72 MARINE ALGAE 
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 arrangément 
of paraphyses, or threads, some of which hold spores, while 
others protrude through a small opening in the outer mem. 
brane. Conceptacles are peculiar to the order Fucacee. 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.) 
