Fig. 9. Ptilota plumosa.
Wrangelia and Naccaria are the only British genera in Wrangeliaceæ. There is only one native species in each, both being rare, the latter especially.
The Helminthocladiæ are also a limited group, of a gelatinous structure; so much so that on being gathered they feel like a bunch of slimy worms, whence the name of the family. Helminthora purpurea and divaricata with Nemaleon multifidum and Scinaia furcellata represent them in Britain. They are nearly all very rare, pretty plants, and very effective as microscopic objects.
The Squamariæ, formerly included in the Corallinaceæ, are a small group of inconspicuous plants resembling lichens, of a leathery texture, and growing on rocks and shells attached by their lower surface.
A single genus only, Polyides, represents the Spongiocarpeæ. Polyides rotundus resembles Furcellaria fastigiata very closely, but differs widely in the fruit, which consists of spongy warts surrounding the frond, composed of spores and articulated threads.
Of the next group represented in Britain, Gelidiaceæ, we have only one plant, Gelidium corneum, very common on our shores, and perhaps the most variable of all vegetable species.
The Sphærococcidæ include both membranaceous and cartilaginous species. Of the latter is Sphærococcus coronopifolius, which cannot easily be mistaken, owing to the numerous berry-like fruits that tip its branchlets. It is rather rare on the northern, but often thrown ashore in large quantities on the southern coasts. The genus Delesseria has four British species, the largest being the well-known D. sinuosa, the fronds of which resemble an oak leaf in outline. The handsomest are D. ruscifolia and D. hypoglossum, which are more delicate and of a finer colour than sinuosa. There are three British species of Gracillaria, in two of which the branches are cylindrical, and in the other flat. G. compressa makes an excellent preserve and pickle, but unfortunately it is the rarest of the three. Nitophyllum is one of the greatest ornaments of this tribe. There are six British species, which are amongst the most delicate and beautiful of our native Algæ.
The Corallinaceæ are remarkable for the property they possess of absorbing carbonate of lime into their tissues, so that they appear as a succession of chalky articulations or incrustations. The most common is Corallina officinalis. There are two British species of Corallina, and two also of the nearly allied genus, Jania. Of the foliaceous group there are likewise two British genera, Melobesia and Hildenbrantia.