GROWTH OF EXOGENS. 39 



it forms bark and upon the other side wood. 

 When this cambium becomes hard the wood 

 portion is called the sap-wood. This sap- 

 wood differs from the heart-wood in being 

 composed of thinner-walled cells and in con- 

 taining more soluble or organic matters, but 

 it is chiefly distinguished by its lighter color. 

 In some trees it does not appear distinct from 

 the heart-wood. On account of the climate 

 of temperate regions the making of cam- 

 bium is arrested every autumn, and when a 

 new layer is formed the next spring a mark 

 is left which defines the annual increase of 

 the trunk. In cold and unpropitious seasons 

 the growth is light and the layer is thin, 

 while in moist and warm years the layer is 

 much thicker. These layers are therefore 

 meteorological records of the years. It some- 

 times happens that a pinching drouth or 

 other cause will entirely arrest the forma- 

 tion of cambium in early summer and subse- 

 quent rains will cause the growth to be re- 

 sumed, but between these two layers a mark 

 will be left and two rings will be formed 

 in one season. The number of rings, there- 

 fore, are not always a true index to the age 

 of the tree. The growth of the trunk causes 

 the dead outside bark to stretch and split, 



