INTRODUCTION. 15 



The occurrence of trees is also influenced by their tolerance — that 

 is, their ability to exist, for a part or the whole of their lives, in dense 

 shade or their requirement of various degrees of shade or of full light. 

 To what extent, however, tolerance — inherent or acquired — may be 

 accounted for by the amount of soil moisture a given species requires 

 can not be stated now. Finally, the characteristic habits and methods 

 of reproduction, by seed or by sprouts, most important factors in the 

 life history of a tree, have much to do with the occurrence of a species. 



It may be said here, in passing, that dendrology, the botany of 

 trees, properly includes a study of the distinguishing characteristics 

 of tree species for the purpose of identification and, naturally, of the 

 affinities which determine their classification into orders and other 

 natural groups. The characteristics of a tree include the definition 

 of both external and internal form characters — the morphology of its 

 trunk, root, branches, twigs, buds, leaves, flowers, fruit, seed — as well 

 as of the anatomical structure of the tissues, including characteristic 

 secretions — gums, resins, etc. — of which these parts are composed. 

 A study of the physiological processes which characterize the life of 

 the tree organism are a part, too, of dendrology. It deals also with 

 the natural range — horizontal and vertical and its peculiar climatic 

 conditions, as well as with the habitat or occurrence — including the 

 character of site and soil the tree chooses either in pure or mixed 

 growths. What the forester has long called silvics, a study of the 

 habits and life history of trees in the forest, therefore falls naturally 

 under dendrology. Silvics, as the basis for all practical silvicultural 

 operations, deals with the factors which influence the life and growth 

 of trees in their natural or adopted habitat. In recent years the new 

 science of ecology, a study of plant associations, has included, in so 

 far as the life habits of trees are concerned, a part of dendrology as 

 one of its natural subdivisions. It appears logical, however, to con- 

 sider dendrology as still including the study of tree associations. 

 This leaves forest ecology in its proper place as a department of 

 general ecology, and at the same time preserves the identity of an 

 essential part of dendrology, a distinct division of general botany. 

 However this may be, the serious student of tree life — dendrology — 

 can make no mistake in taking the broadest view of the field and in 

 striving to familiarize himself with all that pertains to trees, from 

 a study of their distinguishing characteristics to their modes of life 

 and associations. 



A( K X( >WLF.I>(; M EXTS. 



Grateful acknowledgment is here made to Dr. C. Hart Merriam, 

 who placed at the writer's disposal transcripts of his voluminous 

 notes on the distribution and occurrence of California trees. The un- 

 published data thus made available is the result of over twenty years 



