INTRODUCTION 



This KEY is a mechanical device for finding the names of trees by 

 means of characteristics possessed by their leaves, stems and winter- 

 buds. It is mechanical in the same sense as is a crossword puzzle : the 

 leaves are grouped according to similarities in their external structure, 

 and not with regard to their family relationships. 



It is called a FIELD KEY because it is intended to be used where 

 trees are growing, since some of the characteristic organs are too large 

 to be carried away. Because leaves are of primary importance for iden- 

 tification the KEY can be used only when trees are in full leaf. The 

 characters which have to do with stems are always on hand, summer 

 and winter. New winter-buds are fully developed by late summer or 

 autumn ; but sometimes may be found at other times, unopened, on old 

 twigs. 



A tree is a woody plant, 12 feet or more in height, which has one 

 main trunk rising an appreciable distance (a foot or so) above the 

 ground before it branches. Thus it is differentiated from a shrub, which 

 has more than one trunk, or perhaps has none at all above ground. 

 However some shrubs have tree relatives, and these have been included, 

 although their descriptions would not apply necessarily to the shrubs. 



The range of the trees included runs from northern Canada to a 

 line about as far south as central Virginia, and west to the Mississippi 

 River. Many of the trees will extend farther south and west. Some 

 genera will grow over most of the range, while others are restricted to 

 a much smaller territory. Only those hardy in at least the southern 

 part of the area have been listed. 



Both wild and cultivated trees have been classified. The cultivated 

 ones, many of which have been imported from other countries, often 

 escape into the fields and woods by means of their seeds ; and the native 

 forms frequently are cultivated. Thus both kinds may be encountered 

 in the same sort of place. 



The KEY is intended to identify genera and not species : for exam- 

 ple, the pines and the maples arc mentioned, but not the white pine or 

 the sugar maple. Apparent exceptions are due to the presence in the 

 genus of only one species, which may then be named ; to the variation 

 of a species from the rest of its relatives in the genus ; or to the fact 

 that the stated name is applied to all the species in the genus. Occa- 

 sionally large genera, such as PRUNUS, have been subdivided. The 

 same genus may appear several times if its characteristics are so in- 

 definite that a choice in the KEY is difficult, or if its species differ much 

 as to leaves or twigs. In naming the genus the common names are put 

 first, and then the scientific name in a parenthesis. No varieties of 

 species, or hybrids, or doubtful species have been considered in charac- 

 terizing the genera. The KEY includes most of the species mentioned 



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