18 NATURE STUDY REVIEW [9:1— Jan., 1913 



The bark varies in character according to the age of the tree. 

 In the young tree it is smooth, but, as the trunk expands from 

 the growth of the wood within, the covering of dead bark out- 

 side is forced to crack in a variety of ways, giving rise to char- 

 acteristic fissures and ridges which become more prominent as 

 the tree grows older. The bark of few trees such as the Beech 

 and the American Hornbeam remain smooth, their outer layers 

 expanding with the growth of the tree. The barks of others as 

 the Yellow Birch (Fig. 3) stretch and peel ofif in thin papery 

 layers. In the Birches and Cherries the breathing pores (len- 

 ticels) become horizontally elongated to form narrow transverse 

 streaks which are characteristic for these forms. When ridges 

 or scales are formed they may be close and firm and with diffi- 

 culty removed from the trunk as is the case with the bark in the 

 Black Oak group or, on the other hand, they may be easily rubbed 

 ofT as are the scales of the bark of the White Oak and of most 

 members of the White Oak group. Bark of this latter type is 

 called Haky and this distinction between barks that are flaky and 

 those that are not flaky is of considerable importance in classifica- 

 tion. The bark may come oiT in large sheets as in the Shag-bark 

 Hickory and the Sycamore, and the ridges may be long as in 

 the Chestnut or short and run together to form more or less 

 perfect diamond-shaped areas as in the White Ash (Fig. 4). 



Tzmgs — The Horse-chestnut (Fig. 5) may be taken as a con- 

 venient form to illustrate the various markings found on the 

 twig. The large triangular patches resembling somewhat closed 

 horse-shoes in shape are the leaf-scars showing where the bases 

 of the leaf-stalks were attached to the twig before their fall. The 

 little dots corresponding to the nail holes in a horse-shoe are the 

 bundle-scars and mark the location of the so-called fibro-vascular 

 bundles. The leaf-scars are located at the nodes and the portion 

 between the nodes is called the internode. Scattered along the 

 twig are little dots, the lenticels, which are openings that func- 

 tion to a certain extent like breathing pores. Above each leaf- 

 scar is normally produced an axillary bud. The lateral buds are 

 in distinction to the terminal buds at the ends of the twigs. 

 The bud-scales at falling leave a ring or band of scale-scars mark- 

 ing the limit of each year's growth. These bands often remain 

 distinct for many years and by coiintino^ their number the age of 

 the branchlet may be estimated. Thus it can be readily seen that 

 the .figure of the Horse-chestnut represents growth made during 

 three years. 



