ZYGOSPOUE^. 



129 



tinues, the resulting plants become smaller and smaller. 

 AVhen a certain limit is reached, reproduction by another 

 process, namely, by auxospores (Gv. auxano, to increase), 

 takes place, by which the original size is again attained ; 

 two plants approach each other, their valves separate, 

 the protoplasm of the two fuses into one mass, and this 

 grows to a large size, secretes a shell or valves of the 

 normal form. Like the 

 Desmids, the Diatoms loco- 

 mote freely ; but the mech- 

 anism of the process is not 

 understood. They are ex- 

 cessively abundant, and in- 

 habit both salt and fresh 

 water, often forming a yel- 

 lowish layer at the bottom. 

 As fossils they are also 

 abundant, their silicified 

 shells forming vast beds. 

 Their beautiful and very 

 fine markings make them 

 desirable as test objects for 

 the microscope. ^*^ ^^ 



163. Another very common, abundant and beautiful 

 fresh-water Alga, belonging to the division Zygosporese, 

 - is the Spirogyra (Fig. 242, 243). It is a many-celled, 

 filamentous Alga, with the chlorophyll in spiral bands, 

 and a conspicuous nucleus in each cell, which is imbedded 

 in the small central protoplasmic mass, and suspended by 

 extensions to the parietal protoplasm (Fig. 242). The 



Fig. 242. A vegetative filament of Spirogyra. Fig. 243. Spirogyra in tlie process 

 of conjugation. 



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