72 IMPORTANT TIMBER TREES 



son advances the number of cells lessen and the wood becomes 

 much more compact and in most species harder, and this 

 is called the Summer Wood. In some species there is such 

 a distinction in color, porosity, and density between the 

 spring and summer wood that the annual rings can be dis- 

 tinguished without difficulty, and these features more or 

 less affect the character of the wood. As a rule the annual 

 rings indicate the age of the tree, but not always. If a 

 drouth occurs in midsummer, growth will be arrested, and 

 apparently a normal ring will' be formed ; but if wet and 

 warm weather then succeeds, growth will be resumed, and 

 another but thinner ring will be laid on quite similar to the 

 first. This, however, does not frequently occur, and the 

 number of rings is a fair guide to the age of the tree. 



Heartwood and Sap wood. While carrying the min- 

 eral ingredients of the tree's food to the leaves the sap is 

 restricted in its passage, possibly in part to the live portion 

 of the bark, but mainly to a limited number of the young- 

 est annual layers of wood. These vary in number with the 

 species of trees, and also with the conditions environing 

 each individual tree of any given species, but as a rule 

 they are quite uniform in each species. That portion of the 

 tree through which the sap passes is called the Sapwood. 

 After serving for a time for the purposes named, a change 

 takes place in the innermost ring of the sapwood and the 

 sap no longer flows through it, and it then becomes what is 

 known as Heartwood. After this change occurs, that por- 

 tion of the tree ceases to perform any life functions. For 

 nearly all purposes it is dead, and the only service it there- 

 after renders is to support the growing portion of the tree 

 and prevent its destruction by winds. All the heartwood, 

 as it frequently does, may decay and the tree remain alive 

 and be nothing but a shell of sapwood. In most species 

 the color of the heartwood is darker than that of the sap- 

 wood, but not in all, for in some the reverse is the case, 

 and in others there is very little or no difference. 



For nearly all purposes heartwood is preferred to sap- 



