48 TREES 



last the summer, and sometimes persist until spring 

 growth unseats the stalks; sometimes, as in the "live oaks," 

 they hang on three to five years. 



The twigs of oak trees are more or less distinctly five- 

 angled, and the winter buds cluster at the ends. This in- 

 sures a group of young shoots, crowded with leaves, on 

 the ends of branches, and a dense outer dome of foliage on 

 the tree. 



Nearly three hundred distinct species of oaks are recog- 

 nized by botanists, and the Hst is growing. New species 

 are in the making. For instance, a white oak and a bur 

 oak grow near enough for the wind to "cross-fertilize" 

 their pistillate flowers. The acorns of such mixed parent- 

 age produce trees that differ from both parents, yet reveal 

 characteristics of both. They are "hybrids," and may be 

 called new varieties of either parent. Other species of oak 

 are intercrossing by the same process — the interchange of 

 pollen at the time of blossoming. This proves that the oak 

 family is young, compared with many other families, whose 

 members are too distantly related to intercross. 



Though geologically young, the oak family is one of the 

 most important, furnishing timber of superior strength and 

 durability for bridge-building, ship-building, and other 

 construction work. Tanning has depended largely upon 

 oak bark. As fuel, all oak trees are valuable. 



Fifty species of oak are native to North American 

 forests. Twice as many grow east of the Rocky Moun- 

 tains as west of the Great Divide. No species naturally 

 passes this barrier. The temperate zone species extend 

 southward into tropical regions, by keeping to high alti- 

 tudes. Thus we find American oaks in the Andes and 

 Colombia; Asiatic species occur in the Indian Archi- 



