SUNSHINE THROUGH THE WOODS. 317 



shown in Figs. 1 and 2 as a substance quite different from the 

 wood that lies within, and is protected by it. Growth of the 

 stem of ordinary trees takes place in a continuous zone just be- 

 neath the bark, the latter being also supplied with new material, 

 as it may be needed to supply the same formative layer. As the 

 years roll on, the wood first made, while the stem was small, and 



Fig. 4. Cross Section of Tin Oak. Positive. 



now situated near the center, changes its appearance by taking 

 on some color, the shade being determined by the kind of wood. 

 In some of the " precious woods," so called because of their great 

 value for special purposes and possibly their variety, the central 

 or heart wood is nearly jet black, as in the ebony. There is 

 usually a marked difference in the color between the latest formed 

 sap wood lying close under the bark and that formed many years 

 before and now covered by later layers. 



We have come now to consider another point of structure pre- 

 viously hinted at and plainly shown in the negravings, namely, 

 the rings of wood. The tree as it enlarges from year to year 

 leaves in its structure the evident record of its life. Each grow- 

 ing season is marked by a ring of wood, and only under the most 

 adverse circumstances is this deposit omitted, and likewise ex- 

 traordinary events only can lead to the formation of two rings. 

 Therefore with a fair degree of certainty the age of a forest giant 

 can be determined by the number of annual deposits of wood in 

 rings around the common center. 



These deposits become manifest to the naked eye, because of 

 the difference in structure between the spring and autumn de- 

 posits, speaking of course for tree growth in the temperate re- 

 gions. Glance at the papaw stem in Fig. 1, and it will be seen 

 that the lower portion includes the heartwood nearly to the cen- 



