DESMIDS, DIATOMS, AND FRESH-WATER ALG^E. 73 



sion, but never any other. The early spring, as early 

 as the last of March or the first of April, in the writer's 

 locality (New Jersey), is the best time of the year to 

 gather them, or indeed any of the Algae. At that time 

 all these plants seem more vigorous, their vital functions 

 are performed more actively, and the observer is then 

 almost sure to see in some of them the conjugation, or 

 union, of two separate cells and the formation of the 

 spores. This spore formation, however, is much more 

 frequently seen in the thread-like Algae than in the sin- 

 gle-celled desmids. 



There are more than four hundred known species of 

 desmids. Perhaps an undue proportion has been in- 

 cluded in the subsequent list, but nature offers 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 

 directed 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 the following pages 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 turn to 

 this Key, the second line of which describes it, since it 

 is not in ribbons or bands ; he then refers to the lines 

 headed by d, the first one describing his plant as a " cell 

 more or less crescent-shaped," giving the generic name 

 Closterium, 6 being the number of the paragraph fur- 



