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 (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 



