Introduction xiii 



While trees in this group are important in the tropics, they 

 are of Httle use in the temperate zones. 



The Dicotyledons contain our most common and widespread 

 trees such as the oaks, maples, and elms, and they are frequently 

 referred to as broadleaf trees or hardwoods. They differ from 

 the Monocotyledons in having prominent branched (pinnate or 

 palmate) major veins in the leaves, floral parts usually in fours or 

 fives, and fibrovascular bundles disposed in a ring around a pith. 

 While most of the dicotyledonous trees are deciduous (drop their 

 leaves each fall), many are evergreen and have leaves which 

 persist for two or more years. In this text 1 16 genera of Dicotyle- 

 dons are covered, including 160 species for which drawings, maps, 

 and concise descriptions have been prepared, and 293 species 

 which are either briefly described or included in the complete keys. 



FOREST REGIONS OF NORTH AMERICA 



As a result of differing conditions of moisture, temperature, 

 and soil, the forests may vary from one section of the country 

 to another, both in the species of trees present and in the type 

 or character of the forest. In North America six broad, natural 

 forest regions can be easily recognized. Two are restricted to 

 the eastern part, two to the western part, and two regions are 

 transcontinental. These regions are treated in the sections im- 

 mediately following. Foresters generally subdivide these six 

 major regions into a larger number. 



Within each region there are several smaller natural associa- 

 tions or "forest types." These types are usually defined and 

 named by one or more dominating tree species, such as the 

 oak-hickory type in the Central Hardwoods region, or the lodge- 

 pole pine type in the Rocky Mountain region. More than 150 

 such forest types are recognized in the United States alone. 

 Each of the regions will be briefly described, together with 

 the important species found in the regions. The principal forest 

 types for the United States are shown on the Forest Service 

 map in the frontispiece. 



TROPICAL FOREST REGION 



In southern Florida and extreme southern Texas, Arizona, 

 and California are forests made up largely of tropical species. 

 While unimportant, this small area of tropical forest contains 

 many interesting botanical forms. The tropical forest in Florida 

 and Texas is humid in character and is the northern-most 

 range for many species typical of the deep tropics. In Arizona 

 and California, on the other hand, it is a desert climate with 

 largely endemic species. 



