31 



foliage of most of the conifers enriches the winter scenery and af- 

 fords shelter for birds and other animals. Many conifers are highly 

 prized for ornamental purposes and some yield valuable commer- 

 cial products. The Pines, Spruces, Firs, Cedars, and Hemlocks are 

 the commonest examples of this group. In addition to the conifers 

 a few broad-leaf species, such as Rhododendron, Mountain Laurel, and 

 American Holly, are evergreen. A transition from the evergreen to 

 the deciduous habit may be found in the Deciduous Holly and the 

 Laurel Magnolia which are deciduous in the northern and evergreen 

 in the southern states. 



The deciduous trees are commonest in the eastern part of North 

 America while the evergreen are commonest in the western part. 

 The former are usually found in mixed stands, while the latter often 

 occur in extensive pure stands. The hardwood species usually occur 

 on rather fertile soils while the conifers may thrive on more sterile 

 ones. Both the deciduous and the evergreen habits have their ad- 

 vantages. The shedding of the leaves in fall is a protective adapta- 

 tion since it reduces transpiration, danger from snow-break, and 

 damage from noxious gases. The evergreen trees have the advantage 

 of lower summer transpiration and are ready at any season of the 

 year for constructive activity. They are also less subject to damage 

 by frost during the growing season. The advent of forestry may 

 change the structure and distribution of our forests. The present 

 tendency seems to be gradually and cumulatively in favor of the 

 conifers. 



THE AGE OF TREES. 



Some trees reach great size and enormous age while others re- 

 main small and die young. The size and age which a tree attains 

 depend upon the inherent tendency of the species and the factors of 

 the environment. Some species which naturally grow tall and be- 

 come old may remain small under unnatural and unfavorable 

 growth conditions. Other species never become large and old even 

 under the most favorable growth conditions since it is inborn in 

 them to remain small. A definite age limit cannot be fixed for each 

 species but for general convenience we may classify our common 

 trees as short-lived or long-lived. Of the trees native to the State of 

 Pennsylvania the Oaks, Chestnut, Buttonwood, Tulip Tree, White 

 Pine, and Hemlock may be regarded as long-lived trees, and the Pop- 

 lars, Willows, most Birches, and some Cherries as short-lived. Some 

 of the White Oaks found in the original forest of Pennsylvania 

 showed an age of approximately 500 years. Some of the trees of this 

 State reach a great age and enormous size, still none approach such 



