i8 



Trees, Stars, and Birds 



was young reach to the top of the tree ? Do you under- 

 stand that if you could begin with the outermost and 



remove the layers of wood 

 one by one from the trunk 

 of an unbranched tree, you 

 would take off a series of 

 thin-walled hollow columns 

 which taper to a point above 

 and which in the tree fit over 

 one another? The column 

 which grew last year incloses 

 the one that grew the year 

 before, and the columns that 

 will grow in the years to come 

 will be fitted about those that 

 are now standing in place. 

 Springwood and summer- 



FIG. ii. Diagram showing the TTT , 



structure of a tree. The dotted w od - When a pine tree 



part in the center represents the begins its growth in the 



^t^t^ *<**- comparatively large 



length than is the trunk of a tree. Vessels with thin walls are 



formed. Later in the sum- 

 mer, as growth becomes slower, the vessels formed are 

 smaller and have thicker walls ; the openings in them are 

 much smaller than the openings in the springwood. We 

 have in the tree, therefore, alternating layers of light 

 springwood and of denser summerwood. This causes 

 the appearance of rings in the wood, which you have 

 often seen. In different kinds of trees there are often 

 differences in the springwood and summerwood ; these 

 will be discussed in a later chapter (page 43). 



