THE THALLOPHYTES 341 
into the cavity of the sphere where it is surrounded by numerous 
antherozoids (3) and is finally fertilized. 
The product of this fertilization is an odspore or zygote (4) which 
ere long becomes covered with an internal smooth membrane and 
a thicker external spinose coat. The chlorophyll within then dis- 
appears and starch and a reddish- or orange-colored oil make 
their appearance. Up to 40 of these odspores have been 
observed in a single volvox sphere. Not long after the formation 
of these odspores the whole parent colony breaks up and the 
odspores fall to the bottom of the pool to pass the winter season. 
As early as February each odspore germinates to form another 
volvox colony, which repeats the life cycle described. 
Fic. 247.—Vaucheria terrestris, a siphon alga. anth, antheridium (empty); 0, odgonia. 
(Gager.) 
3. ORDER CONFERVALES.—In this order are included a 
variety of green filamentous and membranous forms some of 
which show sexual reproduction. 
Famity ULoTHRIcACE&.—Ulothrix zonata, [Fig. 245 (1)] a 
typical representative of this family, is a filamentous organism 
found growing on stones around ponds, on rocks along the shores 
of lakes, in slow-moving streams, etc. Each filament is 
unbranched and consists of a row of short cells; one of the 
terminal cells, called the rhizoid cell is elongated and serves 
as an attachment structure. Each cell consists of a cell wall 
of cellulose enclosing cytoplasm, a nucleus and a wide, band- 
shaped, green chromatophore, more or less cylindrical in shape. 
The chromatophore (chloroplast) lies next to the cell wall and 
contains pyrenoids or starch-forming centers. The filament 
