. The essentials of botany. Botany. 62 GROUPS OF TISSUES, OR TISSUE SYSTEMS portions of the tropics where there are marked wet and dry seasons have annual growth periods, separated by seasons, where, growth ceases entirely or nearly so. In such cases the first part of the xylem laid down each year consists of a greater proportion of tracheary elements and fewer wood fibers, the proportion of the latter in- creasing as the season progresses. The wall of each successive fiber is thicker and the lumen smaller. Such tracheae as are produced later in the season are smaller than those first formed.
. The essentials of botany. Botany. 62 GROUPS OF TISSUES, OR TISSUE SYSTEMS portions of the tropics where there are marked wet and dry seasons have annual growth periods, separated by seasons, where, growth ceases entirely or nearly so. In such cases the first part of the xylem laid down each year consists of a greater proportion of tracheary elements and fewer wood fibers, the proportion of the latter in- creasing as the season progresses. The wall of each successive fiber is thicker and the lumen smaller. Such tracheae as are produced later in the season are smaller than those first formed. The contrast of these small thick-walled numerous wood fibers, produced at the close of one season's growth, and the large lu- mened tracheary and wood cells Fig 30 —Growth rmg3 formed at the'bcginning of the next, makes a very distinct line and marks off the growth rings, which, as they are usually annual, are of great value in determining the age of a tree. 93. Usually the wood nearest the center of a tree undergoes changes after it has reached a certain age. Among these changes are the deposition in the lumina of the cells of various organic substances, which seem to make the tracheary elements no longer able to carry water, and the death of all living cells ( cells of medul- lary rays, wood parenchyma, etc.), and often a change in color. Such wood is called heart wood, to distinguish it from the water-conducting sap wood, in which the medullary rays and wood parenchyma cells are still alive. Laboratory Studies, (a) By studying successive thin cross- sections of the stem, beginning at the growing point, there will be found the procambial strands, which give rise to the vascular bundles. They appear, in cross-section, as masses of cells of. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original Bessey, Char
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