[Chap. LI SOME FAMILIES OF FLOWERING PLANTS 



703 



Fig. 340. A tropical epiphytic orchid (Dendrobiiim) in cultivation. 



definitely plants of northern latitudes. Both willows and poplars are 

 ancient plants, as is evidenced by the finding of fossils in the rocks of 

 the Cretaceous period. They have hvbridized freely and some present- 

 day species are difficult to classify. There are nearly 200 species which 

 vary in size from low creeping shrubs to large trees. 



The poplars are all trees, but the willows are both trees and shrubs. 

 The arctic willows may be only a few inches in height. The simple 

 naked flowers are borne in catkins, and the staminate and pistillate 

 flowers occur on separate plants. The plants are commercially valuable 

 chiefly as sources of paper pulp, baskets, and some kinds of furniture. 

 They are often planted where their quick growth may prevent erosion 

 or supply shade until more desirable trees can develop. 



The walnut family {Juglandaceae) . The chief representatives of the 

 walnut family are walnuts, butternuts, hickories, pecans. There are 6 

 genera and about 35 species. The species of walnut were more numerous 

 during the Cretaceous period than now. The plants are trees, some in- 

 dividuals reaching heights of 150 feet and diameters of 5 to 6 feet. The 

 wood of the walnut family is especially valuable for furniture, ax han- 



