272 INTRODUCTION TO PLANT GEOGRAPHY [CHAP. 



to have had its origin in minute primitive forms of plant Hfe that 

 flourished in much earher geological times. 



Latexes and Exudates 



Of the products obtained from the milky juice (latex) of various 

 plants, rubber is by far the most important. In spite of the con- 

 siderable employment of synthetic forms, over a million tons of 

 natural rubber are used annually, most being produced in south- 

 eastern Asia. Over three-quarters of the crude rubber consumed 

 goes into tyres and inner tubes, while other important uses are in 

 footwear, packaging, waterproof clothing, road construction, tubing 

 and belting, electrical insulation, etc. Rubber is produced princip- 

 ally from various tropical woody plants of the Spurge, Mulberry, 

 and Periwinkle families, some of which are now cultivated. Besides 

 wild and plantation rubbers produced from the Para (Hevea) Rubber 

 tree, which is by far the most important source of rubber, there are 

 such other natural types, produced from different sources, as Assam, 

 castilla, ceara, guayule, and even dandelion rubbers, the last being 

 cultivable in the cool-temperate belt. 



Other latex products include the non-elastic gutta-percha and 

 balata, obtained from tropical trees and used for insulation, piping, 

 golf-balls, telephone receivers, and many other purposes, and chicle, 

 obtained from the tropical American Sapodilla tree, which is the 

 basis of the chewing-gum industry. 



The gums which exude from plant stems either naturally or in 

 response to wounding, and the resins which are usually secreted in 

 definite cavities or passages, may conveniently be classed together 

 as exudates. Gums are used principally as adhesives, as sizing for 

 paper, in medicine and polishing, in cosmetics and ice-cream, in 

 printing and finishing textiles, as a glazing for paintings, and in the 

 confectionery and paint industries. The chief commercial varieties 

 are gum arabic, obtained from certain Acacias of arid northern 

 Africa, gum tragacanth, from certain Milk-vetches of arid south- 

 western Eurasia, and its substitute karaya, obtained from a tree in 

 India whence several million pounds are exported annually. The 

 related pectins are widely used to make foods jell, in pharmaceuticals 

 and cosmetics, in sizing and adhesives, fibres, films, and other 

 preparations. They come chiefly from citrous and apple wastes, 

 but many other fruits, etc., afl^ord potential sources. 



Resins are more various and tend to be still more important than 



