DEVELOPMENT OF PLANTS 



177 



71. Other Members of the Green Algae. — There are several 

 orders of the Chlorophyceae, some of which are marine, that 

 cannot be considered at this time. Two genera, however, 

 deserve attention because they show significant advances in 

 the evolution of plant life. In Oedogonium, a member of 

 a small order, the Oedogoniales, we find a still higher form 

 of the sexual reproductive process. These plants are of very 

 common occurrence in ditches, streams and springs where the 

 filaments form for a time greenish masses and coatings upon 

 various objects, but finally become detached and free floating 

 (Fig. 115, A). Any of the cells of a filament, save the basal 



Fig. 115. Growth and asexual reproduction of Oedogonium: A, young 

 plant showing basal cell modified as a hold-fast. B, two cells of a filament, 

 in one of which a zoospore is forming and the other cell has opened, per- 

 mitting the escape of the zoospore shown at C. D, zoospore at rest. It 

 becomes attached to an object at its colorless, ciliated end. E, later stage 

 in the germination of the zoospore. — After West. 



one, may form a single zoospore, which are large pear-shaped 

 bodies with a narrow colorless end around the base of which 

 arise a circle of numerous cilia (Fig. 115, B, C). These zoo- 

 spores develop into new plants (Fig. 115, D, E) as in previous 

 cases. The sexual reproduction presents a high degree of 

 specialization. The male gametes are usually formed in pairs 

 in small cells and are similar to but smaller than the zoospores, 

 while the female gametes are developed singly in the ordinary 

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