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BIOLOGY AND HUMAN LIFE 



12. Flax family. This small family has a distinguished member in 

 the very useful flax plant. Besides the fiber we also use the seed, which 

 furnishes a gum similar to tragacanth, the linseed oil which is so impor- 

 tant in the mixing of paints, and the oil cake used as fodder for cattle. 



13. Rue family. 

 The citrus fruits 

 (the lemon, orange, 

 lime, citron, tan- 

 gerine, and pomelo, 

 or grapefruit) are 

 of growing impor- 

 tance as cultivated 

 commercial fruits. 

 From the point of 

 view of diet they 

 are valuable for the 

 acids, mineral mat- 

 ter, and vitamin C 

 which they contain. 



14. Spurge fam- 

 ily. In addition to 

 the beautiful poin- 

 settia this family 

 has three plants of 



Fig. iSi. The peanut plant (Arachis hypogaea) 



This valuable representative of the bean family is a 

 native of Brazil, but is now cultivated in most tropical 

 and semitropical countries for its protein and fat. It 

 is a good hay plant, and large quantities are raised for 

 fattening hogs in the field. After fertilization the flower 

 stalks elongate and the ovary works into the ground 



great value to man. 



The South Ameri- 

 can rubber plant 

 (Hevea) is of in- 

 creasing importance 

 as a source of rubber. Tapioca is obtained from the bitter cas- 

 sava. The pulp of the crushed roots consists of a poisonous 

 milky fluid and masses of starch. The starch settles out but con- 

 tains a poisonous substance. The mass is then dried over fires on 

 iron plates, to destroy the poison, and the starch breaks up into 

 grains or small balls. The castor bean is of value because of its 

 heavy oil, which serves as a lubricant in the intestine. 



