72 MARINE 



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 mem- 

 brane. Conceptacles are peculiar to the order Fucaceae. 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.) 



