GENUS ^Himanthalia^ ^H. lorea^, the sea-thong. A cup-shaped base, from the center of which arises a flat, strap-like frond one fourth to one half of an inch wide and two to twenty feet long, branching dichotomously (or by forking), and dotted with pits, or conceptacles, in which the spores are formed. Found in the extreme North. [pg072] GENUS ^Fucus^ The rockweeds. The plants of this genus grow in thick bunches, and are found in great abundance between tide-marks. The plants are attached by sucker-like disks to the rocks, from which they hang like fringe when the tide recedes; when it rises they float and sway in the water in beautiful bouquet-like forms. In color they are brown or olive-green, in texture thick and leathery, but they sometimes expand into thin membranes. They are many times forked in the same plane, which produces a flat thallus. They often have a distinct midrib. The air-vessels, whose function it is to float the plant, are disposed along the midrib, usually in pairs. The species are named according to the divisions of the frond, and the disposition, or presence, of the air-bladders and the /conceptacles/, or spore-chambers. The conceptacles congregate in particular portions of the frond and give its surface a roughness which is very perceptible; such portions are then known as the /receptacles/. In /Fucus/ this usually occurs on the bulbous extremities of the branches. Under the microscope a section of one of these little pointed spots shows a spherical cavity filled with a beautiful arrangement of /paraphyses/, or threads, some of which hold spores, while others protrude through a small opening in the outer membrane. Conceptacles are peculiar to the order =Fucaceæ=. In them spore-production is carried on in a manner as complicated as is the formation of seeds in flowering plants. Although rockweeds are such a conspicuous feature of sea-shore vegetation, two species only, /Fucus vesiculosus/ and /Ascophyllum nodosum/ (formerly called /Fucus nodosus/), are common on the Atlantic coast, and these do not occur south of New York, owing to the fact that a long stretch of sand-beach extends beyond that point. ^F. vesiculosus.^ Midrib distinct through all the forked branches; margin entire, often wavy; air-vessels spherical or oblong, usually in pairs along the midrib; receptacles on terminal branches, which are swollen and filled with gelatinous matter, heart-shaped or forked, in oblong or pointed divisions; frond tough and leathery, often two feet long. (Plate XIV.) [Illustration: PLATE XV. Fucus serratus. Fucus ceranoides. Ascophyllum nodosum.] [Illustration: PLATE XVI. Phyllospora Menziesii. Halidrys osmunda. Sargassum vulgare.] ^F. serratus.^ Frond distinctly toothed or serrated along the margin; midrib conspicuous through the main axis and forked branchings; no air-vessels. This species is rare on the Atlantic coast. (Plate XV.)