174 SEAWEEDS 
ULVACE. 
General Characters—The Ulvacew are among the 
first seaweeds that meet the collector’s eye on reach- 
ing the shore, since they grow for the most part at 
high-water mark. The thallus is either a flat green 
expanse of tissue, lobed irregularly, or hollow, 
green, tubular and unequal. There are no special 
sporangia, but the ordinary vegetative cells of the 
thallus act as the parent cells of gametes, which 
conjugate, and of zoospores with four cilia. 
The Thallus—In its most simple form, that of 
Monostroma, the thallus consists of a flat layer of 
cells, one cell thick, at least in the upper portion ; in 
Ulva and Letterstedtia uniformly two cells thick, and 
in Enteromorpha also two cells thick; but these 
layers soon separate, and produce a hollow space 
between them, giving rise to the tubular forms that 
are characteristic of the genus. The greater part of 
the basal cells of the Ulvacew grow out into fila- 
mentous processes which become irregularly inter- 
woven, and, while serving as a fixing organ to the 
substratum, add to the thickness of that part of the 
thallus. 
While a certain degree of branching of the thallus 
vecurs in Enteromorpha, it is only in Letterstedtia 
that lateral foliar appendages—if they may be so 
called—of definite growth occur. These  subse- 
quently fall off from the older parts of the main 
shoot, leaving it bare and irregularly toothed at the 
margin. Pringshevmia is a very minute epiphyte 
