^ 



12 TREES 



What was once a delicate cell now becomes a hollow 

 wood fibre, thin walled, but becoming thickened as it gets 

 older. For a few years the superannuated cell is a part of 

 the sap wood and is used as a tube in the system through 

 which the crude sap mounts to the leaves. Later it may 

 be stored full of starch, and the sap will flow up through 

 newer tubes. At last the walls of the old cell harden and 

 darken with mineral deposits. Many annual rings lie be- 

 tween it and the cambium. It has become a part of the 

 heart wood of the tree. 



The cells of its own generation that were crowded in the 

 other direction made part of an annual layer of bark. As 

 new layers formed beneath them, and the bark stretched 

 and cracked, they lost their moisture by contact with the 

 outer air. Finally they became thin, loose fibres, and 

 scaled off. 



The years of a tree's life are recorded with fair accuracy 

 in the rings of its wood. The bark tells the same story, 

 but the record is lost by its habit of sloughing off the outer 

 layers. Occasionally a tree makes two layers of wood in a 

 single season, but this is exceptional. Sometimes, as in a 

 year of drought, the wood ring is so small as to be hardly 

 distinguishable. 



Each annual ring in the chestnut stump is distinct from 

 its neighboring ring. The wood gradually merges from a 

 dark band full of large pores to one paler in color and of 

 denser texture. It is very distinct in oak and ash. The 

 coarser belt was formed first. The spring wood, being so 

 open, discolors by the accumulation of dust when exposed 

 to the air. The closer summer wood is paler in color and 

 harder, the pores almost invisible to the unaided eye. The 

 best timber has the highest percentage of summer wood. 



