GRASS-GREEN SEAWEEDS 53 
In the green alge are found the extreme forms of one-celled 
plants. In Plewrococcus the cell is microscopic in size. In the 
Siphonee the plant still consists of a single cell, but it attains 
large dimensions and develops into forms resembling, in outward 
appearance, leaf, stem, and root (see Caulerpa). 
Other plants consist of single rows of cells, called filaments 
(Confervacez), or of cells arranged in layers or flat surfaces, called 
membranes (Ulvacez), 
ORDER CONFERVACEZ 
The silkweeds. This order is characterized by cylindrical cells 
strung end to end, forming threads or filaments, branched and 
unbranched. The plants inhabit both fresh and salt water, and 
are very abundant and widely distributed. They grow in dense 
tufts, often matted at the base. 
Genus Ulothrix 
A yellow-green, unbranched, decumbent, soft, hair-like fleece 
on the surface of rocks, extending indefinitely. This genus dif- 
fers from Chetomorpha in the character of its filaments, which 
are soft and gelatinous in Ulothria, but bristle-like and wiry in 
Cheetomorpha. 
Genus Chetomorpha 
The frond is filiform; the filaments are coarse, rigid, and 
unbranched. In some species the filaments grow straight and in 
tufts from a definite base; in others they are twisted together 
and are prostrate. Often they are found floating in masses. In 
C. tortuosa the filaments are as fine as human hair, but rigid, and 
so closely interwoven as to resemble a layer of wool on the rocks. 
The cell-divisions give a striped appearance to the filaments when 
dry. 
C. melagontum. This species is dark green, with filaments erect, 
coarse as a double bristle, and wiry; five to twelve inches long. It is 
found in rock pools from Boston northward. It does not adhere to 
paper in drying, and loses its color if immersed in fresh water. (Plate 
I 
C. wrea. Yellowish-green, with filaments erect and less rigid than 
