35 



as a protective covering while the latter is known as the inner or 

 living bark and helps to convey the food which was manufactured 

 in the leaves to various parts of the stem. Thickness of the bark 

 is often determined by the rapidity with which it peels off. Its 

 thickness, together with its larger number of dead, air-containing 

 cells, makes it a very effective protective covering; but the chief 

 protective feature of bark is the formation of corky layers. The 

 chief function of the protective covering in plants is the prevention 

 of excessive transpiration. The regular cork formations in the bark 

 help very much in controlling transpiration. Cork is one of the most 

 valuable elements of the bark. Its structure is complex and vari- 

 able. Cork is impermeable to air and water, a poor conductor of 

 heat, and a preventer of penetration by parasites. Local out-growths 

 of cork like the wings of the Sweet Gum and Hackberry are prob- 

 ably of no value to the plants producing them. In some species the 

 bark is not fully "ripened" at the end of the growing season, con- 

 sequently the subjacent tissues do not have the necessary protection 

 and frequently die back during the winter. The color of the bark 

 varies in different species, in different situations, in different parts 

 of the same species, and with the age of the trees. Young bark is 

 usually green, but it soon loses this color due to the formation of 

 cork and other substances. A few species like Sassafras retain their 

 green color for a relatively long time due to deferred or late cork 

 formation. Gray, brown, and black are the prevailing bark colors 

 while red and white are also common. Color of the bark is very 

 helpful in distinguishing many of our common trees. All the species 

 of Birch native to Pennsylvania may be distinguished from each 

 other by the color of their bark together with a few other bark char- 

 acteristics. The bark on some of the older trunks becomes rough 

 and then the characteristic color of the species may be present only 

 on the branches and young stems. The outer bark may be uniform, 

 mottled, or variegated in color. The interior and exterior parts of 

 the bark may differ in color. Black Oak bark is yellow within and 

 black without, while Hemlock is reddish within and brown to black 

 without. The bark of the Buttonwood is peculiar since it is dark 

 brown without and green, yellow, or white within. The inner bark 

 often becomes very conspicuous due to the complete peeling off of 

 the outer bark. 



Young branches and stems are usually smooth since the bark ex- 

 pands sufficiently to accommodate the increased diameter growth 

 of the interior. Later, in most species, the bark begins to crack, 

 since the growth of the interior is too rapid for the expansion of 

 the bark. In a few species like Beech, Blue Beech, and Balsam 

 Fir the bark remains thin and smooth throughout life. Other species 

 like Basswood and Pin Oak remain smooth for a long time but be- 



