehending the jointed foci, and the remainder of the capsuliferous confervx. It may here be noticed, that Mr Stackhouse at one time, while the French revolutionary wars prevented the naturalists of this country from knowing what was done by their brethren on the Continent, proposed to constitute a very different genus, embracing the broad smooth-fronded foci, under the title of Ceramium ; but that he afterwards dropt that name, and substituted the appropriate one of Palmaria.
The Confervce of Roth are defined as consisting of small tubes, or herbaceous filaments, with granules of fructifica tion scattered on the inside coats of the tubes ; and the C.Yme, as presenting expanded diaphanous membranes, with granules of imbedded fructification principally towards the margins, which the Doctor considered as liable first to de cay, and thus to liberate the seeds.
M. Decanaolle's Method.
M. Decandolle, in the Flora Gallica, and Fiore Fran faisc (1805), has given an arrangement of the submersed alga:, chiefly founded on the writings of Dr Roth, and M. Vaucher of Geneva. Eleven genera are enumerated : Nos toeh, Rivularia, Ulva, Fucus, Ceramium, Diatoma, Chan transia, Conferva, Batrachospermum, Hydrodictyon, and Vaucheria. Of these, Ulva, Focus, and Ceramium, corn prize the sea-weeds.
Ulva includes all those with membranaceous fronds, in which the seeds or capsules are placed under the epider mis, without any means of being discharged but by the de struction of the frond itself. The genus seems rather he terogeneous, and is divided into no fewer than six sections : (I.) Those that are gelatinous within, as F. tomentosus ; (2.) Those that are tubular, as the well-known Ulva cora prcssa ; (3.) Flat, without peduncle, and without midrib, as U. umbilicalis, or laver ; (4.) Flat, with a longitudinal midrib, as F. membranaceus ; (5.) Flat, with a peduncle, as the well-known tangle F. digitatus ; (6.) Flat, marked with transverse zones, as U. pavonia.
Fucus is characterized as flat or filiform, with the or capsules united in groups or tubercles, sometimes late ral, sometimes terminal ; the seeds being discharged by a distinct external pore. This description takes in a part only of the plants usually considered as fuci, particularly F. vesiculosus, serratus, siliquosus; some having, as alrea dy noticed, passed to the genus Ulva, and others, as F. filum, going to the following genus.
Ceranzium is distinguished by having filiform fronds, which are either simple or branched, and with or without articulations, bearing tubercles full of globules, which glo bules appear to be capsules. This includes the species of Dr Roth's second division of Ceramium, with articulations, those of the first being sent back to the genus Focus : it likewise embraces the marine Confervx. The genus Con ferva of the Flore Francaise is confined to those fresh water species which were huddled together by Linnxus, under the name of Conferva bullata.