THK ALG^K. 97 



cell. (See Fig. 52, a.) Each of these halves develops 

 until it is again bilaterally symmetrical, when the process 

 of cell division may be again repeated. 



The Desmids are also reproduced in the sexual way by 

 the conjugation of two individual cells. (See Fig. 55.) 

 The zygospore thus formed grows by the process of cell 

 division. The conjugation seems to take place to give 

 new strength and vigor to the plant. 



References for Reading. Carpenter's "The Microscope," pp. 509- 

 516; Bennett and Murray's " Cryptogam ic Botany," pp. 268-272; 

 Goebel's " Classification of Plants," pp. 50-52. 



The Zygnemeae are very common and familiar objects, 

 some of them forming great masses upon the surface of 

 stagnant pools and ditches. They are commonly known 

 as pond scums. When in active growth the masses are 

 of a vivid though somewhat yellowish green. The plants 

 are unicellular, but as the cells are joined together by a 

 gelatinous covering, they form long unbranched filaments, 

 or linear aggregates. The gelatinous covering gives them 

 a slippery feeling when a mass is taken in the hand. 





FIG. 56. Zygnema. a, a single cell ; 6, part of a filament whioh has resulted 

 from repeated transverse divisions of cells. Magnified. 



There is a large number of species which are distin- 

 guished by the different arrangements of the chlorophyl 

 granules and bands. The disgusting masses of slimy ma- 

 terial are resolved into forms of great beauty when seen 

 under the microscope. In some species the chlorophyl 

 CLARK'S BOT. 7 



