68 MARINE ALGA 
Genus Mesogla@a 
M. virescens. Soft, slimy filaments, with branches and branchlets ; 
olive-green. (Plate XI.) 
M. divaricata. Fronds two to twenty-four inches high; branching 
irregularly, and generally without definite main axis ; branches flexuous, 
solid at first, later hollow; branchlets short and wide-spreading. Com- 
mon from Cape Cod southward; abundant in Long Island Sound. 
(Plate XI.) 
GrEnus Leathesia 
This singular alga resembles a tuber and cannot be mistaken 
for any other plant. Its fronds are gelatinous, fleshy balls, one 
half of an inch to two inches in diameter, at first solid, afterward 
lobed and hollow. It grows singly or in bunches on algee and on 
sand-covered rocks, and is found in summer on every coast. The 
common species is known as L. difformis or L. tuberiformis. 
Genus Myrionema 
Minute alge which grow on other plants and which appear 
like dark spots, or, at first, like stains, on Ulva, Enteromorpha, and 
small red alge. These spots, which to the naked eye appear 
like decay, show, under the microscope, a jelly-like substance 
full of beaded filaments. 
ORDER LAMINARIACEA 
The plants of this order have large and coarse fronds (some 
attaining an immense size), with stems, branching root-like hold- 
fasts, and expanded leaf-like laminee. They are leather-like, not 
articulated, olive-green or brown in color, and sometimes yellow 
and semi-transparent. They grow in deep water and are found 
washed ashore. Sometimes small plants are found in deep tide- 
pools at low-water mark. Some species are perennial. In these 
instances the stems only survive, and the laminw are repro- 
duced annually. The new growth takes place at the apex of the 
stem. The old lamina is pushed off, but is held on the summit 
of the new growth until the latter has matured. 
In the genus Chorda the fronds are cylindrical; in Agarum 
