STEMS AND THEIR FUNCTIONS 157 



usually darker in color than the spring wood. Because of the regular 

 alternation of seasons the wood comes to be made up of concentric- 

 cylinders (rings, in cross section) of spring and summer wood. A pair 

 of such layers constitutes an annual ring, 1 and the age in years of any 

 particular part of the tree may be determined by counting them. In 

 tropical regions with uniform temperature and rainfall throughout the 

 year, most trees do not produce annual rings ; but any regular alternation 

 of favorable and unfavorable conditions, such as wet and dry seasons, 

 results in their formation. Another well-known feature of the xylem is 

 its differentiation into heartwood and sapwood in many trees. The heart- 

 wood is the older, central portion of the xylem. It no longer functions as 

 part of the vascular system, for its cells have become impregnated and 

 darkened with resins and tannin, increasing its density and strength but 

 blocking the water channels. It is thus transformed into an exclusively 

 mechanical tissue. The sapwood is the younger outer portion of the xylem: 

 its main function is water transport. 



The "bark" of the stem is made up of all the tissues that lie outside 

 of the xylem — the phloem, the cortex, and (in younger stems) the epi- 

 dermis. It can readily be stripped from the wood, since the layer of 

 delicate, thin-walled cambium cells is easily ruptured; "peeling" a stem 

 destroys the cambium. 



During periods of growth the cambium adds to the phloem from the 

 inside. As the diameter of the xylem cylinder increases, the original con- 

 tinuous enclosing cylinder of phloem becomes too small to encircle the 

 stem and breaks apart. Since the cambium, by radial cell division, keeps 

 pace with the growth of the xylem cylinder and always forms a continuous 

 layer outside it, the new phloem which the cambium produces at any 

 given growth stage is always just large enough to enclose the cambium 

 and xylem at that stage. The broken remnants of the earlier phloem 

 layers, crumpled and discontinuous, are forced outward by the enlarge- 

 ment of the xylem and phloem cylinders beneath them. The spaces left 

 between these remnants become filled with cortical parenchyma. 



Beneath the original epidermis there develops a cork cambium, as in 

 the root. The impermeable layer of cork cells produced by this cambium 

 causes the death of the epidermis, and becomes itself the outer protective 

 sheath of the stem. In smooth-barked trees the original cork cambium 

 persists, increasing in diameter as the trunk enlarges. It continuously 

 replaces from within the thin cork layer that forms the outer bark, as 

 fast as the older parts of this layer crumble off from the surface. In rough- 

 barked trees deeper layers of cork cambium form one after another beneath 



1 The figure or pattern seen on the cut surfaces of wood is caused by the annual 

 rings — narrow parallel stripes if the log was sawed radially, broad bands or parabolas 

 or ellipses if the log was cut tangentially. 



