DESMIDS, DIATOMS, AND FRESH-WATER ALGjE. 67 



summer are Algae. The reader need have no trouble to 

 recognize them as Algae after a little experience, but 

 since he at first may be somewhat uncertain as to 

 which of the three classes of plants his specimen may 

 belong, the following Key has been constructed to aid 

 him. To use it, compare the plant with it in the fol- 

 lowing way: 



Suppose the specimen is a single cell, shaped like a 

 crescent, as described in the first sentence of the Key. 

 The reader will notice (a) at the end of the line, mean- 

 ing that he shall now seek a description somewhere in 

 the table below with a at the beginning of the line. 

 Finding three such lines, he reads the first,. "Color 

 green," which is the color of the specimen under the 

 microscope; "the plant a floating hollow sphere," 

 which does not describe it, since it is crescent-shaped. 

 He then reads the second "a" line: "Color green, the 

 plant not a hollow sphere," which is of course correct, 

 as his plant is not a sphere. The {l>) at the end refers 

 to another line below headed by 6. There being but 

 one such, the plant must be a desmid; but to learn 

 which of the numerous desmids it is, he turns to Sec- 

 tion I. of this chapter, where is another Key to help 

 him find the name of the genus. 



Again, suppose he obtains a floating mass which, 

 when lifted on the hand or in a dipper, he sees to be a 

 fine, delicate green net. To find the section to which 

 this belongs, read each numbered sentence at the be- 

 ginning of the Key: i will not do, since fhe specimen 

 . is not spherical, crescentic, nor circular; 2 will not do, 

 because the plant is not in long threads; 3 and 4 do 

 not describe it, because it is neither star-shaped nor 

 formed of oval cells with two bristles on each end; but 



