THALLOPHYTES 
21 
FIG. 34. Pleurococ- 
cus: the single cell with 
Pleurococcus (fig. 34). The globular cell has a definite wall, a central 
nucleus, and a large lobed chloroplast whose lobes suggest several chlo- 
roplasts. These cells multiply rapidly by division, 
and they may be seen clinging together in irregular 
groups. No other method of reproduction is 
known, so that the life history is exceedingly 
simple. 
The other forms selected as illustrations are 
colony formers, this tendency being as striking as 
among the Volvocales. 
Scenedesmus. - This 
form represents the sim- its nucleus and large 
plest colony, consisting of cW r P last . and f el1 - 
. . groups of various sizes. 
two to eight cells lying 
side by side, the end cells often with more or 
FIGS. 35, 36. Scenedes- less conspicuous appendages (fig. 35). Each 
mus: 35, colony of four ce \\ divides internally to form a new colony 
cells; 36, cell forming anew ,/- , x , .| j f j 
colony ( 8" 3 '' a no otner method of reproduction 
is known. 
Pediastrum. The colony in this form is a floating or suspended, 
more or less star-shaped plate of polygonal cells, sometimes as many 
as sixty-four in number (fig. 37). Within any cell distinct zoospores 
are formed, which escape from the mother cell inclosed by a delicate 
membrane and then become 
arranged into a new colony 
(figs. 37-39). Sexual re- 
production is also present 
in its simplest form, certain 
cells forming zoospore-like 
cells, smaller and more 
numerous than the zoo- 
spores, which function as 
gametes (figs. 40-44) . Pedi- 
astrum, therefore, is isoga- 
mous, forming zygospores 
that are resting or protected 
cells. 
.__. . ,. rpi_ FlG. 37. -Pediastrum: a colony of polygonal 
Hydrodictyon. - This is cel , Sj someo{ which are f orm i ng new colonies; two 
the well-known water net, colonies escaping from, the mother colony. 
