[Chap. XLVII THE ALGAE 621 



ditches. Cell division in it is unaccompanied by the formation of cross 

 walls and thus the filaments are tubular and multinucleate. The motile 

 spores of Vaticheria are unusual in that a number of nuclei are present 

 in a so-called "compound zoospore," from which a new filament develops 

 almost immediately. Most species of Vaucheria have an oogonium with a 

 single egg, and an antheridium containing several sperms. These sex 

 organs are formed at the ends of short lateral branches (Figs. 281 and 

 282 ) . The oospore resulting from fertilization remains dormant for some 

 time before a new filament develops from it. Reduction division probablv 

 occurs during the formation of gametes as it does in animals. If it does, 

 the nuclei of the zygote, filament, and all of the motile spores have the 

 diploid number of chromosomes. 



The algae described above and manv others are usuallv available in 

 their natural habitats, and the details of their life histories ma\' be studied 

 from living or preserved specimens. The desmids are either unicellular 

 or filamentous forms, a few of which are illustrated in Fig. 284. 



The stoneworts. These plants are mentioned here largelv because of 

 their importance in the formation of marl deposits. The stoneworts 

 (Charales) are rather widely distributed in both fresh and salt waters; a 

 few species inhabit both. They are attached to the soil beneath the water 

 and often fomi extensive meadows several feet below the water surface. 

 The plants become incrusted with calcium carbonate in limestone re- 

 gions, and as the plants die, marl accumulates. About one hundred 

 species have been described. 



The plant body consists of a cylindrical axis bearing a whorl of 

 branches at its several "nodes." The plants are nearly alwavs erect and 

 may vary in height from an inch to 3 or 4 feet. The stoneworts constitute 

 a group verv distinct from all other algae. 



The Blue-green Algae: Myxophyceae 



The blue-greens differ from all the great groups of algae in that the 

 nuclear substances of the cell are not organized in a well-defined struc- 

 ture in the cytoplasm as in other green plants. Hereditarv units of 

 matter comparable to genes must be present, however, for the species 

 are hereditarib' different. Chlorophvll and other pigments are dispersed 

 throughout the cell and not confined to plastids. No motile spores are 

 formed and no sexual reproduction occurs. Onh^ about half the blue- 

 green algae are really blue-green in color. The others vary greatly in 

 color owing to different amounts of blue, red, green, and vellow pig- 



