59 



rather than variable characteristics. This publication will no doubt 

 come into the hands of different classes of people, some of whom 

 will recognize at a glance the genus to which a certain tree belongs, 

 while others will not have the slightest idea as to what it is. An 

 attempt has been made to satisfy both types of persons. The former 

 can go at once to that portion of the publication where the genus 

 under consideration is treated and by the use of the "Key to the Spec- 

 ies" determine the exact species which they have at hand, while the 

 latter should begin at the "Key to the Families" found on page 63, 

 and use the key until the family to which it belongs is found, then go 

 to the family and use the "Key to the Genera" and the "Key to the 

 Species" until the species is determined. With a little practice one 

 will find it easy to use such simple keys. 



Before attempting to use a key, it is necessary that good material 

 be available. Parts of trees vary considerably,' depending upon the 

 environments in which they were developed. An abnormal environ- 

 ment will produce abnormal organs, and if these should be the parts 

 with which you are attempting to identify the species through the 

 use of the keys, it is natural that it would be a difficult task. Struc- 

 tural variations are commonly found in leaves, flowers, fruit, bark, 

 as well as other plant organs. Upon the same tree or even the same 

 branch one may find three or more distinct varieties of leaves. On 

 account of this variation, which often makes identifi(!ation difficult, 

 abundant material should always be at hand, and especially that 

 which is normal in appearance. The keys are based upon normal ma- 

 terial and may not fit variable forms. Only by years of constant and 

 careful study of trees will one be able to distinguish accurately be- 

 tween normal and abnormal material ; but by carefully observing and 

 constantly studying the trees one will unconsciously absorb many de- 

 tails concerning them which can be appreciated but not described. 

 This unconscious absorption of appreciable but indescribable detail 

 in trees has a greater significance than we attribute to it at first. 

 The writer, in conducting field work (Fig. 7) for five years in connec- 

 tion with a course in Dendrology given at the Pennsylvania State 

 Forest Academy, finds that the students learn to notice many differ- 

 ences between species, which differences they cannot describe. 



The keys are subdivided into three classes, viz : "Key to the Fam- 

 ilies," "Key to the Genera" and "Key to the Species." The "Key to 

 the Families" is found on page 63, preceding the description of any 

 of the species. The "Key to the Genera" is found under the descrip- 

 tion of each family which contains more than one genus; and the 

 "Key to the Species" is found under such genera which contain more 

 than one species. The reason for subdividing the keys into three 

 classes instead of combining all three into a general key to genera 



