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 hght 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, hike 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 are 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 hght. Many 
of our deciduous trees also exhibit quite regular, conical-shaped 
erowns. 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 eylinder 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 
erowth 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 hghter 
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 
