70 AQUATIC MICROSCOPY FOR BEGINNERS. 



actively, and the observer is then almost stire to see 

 some of them in conjugation, or union of two separate 

 cells, and, it may be, the formation of the spores. 

 This spore-formation, however, is more frequently 

 seen in the thread-like Algae than in the single-celled 

 desmids. 



There are more than four hundred known species of 

 desmids in.this country. Perhaps an undue proportion 

 has been included in the following list, but Nature of- 

 fers them so freely and abundantly, and they are so 

 attractive, that they must be their own excuse. 



The following Key to the genera is to be used as di- 

 rected for the "Key to the Desmids, Diatoms and 

 Fresh-water Algae," except that when the name of the 

 genus has been found, the reader should then refer to 

 the paragraph on a subsequent page headed by that 

 name, where he will find one or more species described 

 and figured. Thus, if he has a green, half-moon- 

 shaped plant under the microscope, to learn its name 

 let him turn to this Key, the second line of which de- 

 scribes it, since.it is not in ribbons nor bands; he 

 then should refer to the lines headed by d, the first 

 one of which describes the plant as a "cell more or less 

 crescent-shaped," and giving the generic name Clos- 

 terium, 6 being the number of the paragraph further on 

 in this section of the chapter, where several species 

 are noticed. 



Keii to Genera of Desmids. 



1. In ribbons or narrow bands {a). 



2. Not in ribbons nor bands ((/). 



a. In a transparent, jelly-like sheath (b). 

 a. Not a jelly-like sheath ic). 



