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ULVACEAE. 
Frond consisting of a membrane of one or two layers of cells; in 
the simplest forms of merely two rows of cells, side by side; or in 
some parts of the frond of only a single series of cells. Membrane 
forming a tube or sac,or a flat expansion; simple or branching. 
Near the base of the frond the cells send down rhizoidal prolonga- 
tions to the substratum, often uniting to form a thickened stipe ; 
otherwise than this there is no specialization of cells. Asexual 
propagation by oval zoospores with four cilia, which may be formed 
in any ordinary cell of the frond, by successive division.  Fructifica- 
tion by zoospores similar to the asexual, but noticeably smaller, and 
with two cilia; after the union of two such zoospores, with no appar- 
ent distinction of sexes, a new plant immediately begins to develop. 
Of world-wide distribution: the plants usually gregarious, often 
growing in great quantities. "They are specially plants of the litoral 
zone, occasionally extending down for a short distance in the sub- 
litoral. 
KEY TO THE GENERA. 
Frond membranaceous, of two layers of cells Urva. 
Frond originally tubular, sooner or later opening at the top, and usually dividing 
into a membrane of a single layer of cells MONOSTROMA. 
Frond tubular, or ruptured only by external causes ; membrane parenchymatous 
ENTEROMORPHA., 
Frond tubular, very gelatinous, the cells in distinct longitudinal series, which are 
only loosely connected with each other ILEA. 
ULVA L. 
Frond membranaceous, flat, consisting of two layers of cells, in any 
of which, excepting those in the thickened base, zoospores may be 
formed, issuing through an opening in the surface of the frond. 
