8 Bulletin of the University of Texas 



The water, with its dissolved mineral salts taken up by the roots, 

 passes up through the stem and its branches out into the leaves. 

 The stem responds to the stimulus of gravity and light by grow- 

 ing away from the pull of gravity and toward the light. This 

 response is exactly the opposite to that made by the root, and 

 results in an elongated aerial organ whose chief function is to 

 display the foliage leaves to light. The lateral branches of the 

 stem, like the branches of the root, grow in various directions. 

 The branches may be almost horizontal, drooping, or ascending. 

 In the coniferous trees, such as the pines, the main stem does not 

 branch equally, but extends upward through the crown to the 

 top, and the lateral branches iare arranged in regular whorls, 

 which 'decrease in length from the base to the apex of the stem. 

 Such trees form a more or less symmetrical, elongated, cone- 

 shaped body, which affords the optimum exposure to light. Many 

 of our deciduous trees also exhibit quite regular, conical-shaped 

 crowns. The lombardy poplar, the sour gum, and many species 

 of oak and maple furnish examples. In some trees the main trunk 

 does not extend upward through the crown, but is soon lost and 

 the crown becomes rounded or spreading; the bur oak, the hack- 

 berry and the American elm are examples of this type. 



In transverse cuts across the stem or any of its branches, four 

 distinct main parts may be observed : the central pith, the woody 

 cylinder, the cambium or growing layer, and the bark. 



The woody cylinder consists of a series of annual rings arranged 

 more or less in concentric order. Each ring represents a single 

 season of growth, and by counting the rings the age of the tree in 

 years can be determined with almost perfect accuracy. The ring 

 is made up of two layers, of which the thin, firm layer represents 

 the fall growth, while the more porous large vessels develop in 

 the spring and early summer. It sometimes happens that if 

 growth is checked during the growing season more than one ring 

 is formed, but this can usually be distinguished by careful exami- 

 nation. The outer layer of wood, the sap wood, is usually lighter 

 in .color and more readily subject to decay than the central layers 

 or heart wood. This is due to the deposition in the vessels of the 

 heart wood of various secretions which render them impervious 

 to water and exert an antiseptic action on the organism of de- 

 cay. The sap wood is the chief water-conducting avenue of the 



