Addilioiial Pa[H'rs. 



317 



growth into annual rings, which serve as a means of computing the age 

 of a tree. Serious injury to the tree often follows the death of the cam- 

 bium" cells, which may be caused in a number of ways, e. g'., by heavy ob- 

 jects striking the tree, by driving nails into the tree, the action of wood- 

 destroying fungi, etc. 



The Leaf-fall and the Winter Rest. — During the months of autumn, 

 the leaves of our broad-leaved trees fall off, and during the winter months 

 the trees are in a dormant condition. The leaves of the Evergreens as- 

 sume a darker color and suspend activity, although they still remain on the 

 tree. 



What causes the fall of the leaf? Frost plays some part, but it is 

 not the only cause. Early in the autumn a layer of cork cells containing 



Fig. 3. An Oak showing the results of ignorant pruning. This tree will he iuevit;ihly at- 

 tacked by a host of fungous and insect enemies. Photographed In December. 



.water is formed at the base of each leaf-stalk. When the water in these 

 cells freezes, it tears apart the cell walls and allows the leaf to fall. If 



