Trees and How to Know Them [ 343 



Lacking water, the green pigment of the leaves begins to fade 

 and is gradually replaced by yellow and orange pigments that have 

 been present all along but in smaller quantities than the green. 

 Red coloring has a different origin: It is formed in the cell sap 

 by the same sort of "dye" that colors red cabbage and beets. You 

 can look for lovely red displays on sugar maples, white oaks, and 

 sumac. 



Poplar, hickory, and linden are some of the trees that have 

 golden-yellow fall coloring. The green pigment of evergreens is 

 so hardy that as we might guess from the name of this tree it is 

 not affected by winter conditions. 



How Leaves Die and Drop Off: While the leaves are changing 

 color, a thin corklike layer of cells develops between the leaf stems 

 and the twigs to which they are fastened. This layer of weak tissue 

 reduces or shuts off completely the flow of sap to the leaf. This not 

 only contributes to the death of the leaf it also weakens its attach- 

 ment so that it falls at a slight breeze or even from its own weight. 



How Knots and Knotholes Are Formed 



Trees Prune Their Own Branches: Children often have the 

 opportunity to watch trees being pruned in city parks or on 

 suburban lawns. But they are surprised to learn that trees growing 

 under natural conditions are also pruned. The trees do this 

 pruning themselves! One process, called natural pruning, works 

 like this: Lower branches become undernourished because exces- 

 sive shade prevents their leaves from manufacturing food, with 

 the result that these branches die and drop off. 



In willows, poplars, and other trees, layers of weak tissue, 

 similar to those that cause leaves to fall, form somewhere along 

 certain branches sometimes at the base. After a while the 

 branches break off, even though many fresh leaves may still be 

 attached to them. This process is known as self-pruning. 



Knotholes and Peepholes: When a branch is lost to a tree by 

 pruning, the remaining short stump of branch eventually becomes 

 overgrown by the trunk. If the tree is felled and cut for lumber, 



