SOME FAMILIES OF FLOWERING PLANTS 



705 



[Chap. LI 



to 12 feet. There are 5 genera and about 600 species. Some of the 

 oaks have hybridized freely and are extremely difficult to classify. The 

 members of this family also apparently had their origin during Cre- 

 taceous times. 



Fig. 342. Catkins of staminate flowers on a branch of white oak {Qiiercu.s alba). 



Economically the family is probably the most valuable of all woody 

 flowering plants. The wood is particularly durable and is important in 

 making furniture, floors, ships, posts, railroad ties. The plants yield 

 nuts and cork, and many are used for ornamental planting. 



The elm family ( Ulmaceae ) . The family includes some 13 genera and 

 140 species, of which the elms and hackberries are the best known. 

 The plants are either shrubs or trees, common in the subtropics and 

 in temperate regions. The elm is especially valued as a shade tree, al- 

 though its existence is at present threatened by parasitic fungi and 

 viruses. The trees are not particularly valuable for their wood. 



Related plants, and by some of the early authors included in this 

 family, are nettles, mulberries, figs, hemp and hops. 



The mistletoe family ( Loranthaceae ) . This curious family consists of 

 about 20 genera and 500 species, chiefly tropical and subtropical but 

 occasionally found in temperate regions. The plants are either com- 

 pletely or partly parasitic on other woody plants. They are usually less 

 than a foot in height and often appear to be dichotomously branched. 

 The root is a modified haustorium and through it part or all of the 



