49 



important as wind, must still be regarded as an agent of seed dis- 

 persal. It transports some seeds over great distances, especially 

 those which will float or are inclosed in bladder-like inclosures like 

 the American Hop Hornbeam (Plate IX, 10), or the Bladder Nut, a 

 small shrub very commonly found along our streams. Gravity on 

 slopes, is a minor agent of seed dispersal, but sometimes does effective 

 work, especially with heavy seeded species like Oak and Beech. 



9. Wood: 



Wood, next to food, and clothing, is probably the most useful and 

 indispensable material which man uses. It is found in many of the 

 higher plants but becomes of commercial importance only in the sper- 

 matophytes or seed-bearing plants. In the timber-producing trees it 

 is found in the roots, branches, and stems. The wood derived from 

 the roots is limited in quantity and inferior in quality. The branches 

 produce wood which, in some respects, very closely resembles that of 

 the stem, but is inferior on account of its sm<aller size, irregular shape, 

 and more knotty structure. The wood obtained from the stem is of 

 the greatest utility and value on account of its desirable dimensions 

 and satisfactory structure. The stem should not only yield a large 

 quantity of wood but also a superior quality. The quality of wood 

 which a stem will yield depends largely upon its age, inherent ten- 

 dencies of the species, and i^ts environment during its development. 

 High grade material is usually obtained from the stems of valuable 

 species which have attained a large size, are free from lateral 

 branches, and possess little stem taper. The form and character of 

 the stem are dependent on the environment. A suitable environment 

 may be created by applying the fundamental principles of forestry 

 which will not only increase the productivity of our forests but also 

 the quality of the yield. 



In order to identify the different kinds of woods it is necessary 

 to study them from the following three sections: cross, radial, and 

 tangential (Plate XI, 7). An examination of a cross-section of a 

 woody stem will show that the major part of the structure consists 

 of wood which is covered with bark on the outside and has a narrow 

 cylinder of soft tissue known as pith running through the center 

 (Plate XI, 1). 



The woody portion of most of our trees, especially the older ones, 

 may be differentiated into two parts on the basis of colors. The 

 central colored part is known as the heartwood, while the outer al- 

 most colorless part is known as the sapwood. A narrow zone of cells 

 located between the sapwood and the bark is known as the cambium 

 (Plate XI, 1). All the wood elements have their origin in this zone. 

 For sometime after their origin these elements are living, but later 



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