STEMS AND BRANCHES AND THEIR USES 209 



does begin, the sapwood is not turned into heartwood one 

 ring each year as we might expect ; but the thickness of the 

 zone that is changed varies from year to year with the condi- 

 tions of growth and with the age of the tree. The older the 

 tree, the more rapidly the change takes place ; for this reason, 

 the zone of sapwood is narrower in old trees than in young 



FIG. 130. Part of a cross section of a three-year-old stem of basswood ; 

 a, spring wood formed during the past year; b, summer wood; c, bast. 

 The large openings in the spring wood of each annual ring are sections of 

 vessels. 



ones. In some trees, as in the oak, the distinction between 

 heartwood and sapwood is very sharp. This is especially 

 true of the ebony, whose sapwood is white and its heartwood 

 black. In some, including the ash, although the heartwood 

 is harder and more solid than the sapwood, it remains of much 

 the same color. In others, like the birch and the poplar, no 

 material change of any kind takes place in the wood ; it re- 

 mains soft, light in weight, and unchanged in color. 



228. Spring and Summer Wood. Trees differ much in 

 the thickness of their annual rings. Even in the wood of a 

 single tree, some rings are much thicker than others. Differ- 



