GRASS-GREEN SEAWEEDS 55 
consists of a single layer of cells it is Monostroma (“one 
layer”); when it consists of a double layer it is Ulva; when the 
layers separate, the thallus becomes hollow, and it is then En- 
teromorpha. These plants are mostly a brilliant grass-green 
in color, are silky in texture, and are attached by a small disk to 
rocks and stones. They abound everywhere, and are commonly 
known as green laver. 
Genus Ulwa 
Frond a thin, silky, flat membrane, sometimes leaf-like, again an 
extended surface of no definite shape. These are the largest 
green alge. They are common everywhere. 
U. lactuca, the sea-lettuce. Frond a flat membrane of various shapes, 
sometimes orbicular, again deeply incised, often ribbon-like; margin 
always much waved or ruffled. In the variety rigida the frond is oval 
in outline, not very large, and quite firm or rigid. The species is found 
on rocks exposed to the action of the waves. (Plate IV.) 
U. latissima. Frond a fiat, expanded membrane of indefinite shape, 
but general outline oval, never ribbon-like; attains a size of twelve to 
twenty-four inches; often deeply lobed, very waved, often perforated 
with holes; membrane brilliant green, thin, smooth, glossy. It grows 
apart or in tufts, and is found everywhere, in all stages of growth. It 
is the largest species of Ulva, and is very common on muddy shores. 
GEenus Enteromorpha 
Fronds tubular, simple or branched, sometimes inflated. The 
tubes vary in size in different species and also in the same spe- 
cies, some being fine like a hair, others large, and flat or inflated. 
The genus is widely distributed and very abundant. Species of 
Enteromorpha grow on the bottoms of ships, and in nautical 
language are called grass. 
E. clathrata. Fronds thread-like, tubulat, branched, and branches 
beset with numerous fine branchlets; densely tufted, soft. Common 
everywhere. (Plate IV.) 
E. compressa. Fronds long, slender, branched, tufted; branches 
simple, compressed, extending from main central branch, obtuse at ends, 
but attenuated at base. The species is very abundant everywhere, and 
is a useful plant for the aquarium. (Plate V.) 
E. intestinalis. Single, long, inflated tubes or sacs, obtuse at the 
apex, very attenuated at the base; fronds often crimped and twisted,’ 
resembling an intestine, whence the name. - (Plate V.) 
