Cystoclonium (Hypnæa) purpurascens is a very common weed, growing on other algæ between the tide-marks, and sometimes reaching a length of two feet. Its cartilaginous, purple fronds are much branched, and become almost black when dried. The spores are embedded in the smallest branches, and the tetraspores are arranged among the superficial cells.
The genus Phyllophora contains a few British weeds with a stiff, membranous frond, bearing leaf-like appendages, and supported on a stalk. The tetraspores are contained in external wart-like swellings. The commonest species is P. membranifolia, the fronds of which are divided into wedge-shaped segments, and grow in tufts from an expanding root. The spores are contained in stalked sporangia, and the tetraspores are near the centres of the segments. Another species—P. rubens—has a shorter stem, and grows in deep and shady rock pools. Its fronds are densely tufted; and, as the plant grows, new series of segments are formed at the tips of the older ones. A third species (P. palmettoides) has a very bright-red frond and an expanded root.
The order Rhodymeniaceæ includes a number of red or purple sea weeds with flat or thread-like unjointed, cellular fronds, the surface cells forming a continuous coating. The spores are lodged in external conceptacles, and are at first arranged in beaded threads. The tetraspores are either distributed among the surface cells, collected in clusters, or situated in special leaflets.
The typical genus (Rhodymenia) contains two red, membranous weeds, the commoner of which is R. palmata (Plate VIII.), so common on the Scottish and Irish coasts, where it forms an important article of diet, and is known as the Dulse or Dillisk. It is also widely distributed over the English coasts. Its broad, fleshy fronds are divided into finger-like lobes, and are either sessile or supported on a stalk that proceeds from a small discoid root. The frond is very variable in form, being sometimes divided into very narrow segments, and sometimes quite undivided. One variety has a number of small stalked leaflets on its margin (see [Plate VIII.]); and another is very narrow, with wedge-shaped irregular lobes. R. palmetta is a smaller and less common species that grows on rocks and large weeds in deep water. The tetraspores form crimson patches on the tips of the lobes.
Maugeria (Delesseria) sanguinea (Plate VIII.) is a large and beautiful weed, of a blood-red colour, that grows in the lower rock pools or beyond low-water mark, under the shade of high rocks or hidden by the olive tangles. Its frond is thin and membranous, with a well-defined midrib. The spores are contained in globular stalked conceptacles, usually on one side of the midrib; and the tetraspores may be seen in pod-like leaflets attached to the bare midrib during the winter.
Passing over some of the rarer membranaceous Rhodymeniaceæ, we come to the beautiful Plocamium, distinguished by its linear compressed crimson fronds, which are pinnate, with comb-like teeth, the branchlets being alternately arranged on either side in threes and fours. The spores are on radiating threads, in globular conceptacles; and the tetraspores are in the outer divisions of the frond. We have only one species of this beautiful genus, and that is P. coccineum, which is of such a brilliant colour that it is always a favourite with collectors.
Fig 251..—Plocamium
Our last example of the order is Cordylecladia (Gracilaria) erecta, with threadlike, cartilaginous frond, irregularly branched and cellular in structure. The fronds arise from a disc-like root; and bear spores in thickly-clustered spherical conceptacles, and tetraspores in lanceolate pods at the tips of the branches, both in the winter. It is a small weed, and grows principally on sand-covered rocks near low-water mark.