THE STRUCTURE AND GROWTH OF STEMS 177 



this wood, to think of all the intricate processes and forces 

 of which we know so little except that they have been 

 concerned in this matter — this stirs our wonder and our 

 admiration. 



If you were there when the tree fell, you smelled its 

 fragrance and perhaps saw the sap oozing from that wood 

 which lies just beneath the bark. You noted that the. 

 wood is divided into two quite distinct zones. The outer 

 and usually lighter zone is the sapwood; the rest is heart- 

 wood. It is common to speak of the sapwood as alive, 

 and of the heartwood as dead, but this is not exactly ac- 

 curate. Many of the cells through which sap runs are 

 already dead. So, though the trunk as a whole may have 

 endured for centuries, only a comparatively small part of 

 it has been alive at any one time. This live zone lies be- 

 tween the outer bark and the heartwood, and the most 

 active part of it is, as you know, the cambium. 



By understanding the way in which the cambium works 

 we can understand how the rings in the wood are formed. 

 Perhaps you have heard that each one of these rings indi- 

 cates a year of growth, and that by counting the rings you 

 can tell the age of the tree. This is not strictly true. 

 These rings are often called annual rings, which implies, of 

 course, that each one of them represents the growth of one 

 year. It is better to call them growth rings, for more than 

 one may be formed in a year. 



It is true that a ring usually represents a season's growth, 

 but that does not explain why the ring is there. The fact 

 that a bricklayer stops work at night does not mean that 

 the wall of bricks he is building will show by lines where 

 he left off at night and where he began in the morning. 

 So with the rings. They are not caused by the fact that 



