37 



often hairy. As they develop during the season they become firmer 

 and often lose their green color and their hairs. The direction of 

 the new growth is variable. In many species it takes at first a 

 drooping direction and later, as its elements become firmer, it as- 

 sumes a horizontal or ascending position. The new growth of the 

 Pines is conspicuous in that it grows in an erect direction at first 

 and later becomes horizontal or drooping. 



The taste, smell, and color of the twigs, are helpful in distinguish- 

 ing some of our common species. The twigs of some species as the 

 Black Birch, Spice Bush, Sassafras, and Wild Cherry have a char- 

 acteristic taste and smell. The color of the twigs may be green 

 as in the Sassafras, red as in the Basswood and Red Maple, or brown 

 as in the Sugar Maple. Many other different colors and combina- 

 tions of color aid materially in distinguishing our trees. 



Some twigs are rough while others are rather smooth. They 

 may be roughened by hairs, lenticels, raised leaf-scars, bud-scale 

 scars, warty or resinous exudations, corky projections, or decurrent 

 projections of the bark. If we examine a young twig just after it 

 has emerged from the bud we will find that it is usually green in 

 color. At the end of the first season's growth a thick bark has 

 usually developed which is no longer green on the surface, but, by 

 cutting a cross section of a twig, one will often find that the inner 

 bark is still green. This green tissue develops chlorophyll and manu- 

 factures food just as* does the green tissue of the leaves. As the 

 bark increases in thickness the chlorophyll decreases, eventually dis- 

 appearing entirely from the stem. In order that this green tissue 

 in the bark may function it is necessary that gases be exchanged 

 through the bark. Special structural modifications on the bark 

 known as lenticels (Figs. 96 and 98) make possible this exchange 

 of gases just as the stomata on the leaf-surfaces allow and even regu- 

 late the exchange of the gases of the leaf. 



The lenticels are very numerous and conspicuous on some species, 

 while on others they are rare and inconspicuous. They are raised on 

 some species like the Elder, while on others they are even with the 

 bark. Their color varies. They may be white, gray, pinkish, yel- 

 low, brown, or black. In outline they are usually circular or slightly 

 elongated. In the Cherries and Birches they are confluent, a char- 

 acteristic which results in the horizontally elongated lines of lenti- 

 cels (Figs. 96 and 98) so common on their trunks. 



The duration of the lenticels varies with the species and its en- 

 vironment. As a rule the more rapidly bark is formed the shorter 

 is the duration of the lenticels. On some species it is difficult to find 

 lenticels on any but the last season's growth while on others they 

 may persist for some years, The exfoliation of the bark causes their 



