The Corallines are florid Algæ, which absorb such a quantity of carbonate of lime from the surrounding water, that they become rigid, hard, and often stony. They are purple or pink when fresh, white and sometimes brittle when dry, and are propagated by strings of spore threads rising from the base of the nuclei which are enclosed in conceptacles or spore cases, open at the top. Some are articulate, composed of closely compacted threads, as the Corallina officinalis, a pretty little branched and bushy plant, most luxuriant in deep water, and particularly abundant in the rocky pools. Its urn-shaped spore sacs are attached to the tips or sides of the branches; fig. 24 _c_ is a vertical section of one of them magnified, and _d_ is a membrane of the same, more highly magnified, with impressions of the external cells. The joints of the articulate corallines, which are flexible and vary much in length, are either free from carbonate of lime, or ornamented with calcareous plates; it is through these open spaces that the plant is believed to obtain nourishment. The forms of the corallines are varied beyond description; many are mere amorphous crusts on stones and sea weeds, increasing from the centre outwards as in the lichens, others are lobed and branched like real corals. Corallines ascend to very high latitudes, but abound most in warm and tropical seas: either free, or coating pebbles at vast depths, they form the last zone of vegetable life.
The Laurenciaceæ have fronds which are soft and thread-like, or solid, fleshy, and inarticulate; both are repeatedly branched. The colour of these plants is purple or a dullish red, but they are extremely sensitive to the influence of light and air, changing through every shade of orange, yellow, or green, according to the exposure, and like many other florid Algæ they lose their colour in fresh water. They are amongst our commonest sea weeds. The Laurencia pinnatifida is the pepper dulse of Scotland, and is also native on the eastern and western coasts of North America. Species have been found at the Cape of Good Hope, Australia, and New Zealand. The fructification in this section is quite peculiar. They have tetraspores lodged in the branchlets; and egg-shaped conceptacles with a terminal pore, enclosing nuclei with pear-shaped spores radiating from a fibro-cellular mass at their base. The antheridia, which differ in the different species, attain a greater degree of complication than in other tribes. In Laurencia tenuissima they form curious lateral twisted plates of a greyish tint, bordered with large cells. The plate is occupied by the productive cells of a much smaller size, evidently springing from a cellular branched axis. In Laurencia pinnatifida instead of a plate there is a somewhat hollow cup-shaped disc, formed of dart-like vertical groups of pale cells surmounted by two or three larger oily-looking sacs filled with yellow pigment. These bodies are sometimes forked, and appear to shoot out from the mass. L. dasyphylla presents a third modification, the antheridium being a sac, and the dart-like groups of cells being ejected from the minute terminal orifice. The moving particles produced in the cells of these three forms, differ a little in shape, and as they do not germinate they are believed to be spermatozoids, though no cilia have been found on them.