The Bulbochæteæ constitute a small group, some half-a-dozen species being British. They are freshwater plants, composed of articulate branched filaments, with fertile bulbshaped branchlets. The endochrome is believed to be fertilized by bodies developed in antheridia, the contents of each fertilized cell dividing into four ovate zoospores.
The last two groups of green sea-weeds consist chiefly of marine plants. Of these the first, Siphoneæ, is so called because the plant, however complicated, is composed invariably of a single cell. It propagates by minute zoospores, by large quiescent spores, or by large active spores clothed with cilia. It includes the remarkable genus Codium, three species of which inhabit the British seas. In Codium Bursa the filamentous frond is spherical and hollow, presenting more the appearance of a round sponge or puff-ball than a sea-weed, and is somewhat rare. Another species greatly resembles a branched sponge, and the third forms a velvety crust on the surface of rocks. Another genus, Vaucheria, is of a beautiful green colour, forming a velvety surface on moist soil, on mud-covered rocks overflowed by the tide, or parasitic on other sea-weeds. The most attractive plants of this family are however those of the genus Bryopsis, two of which are found on the British shores. The most common one is B. plumosa, the fronds of which grow usually in the shady and sheltered sides of rock pools.
The fronds of the last of the green-weed groups, the Ulvaceæ, are membranous, and either flat or tubular. Two of them, Ulva latissima, the green, and Porphyra laciniata, the purple laver, are among the most common sea-weeds, growing well up from low-water mark. The propagation in all of them is by zoospores. An allied genus, Enteromorpha, is protean in its forms, which have been classed under many species. They may, however, be reduced to half a dozen. Some of them are very slender, so as almost to be mistaken for confervoid plants.
With the Rhodospermeæ we enter a sub-order of Algæ, exclusively marine, the plants in which have always held out great attractions to the collector. In structure they are expanded or filamentous, nearly always rose-coloured or purple in colour. Of the fourteen groups into which they are divided by Harvey, the first is Ceramiaceæ, articulate Algæ, constituting a large proportion of the marine plants of our shores. Of the genus Ceramium, C. rubrum is the most frequent, and it is found in every latitude, almost from pole to pole. It is very variable in aspect, but can always be recognized by its fruit. C. diaphanum is a very handsome species, growing often in rock pools along with the other. There are about fifteen native species altogether, some of them rare, and all very beautiful, both as displayed on paper and seen under the microscope. Crouania attenuata is a beautiful plant, parasitic upon a Cladostephus or Corallina officinalis. It is however extremely rare, being only found in England about Land's End. A more common and conspicuous, but equally handsome plant is Ptilota plumosa ([Fig. 9]), which is mostly confined to our northern coasts; although P. sericea, a smaller species, or variety, is common in the south, and easily distinguished from its congener, which it otherwise greatly resembles, by its jointed branchlets and pinnules. Callithamnion, Halurus and Griffithsia, articulate like Ceramium, furnish also several handsome species. ([Fig. 5.])
Fig. 5. Species of Callithamnion.
Fig. 6. Chondrus crispus.