Our next genus (Halurus) contains a common weed of the south coast which was once included in Griffithsia. It is the Equisetum-leaved Halurus (H. equisetifolius), so called because its branches are regularly whorled round the nodes of the jointed branches, thus resembling the semi-aquatic Mare’s Tail. Its frond is tubular, and the spore-clusters are situated on the tips of the branches, surrounded by a whorl of small branchlets.
The genus Pilota has a slightly flattened cartilaginous frond, divided pinnately, and the axis surrounded by a cuticle of two layers of cells. The spore-clusters, at the tips of the branches, are surrounded by a whorl of branchlets. It contains only two British species, one of which (P. plumosa) is a very feathery species, with comb-like branchlets, growing on the stems and fronds of other weeds found on our northern shores. The other (P. elegans), with narrower fronds, in long flaccid tufts, is found all round our coasts.
Our last genus of the Ceramiaceæ is the large and typical one Ceramium, which contains about a dozen British species in which the frond is threadlike, jointed, branched or forked repeatedly, with the tips of the branchlets usually curled. The spore-clusters are enclosed in transparent sessile sacs, surrounded by a whorl of very short branchlets; and the tetraspores are embedded in the cortex, but distinctly visible. As a rule the fronds are very symmetrical, and the branches radiate in a regular fan-like manner.
In one species of the genus the frond is completely covered with cortex cells, and at each node of the frond there is a single spine which, although so small as to be invisible without a lens, so effectually locks the threads together that they form an entangled mass that is not easily arranged to the satisfaction of the collector. The species referred to is C. flabelligerum—the Fan-bearing Ceramium—and is very rare except in the Channel Islands.
Other species are armed with one or more spines at the nodes, but the nodes only are covered with cortex cells, which render them opaque, while the internodes or joints are transparent. In this group we have C. ciliatum—the Hairy Ceramium, with reddish-purple segments, and a regular whorl of hairs, directed upwards, round each node; each hair or spine consists of three segments. This plant is common during the summer and autumn, and may be found in the tide pools at all levels, either attached to the rocks or parasitic on other weeds. The same section contains C. echinotum, with rigid, forked fronds, and irregularly-scattered one-jointed spines; it is common on the south coast, where it may be found on the rocks and weeds of the upper tide pools; and C. acanthonotum, also common in the rock pools, with a single strong three-jointed spine on the outer side of each filament. The last-named weed is found principally on the northern shores, especially on rocks covered with the fry of the common mussel.
Other species are characterised by transparent internodes as above described, but have no spines at the joints, and may thus be easily floated on to a sheet of paper without the troublesome matting of their fronds. These include the Straight Ceramium (C. strictum), with erect and straight branches growing in dense tufts, and conspicuous tetraspores arranged round the nodes of the upper branchlets, C. gracillimum, of the lower rock pools, with very slender gelatinous fronds, swollen nodes and small fan-shaped branchlets; C. tenuissimum, closely resembling C. strictum in general appearance, but distinguished by having its tetraspores only on the outer side of the nodes; and the Transparent Ceramium (C. diaphanum), which may be found throughout the year on rocks and weeds in the rock pools. The last species is the largest and most beautiful of the genus, and may be readily recognised by its light-coloured, transparent stem with swollen purple nodes, and its conspicuous spore-clusters near the tips of the filaments.
Fig. 250.—Ceramium diaphanum
Our last example of the genus is the Common Red Ceramium (C. rubrum), which may be found in the rock pools at all levels. It is very variable in form, but may be known by its contracted nodes, in which the red tetraspores are lodged, and its spore-clusters surrounded by three or four short branchlets. It differs from most of the other species in having both nodes and internodes covered with cortex-cells, and hence the latter are not transparent.