314 



THALLOPHYTES 



special cells and often on different plants. Also in the higher 

 forms, the introduction of a case around the oospore, 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 

 w^hich 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 



Fig. 270. — - Desmids. a and b, two ccmmon 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 



