342 PHARMACEUTICAL BOTANY 
grows in length by the fission of its various component cells. 
After attaining a certain size it reproduces either asexually 
or sexually. Asexual reproduction takes place by certain cells 
becoming altered in their protoplasmic contents, through divi- 
sion, to form rounded or pear-shaped zodspores. Each zodspore 
contains a red pigment spot and bears four cilia (protoplasmic 
outgrowths). The zodspores escape into the water by lateral 
openings in the walls of cells containing them. They swim 
rapidly about, propelled by their cilia, and ere long attach 
themselves to various objects and grow into Ulothrix filaments. 
_ The sexual method of reproduction is effected through the produc- 
tion of many gametes, in cells of the filament. These resemble 
the zodspores in shape but differ from them in being smaller 
and possessing but two cilia. These escape into the water 
and, after swimming about for a short time come together 
in pairs and fuse with one another. The product of the fusion of 
each pair of these like gametes is termed a zygospore. The zygo- 
spore swims about but finally comes to rest, remaining quiescent 
for a considerable length of time. It then enlarges and its 
protoplasmic content divides to form several zodspores which, 
escaping from the cell, swim about for a while and finally, 
attaching themselves to objects, grow into filamentous Ulothrix 
organisms. 
Closely related to Ulothrix is the sea-lettuce, Ulva lactuca (Fam. 
Ulvacee), a membranous, leaf-like form found attached to rocks 
along the shores exposed to the action of the waves. 
4. ORDER ConjuGALEs.—To this order belong the desmids 
and pond scums which are distinguished from other green alge 
by presenting no motile stages in their life histories. No zoé- 
spores are, therefore, produced by members of this order. T hey 
are all of fresh-water habit and reproduce by conjugation. 
Famity Desmipace#.—The desmid family includes a number 
of genera of unicellular as well as filamentous green plants that 
present a variety of shapes. Each unicellular desmid [see 
Figs. 245 (4, 5)] is characterized by being composed of two like 
halves frequently separated from each other by a constriction 
called the isthmus. In each half there is a chromatophore contain- 
ing pyrenotds. The nucleus is found in the isthmus. Reproduc- 
