18 INTRODUCTION. 



classes and of the genera are to the numbers preceeding those classes 

 and genera in this list; those following the species are in every case 

 for reference to the pages on which will be found the extended de- 

 scriptions in the monographs already so often referred to, where ref- 

 erences will also be found to the illustrations. 



The measurements of the Algse are given in parts of amikron (/i) ; 

 in the Desmids, by both fractions of an inch and of mikrons. The 

 former will necessitate a little calculation, if the reader is not en- 

 tirely familiar with the minute space called a mikron and represented 

 by the Greek letter fA.. It measures the ^g^^ ^, of an inch in length, and 

 the calculation needed to bring it to the fractions of an inch as given 

 in the measurements of the Algse, are slight. Twenty-five mikrons, 

 for instance, being g g^^, ;, inch, or -j-ifW inch. It is only necessary 

 therefore to use the number of mikrons as given with the Algae for 

 the numerator of a fraction of which the 25000 forms the denomi- 

 nator, and reducing the whole to the lowest terms. In the Desmids the 

 calculations have been made, and the measurements given in both 

 forms. The student will therefore need an eye-piece micrometer, but 

 it may be ruled to parts of an inch. The use of the mikron, like the 

 use of the metric system, has not and probably never will come into 

 popular favor. 



Does the reader know how to use a key of this kind ? To some 

 this appears to be a mysterious thing, although a little inspection of 

 the key itself should make it plain. It is simply a matter of exami- 

 nation, comparison and rejection. ' With the specimen in hand, which 

 in this instance means under the microscope, begin at the beginning 

 of the key and compare the description in the first sentence with the 

 object under the instrument. Do the two agree in every particular? 

 If not, leave that reference and go to the next having the same letter 

 or other symbol at its front. If that should describe the specimen, 

 notice the letter in the parenthesis at the end of the line, and seek 

 that same letter at the beginning of a line in some other part of the 

 key. You will probably find several lines beginning with that letter. 

 Starting at the first, compare the description there given in a word or 

 two with the actual specimen, and if it does not agree pass to the line 

 next below. If at the end of this line there is another reference let- 

 ter in parenthesis, turn to that, and so continue until at the end of 

 some line you find the generic or the specific name of the plant. Mis- 

 takes are easily made, here as elsewhere. If you should happen to 

 make one at the start every step forward will lead you further and 

 further astray. With care and intelligent observation, errors may be 



