The Growth of a Tree 



season's growth, the annual ring of wood on one side of the cam- 

 bium and the annual layer of bark on the other. 



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 between 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 bark 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 neighbouring ring. The wood gradually merges from a dark 

 band full of large pores to one paler in colour and of denser texture. 

 It is very distinct in oak and ash. The coarser belt was formed 

 first. The spring wood, being so open, discolours by the accumu- 

 lation of dust when exposed to the air. The closer summer wood 

 is paler in colour and harder, the pores almost invisible to the 

 unaided eye. The best timber has the highest percentage of 

 summer wood. 



If a tree had no limbs, and merely laid on each year a layer 

 of wood made of parallel fibres fitted on each other like pencils 

 in a box, wood splitting would be child's play and carpenters 

 would have less care to look after their tools. But woods differ 

 in structure, and all fall short of the woodworker's ideal. The 

 fibres of oak vary in shape and size. They taper and overlap 

 their ends, making the wood less easily split than soft pine, for 

 instance, whose fibres are regular cylinders, which lie parallel, 

 and meet end to end without "breaking joints." 



