[Chap. LI SOME FAMILIES OF FLOWERING PLANTS 



709 



(Fig. 346), and by such trees as black locust, honey locust, Kentucky 

 coffee tree. About two-thirds of the legumes belong in this family, the 

 others to the nearly related mimosa and senna families. 



Fig. 346. The subterranean fruits on a peanut plant. Note also the root nodules. 



Nitrogen-fixation occurs in the roots of legumes through symbiotic 

 relationships with certain bacteria (Chapter XLIII). The pea flower 

 has been discussed in Chapter XXXII. Legumes contain much protein 

 and thus are important food of animals, supplementing the largely 

 carbohydrate seeds and fruits of the grasses. Legumes are yery yalu- 

 able as both forage and hay crops. They are important sources of oils, 

 honey, drugs, and timber, and some species such as sweet peas have 

 decorative value. 



The recent rise in importance of the soybean in the United States 

 deserves special mention. In 30 years the acreage devoted to soybeans 

 has increased from about 50,000 to nearly 6 million. These plants have 

 been extensively cultivated in China for centuries, but were not intro- 

 duced into this country until 1804. The uses of the soybean are astound- 

 ingly varied: pasture, silage, soil improvement; celluloid substitutes. 



