43 



estimated that the lower leaf surface of Black Walnut contains 

 about 300,000 per square inch. The leaf is not only peculiarly modi- 

 fied for the reception of light and the absorption of gases, but also 

 for the loss of water. This process of losing water in the form of 

 vapor through the stomata is known as transpiration. The large 

 amount of water given off by trees is usually not appreciated. The 

 Austrian Forest Experiment Station has published data which show 

 that an open-grown birch tree with 200,000 leaves transpired on hot 

 summer days from 700 to 900 pounds. Assimilation, the fourth of 

 the functions named above, comprises a series of changes which are 

 necessary to transform the raw or newly manufactured food material 

 into actual plant tissue. 



6. LEAF-SCARS AND BUNDLE-SCARS: 



Most of our trees and shrubs, except the cone-bearers, shed prac- 

 tically all their leaves in autumn. Those which shed their leaves 

 in this manner are known as deciduous trees, while those which re- 

 tain them for two or more seasons are known as evergreen trees. 

 When the leaf falls a scar is left at the point of its insertion. The 

 leaf-scars vary in size, form, position, occurrence, and the number 

 of vascular bundles which they contain (Plate VI). They may oc- 

 cur singly, in pairs, or in whorls, just as the leaves which precede 

 them. They appear at points on the twigs known as nodes. The 

 portion of the twig between the nodes is called the internode. They 

 may be large, medium, or small in size depending upon the species. 

 If they occur in pairs on opposite sides of the twig they may be so 

 large that they completely encircle the stem, or only a portion of 

 it. Their form may be round, oval, elliptical, heart-shaped, shield- 

 shaped, crescent-shaped, lobed, or triangular. They may be raised, 

 depressed, or even with the surface of the twig. Their surface may 

 be flat, concave, smooth, or wavy. 



The leaf -scars contain bundle-scars. The bundle-scars mark the 

 position of the vascular bundles which formed a connection between 

 the leaves and the twigs. They carry liquid material to and from 

 the leaves. Two distinct portions may be distinguished in these 

 vascular bundles ; the woody portion which serves to carry water into 

 the leaf, and the sieve-tube portion which serves to carry plant food 

 from the leaves where it was manufactured, down into the twigs, 

 branches, and stem. These bundle-scars vary in size, form, and num- 

 ber in a leaf-scar, and the manner in which they are distributed. 

 Some of our common forest trees have only one bundle-scar in a leaf- 

 scar, while many have three, and others four, five, to many. The 

 number is constant in some species and variable in others. The in- 

 dividual bundle-scars usually are circular in outline but may be 

 linear, crescent-shaped, or irregular. Where more than one is found 



