The order Dictyotaceæ contains the olive weeds with inarticulate fronds, and superficial spores disposed in definite lines or spots. In the typical genus (Dictyota) the frond is flat and forked, somewhat ulva-like and ribless, and the spores are produced in little superficial discs just beneath the cuticle. There is only one British species—D. dichotoma—but that is a very common one, and it assumes a great variety of forms as regards the shape and division of its fronds according to the situation in which it grows, the fronds being broadest and strongest in the deepest water. The root is covered with woolly fibres, and the frond is regularly forked.
One of the most interesting algæ of this order is the Turkey-feather Laver (Padina pavonia), which is the only British representative of its genus (see [Plate VII.]). Its very pretty fan-shaped fronds are of a leathery nature, curved, fringed along the upper margin, and marked with concentric lines. They often bear small leaflets, and are partially covered with a powdery substance which renders them beautifully iridescent when in the water. The root has woolly fibres, and the spores are arranged in lines along the upper margin. This weed seems to be confined to the south coast, where it may often be seen in the deeper tide pools; though in some of the sandy bays of the Isle of Wight it may be seen in shallow pools, and even in places left exposed to the air at low tide.
The genus Zonaria contains a British species (Z. parvula) that covers the rocks in round patches; and though moderately common is not very frequently seen by collectors on account of the fact that it grows in the deep crevices of rocks at or near low-water mark. Its frond is flat and membranaceous, more or less divided into lobes, without veins, and rather obscurely divided into concentric zones. It is attached to the rock by fibres that proceed from the under surface of the frond, and the spores are arranged in clusters beneath the superficial cells.
Cutleria multifida, though not very abundant, is to be found on most of our coasts; but since it grows almost exclusively beyond low-water mark, it should be looked for on the beach after storms, or in the fishermen’s nets. The frond is olive-green, fan-shaped, rather thick, and irregularly divided into forked branches; and it has a beautifully netted surface. The spore-cases may be seen scattered over the surface of the frond as so many black dots, and the antheridia are elongated, cylindrical bodies attached to tufted filaments on all parts of the frond.
In the genus Stilophora the root is discoid; the frond cylindrical, hollow, and branched; and the spores arranged in clusters over the surface. One species (S. rhizodes) is occasionally to be seen on the south coast. It is of a yellowish colour, from six to twenty inches long, and may be known by its long thread-like branches, with scattered, forked branchlets, and by the wart-like projections of the stem which contain the spores. This weed is often the source of some disappointment to the collector, for it soon turns to a jelly-like mass when removed from the water, and should therefore be mounted as soon as possible after it has been collected.
The fennel-like Dictyosiphon fœniculaceus is abundant in tide pools, where it may be seen in its best condition during spring and early summer. Its root is a small disc, the frond is tubular, thread-like and branched, and the branches bear hooked branchlets. The spores are naked, and distributed either singly or in clusters over the surface of the frond.
Our next genus—Punctaria—contains a few British species with a shield-shaped root, and a flat, membranous, undivided frond, without a midrib, and having the spores disposed as minute dots over the surface. A plantain-like species (P. plantaginea) has broad, leathery, lanceolate fronds, of a dark olive-brown colour, usually from six inches to a foot in length. Two other weeds—the broad-leaved P. latifolia of the tide pools, and the slender, tufted P. tenuissima, which is parasitic on Zostera and soe algæ, are sometimes regarded as mere varieties of P. plantaginea.
In the genus Asperococcus the root is shield-shaped, and the frond is a membranous tubular sac of two distinct layers. The colour is pale green, and the general appearance very similar to that of Ulva. The spores are arranged in small oblong clusters which appear as dark dots on the surface of the frond. A. compressus has slightly swollen flat fronds of a linear lanceolate form, tapering below. It grows in deep water, but is often washed up during storms. A second species—A. Turneri—has large, puffy, green fronds, contracted at intervals, and grows in tufts on rocks between the tide-marks, being specially partial to muddy shores. The genus also includes the prickly A. echinatus, the long, thin fronds of which grow in dense tufts in deep water.
The last genus of the order is Litosiphon, a parasitic group characterised by a cylindrical, cartilaginous, unbranched frond, with scattered, naked spores. A very small species (L. pusillus) with tufted green fronds grows parasitic on the fronds of Chorda and the stems of Laminaria. It is only two or three inches long, has a reticulated surface, and is covered with minute jointed fibres. A still smaller species (L. laminariæ), seldom exceeding half an inch in length, forms brown tufts on Alaria, and the rounded apex of its frond is covered with minute fibres.
The order Laminariaceæ contains olive, inarticulate algæ, mostly of large size, and generally growing in deep water beyond the tide-marks. Their spores are superficial, either covering the whole surface of the frond or collected into indefinite cloudy patches.