SOME IMPORTANT FAMILIES OF ANGIOSPERMS 301 



although the flowers of orchids take on a great variety of 

 forms. The orchid family is the largest among the mono- 

 cotyledons, having over 7000 species, but few of the species 

 are plentiful and some are extremely rare. The oddity and 

 rarity of many orchids have made them favorite objects 

 of cultivation, and by long breeding some wonderfully beau- 

 tiful varieties have been developed. The lady's-shpper is 

 among the best-known of the orchids that grow wild in 

 temperate countries. The underground stems and roots of 

 many (if not all) orchids are inhabited by fungi that are in 

 partnership with the orchid plant and that assist it in securing 

 food. In some cases it is found that a seed cannot develop 

 into a plant unless it is infected by one of these fungi. Some 

 orchids, like the coral-roots, have no chlorophyl and live a 

 completely saprophytic life with the help of the fungi in their 

 tissues. Many of the tropical orchids are epiphytes. The 

 flavoring extract known as vanilla is obtained from the fruit 

 of a tropical American orchid that is much cultivated in 

 tropical countries ; salep, used both as a food and as a drug, 

 consists of the dried tubers of various European and East 

 Indian orchids. Not many members of the family, however, 

 have been found useful for other than ornamental purposes. 



Dicotyledons 



309. Forest Trees. — Certain small families, which are 

 among the more primitive of the dicotyledons, include sorne 

 of the common trees and shrubs. Thus the poplars and wil- 

 lows belong to the willow family ; the walnuts, butternut, 

 pecan, and hickory to the walnut family ; the birches, iron- 

 wood, hop-hornbeam, hazels, and alders to the birch 

 family ; and the beeches, chestnuts, and oaks to the beech 

 family. 



310. The Nettle Family. — This large family also includes 

 many trees, among them the elms and the hackberries. Its 

 members have small flowers, simple in structure ; the stamens 



