70 MICROSCOPY FOR BEGINNERS. 



slippery streamers usually so abundant in still water dur- 

 ing the summer are Algse. The beginner need have no 

 trouble to recognize them as Algse after a little experi- 

 ence, but since he at first may be somewhat uncertain 

 as to which of the three classes of plants his specimen 

 belongs, 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, meaning that he 

 shall now seek a description somewhere in the table be- 

 low with a at the head 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 right, as his plant is not a sphere. 

 The (>) at the end refers to another line below headed 

 by 5. There being but one such, the plant must be a 

 desmid ; but to learn which of the numerous desmids it 

 is, he turns to Section I. of this chapter, where is another 

 Key to help him find the name of the genus. Again, sup- 

 pose he obtains a floating mass which, when lifted on the 

 hand or in the dipper, he sees to be a fine, delicate green 

 net. To find the section to which this belongs, read each 

 numbered sentence at the beginning of the Key : 1 will 

 not do, since the specimen is not spherical, crescentic, 



