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- 
HF mened tracheary and wood cells 
Fia. 30.—Growth rings formed at the beginning of the next, 
ris Wey on 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 (e.g. 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 
