60 



and species, was the fact tliat a combined key is often difficult to use 

 on account of its great length, and tedious to operate for those who 

 can recognize the family or genus at a glance but do not know the 

 species. Besides, keys to the genera and keys to the species are more 

 serviceable when placed close to the written description and its ac- 

 companying plate than if they precede the descriptive material of all 

 the species. 



The three classes of keys are constructed on the same plan; conse- 

 quently, they can be used in the same manner. To use them it is 

 necessary to make a choice for the most part between two alternatives 

 stated in two paragraphs preceded by the same number. The choice 

 leads to another number or to a family, a genus or a species followed 

 by the page upon which a further description is found. The Sugar 

 Maple may be taken as an example to show how to use the key. Un- 

 der ^'Key to the Families," page 63, we start with 1. We have the 

 choice between trees with "Leaves narrow, needle-like, awl-like, or; 

 scale-like, usually persistent except in the genus Larix" and trees 

 with "Leaves broad, flat, rarely five times as long as wide, usually; 

 deciduous." We select the latter, which is followed by 2. Under 2 

 we have the choice between "Leaves opposite or whorled^ i. e. 2 or 3 

 occur at a node" and "Leaves alternate, i. e. only one occurs at a 

 node." We choose the former, which is followed by 3. Here we have 

 the choice between "Leaves or at least most of them three at a node" 

 and "Leaves always two at a node." We select the latter, which 

 is followed by 4. Here we have the choice "Leaves simple" and 

 "Leaves compound." We select the former, which is followed by 5. 

 Here we have the choice between "Leaves palmately lobed" and 

 "Leaves not lobed." We select the former, which is followed by 

 Aceraceae, which is the family name for the Maples. This is followed 

 by a number which indicates the page upon which a further descrip- 

 tion of the family may be found. At this point it is advisable to 

 check one's self. This can be done by carefully studying the descrip- 

 tive matter of the family indicated in order to find out if the de- 

 scription corresponds to the species under consideration. If the 

 description does not correspond it is advisable to go back to th 

 "Key to the Families" and attempt to find the mistake. If the d 

 scription does correspond it is reasonable to think that the "Key t 

 the Families" was used correctly. If you feel certain that this is th 

 correct family you should go to the "Key to the Genera," or to th 

 "Key to the Species." No "Key to the Genera" is given under th' 

 family because it contains only one genus. Under the "Key to th 

 Genera" and the "Key to the Species" the same method of procedu 

 should be used that was used under the "Key to the Families." O 

 account of the wide variation between the distinguishing characteris 



