314 THALLOPHYTES 
special cells and often on different plants. Also in the higher 
forms, the introduction of a case around the odspore, and a new 
structural stage between fertilization and the formation of new 
plants, suggests a relationship to the higher plants. On the other 
hand, the simpler forms resemble some of the Protococcales from 
which the Confervales have probably been evolved. 
Conjugating Algae (Conjugales).— This group is so named 
because of the peculiar conjugating habit, in which the contents 
of two cells fuse to form zygospores. Some are unicellular but 
many are filamentous. They include Spirogyra and others that 
Fic. 270. — Desmids. a and b, two eccmmon species of Desmids highly 
magnified; at the right of c, a Desmid dividing, and at the left of c, each 
daughter cell resulting from the division developing a new half; at d, the pro- 
toplasts of two Desmids are escaping and conjugating. Redrawn from Curtis. 
are very common nearly everywhere in fresh water. They are 
free floating, and the filamentous forms often form extensive 
floating mats, which are buoyed up by the oxygen entangled 
among the filaments. Some, owing to the shape and arrange- 
ment of their chloroplasts, are attractive plants under the micro- 
scope. One peculiar feature of the group is that, although the 
plants are aquatic, there are no ciliated cells of any kind. 
Desmids. — The simplest of the Conjugales are the Desmids, 
which are unicellular floating plants that exhibit a variety of 
shapes and some are extremely beautiful (Fig. 270). They are 
