A third species—the Obtuse Laurencia (L. obtusa)—is widely distributed on our coasts, and may be known by its thread-like bipinnate fronds with short blunt branchlets, cup-shaped at the tips. It is parasitic on various other weeds.
The genus Lomentaria includes a few weeds with tubular fronds that are constricted at intervals, and divided internally by transverse membranous septa. The spores are pear-shaped and lodged in spherical cells; and the tetraspores are scattered on the surface of the branches. One species called the salt-wort (L. kaliformis) is widely distributed. Its colour is pink, sometimes yellowish, and it grows on rocks or stones, and sometimes on other weeds. It may always be known by its spherical fruit, without any visible opening, containing crimson pear-shaped spores. Another species (L. ovalis), found on the coasts of Devon and Cornwall, may be recognised by its solid branched frond and little oval leaf-like branchlets, which are hollow, jointed, and divided by partitions internally.
The one remaining order of the red-spored sea weeds is the Rhodomelaceæ, which has either a jointed or a many-tubed axis, and the surface divided up into little definite areas. The fronds are either leafy or thread-like, and the prevailing colours are red, reddish brown, and purple. The spores are pear-shaped, and occupy the terminal cells of tufted threads in external, globular, or rounded conceptacles; and the tetraspores are lodged in special receptacles, or in special modified branchlets. The order contains some of our most beautiful weeds, while some of its members are of a very dark colour and unattractive form.
The typical genus—Rhodomela—contains two British species with dark-red, cartilaginous fronds, cylindrical, unjointed, and irregularly branched; and the tetraspores imbedded in the tips of the slender branchlets. The name of the genus signifies ‘red-black,’ and is applied on account of the tendency of the dark-red fronds to turn black when dried.
R. subfusca is very common on all our coasts. It has rigid fronds, irregularly branched; and is in its best condition during the summer. The other species—R. lycopodioides—has long undivided branches with thickly-set and freely-divided branchlets.
When turning over the fronds of different species of the larger olive weeds we commonly find them more or less clothed with tufts of filamentous plants, sometimes small and delicate, and sometimes larger and of more robust growth, varying in colour from a purplish brown to a dark violet, and the articulated filaments more or less distinctly striated with parallel lines. These weeds belong to the genus Polysiphonia, and derive their generic name from the fact that the threadlike fronds are composed of several parallel tubes. The surface cells are also arranged in regular transverse rows, and it is this which gives rise to the striated appearance above referred to.
Over twenty species of Polysiphonia are to be found on our shores, where they exist at all levels between the tide-marks. They are distinguished from one another partly by their general form and mode of growth, and also by the number of tubes in their threadlike fronds.
Although they would not always be considered as lovely weeds and are often anything but beautiful when dried and mounted, yet in their fresh condition they are generally pretty objects, and their microscopic structure is particularly interesting on account of the beautiful and symmetrical arrangement of their siphons and tubes.
If the reader is the fortunate possessor of a compound microscope, it will amply repay him to make transverse sections of the fronds for examination. A short length of the frond should be inserted into a slit cut in a piece of carrot or elder pith; and, while thus supported, very thin transverse sections may be easily cut with a sharp razor, care being taken to keep both razor and object very wet during the process. Allow the sections to fall into a vessel of water as they are cut, and then select the thinnest for examination, mounting them in a drop of water in the usual way.
Specimens in fruit should always be obtained when possible, so that the nature of the fructification may be observed. Two kinds of spores may be seen in each species, but, as is usually the case with the red sea weeds, on different plants. Some are small pear-shaped bodies, enclosed in oval cells at the tips of the fronds; and the others are arranged in clusters of four in swollen parts of the threads.