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 Viva; 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 Viva 



Frond a thin, silky, flat membrane, sometimes leaf -like, again an 

 extended surface of no definite shape. These are the largest 

 green alga3. They are common everywhere. 



17. 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 flat, 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. 



GENUS 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, tubular, 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.) 



