Fig. 11. Padina pavonia.
The Mesogloieæ in a fresh state resemble bundles of green, slimy worms. There are three British species, two of which are not uncommon. Although so unattractive in external aspect, they, like many others of the same description, prove very interesting under the microscope. One of the cartilaginous species, Leathsia tuberiformis, has the appearance, when growing, of a mass of distorted tubers.
The species of Elachista, composed of minute parasites, are, as well as unattractive like the Mesogloieæ, inconspicuous, but are beautiful objects when placed under the microscope. Myrionemæ are also parasitic, and even smaller than the plants of the preceding genus.
In the Dictyoteæ the frond is mostly flat, with a reticulated surface, which is sprinkled when in fruit with groups of naked spores or spore cysts. This tribe includes not a few of the most elegant among the Algæ. In structure they are coriaceous, and include plants both with broad and narrow, branched and unbranched fronds. In Haliseris there is a distinct midrib. The largest of the British Dictyoteæ is Cutleria multifida, sometimes found a foot and a half long; and the best known is doubtless Padina pavonia, much sought after by seaside visitors where it grows. Its segments are fan-shaped, variegated with lighter curved lines, and fringed with golden tinted filaments. ([Fig. 11.]) Owing to its power of decomposing light, its fronds, when growing under water, suggest the train of the peacock, whence its specific name. Taonia atomaria somewhat resembles Cutleria, but exhibits also the wavy lines of Padina. The plant of this group most often cast ashore is Dictyota dichotoma. It makes a handsome specimen when well dried, and is interesting on account of the manner in which it varies in the breadth of its divisions. The variety intricata is curiously curled and entangled. Dictyosiphon fœniculaceus, the solitary British example of its genus, is a bushy filiform plant, remarkable for the beautiful net-like markings of its surface. The Punctariæ have flattened fronds, marked with dots, which sufficiently distinguish them from all the others. A small form is often found parasitic on Chorda filum, spreading out horizontally like the hairs of a bottle brush. Asperococcus derives its name from its roughened surface, occasioned by the thickly scattered spots of fructification.
The Laminariaceæ are inarticulate, mostly flat, often strap-shaped. Their spores occur in superficial patches, or covering the whole frond. The plants of this order, as we have already seen, include the giants of submarine vegetation. In point of mass they constitute the larger part of our native Algæ, although they number only a few species. They are popularly known as tangle or oarweeds, and the stems of Laminaria saccharina and the midrib of Alaria esculenta are used as food.
The Sporochnaceæ are a small but beautiful tribe, inarticulate, and producing their spores in jointed filaments or knob-like masses, and remarkable for their property of turning from olive brown to a verdigris green when exposed to the atmosphere.