INDUSTRIAL USES OF PLANTS 387 



and is widely used as a mounting medium in the preparation of 

 plant and animal tissues for microscopic study, and as a cement 

 for lenses. It is used to a lesser extent as an antiseptic in medicine. 



True gums are formed as a result of internal breakdown of 

 cellulose tissues. Unlike resins, gums are soluble in water but in- 

 soluble in alcohol. They are used in paints and candies, as adhe- 

 sives, in medicine, and in textile and paper sizing. Gum Arabic is 

 obtained as small hardened drops or "tears" from a small species 

 of Acacia — a tree member of the Pea Family — native to the dry 

 regions of North Africa. Thin strips of bark are torn from the 

 trees to stimulate gum exudation, and collections are made in 

 three to eight weeks. 



Gum tragacanth comes from a thorny shrub of Western 

 Asia and Southeastern Europe. Gum flow is stimulated, as above, 

 by puncturing or tearing off strips of bark, and the resultant 

 exudate is collected, purified, and marketed. In medicine, both 

 of these gums are used as adhesives to hold pills and lozenges 

 together, and as emulsifying agents, to keep immiscible liquids 

 and insoluble powders suspended. 



Tannins and Dyes 



The art of tanning leather to preserve it and increase its 

 wearing qualities is by no means a development of our modern 

 civilization, since records point to the use of such tanning 

 materials as long as three thousand years ago by the Chinese. 

 The American Indians used oak bark to toughen and preserve 

 skins of buffalo and other of the animals upon which they sub- 

 sisted. Oak bark was first used by early American colonists, but 

 while it was generally preferred to that of hemlock, the latter 

 became more widely used because of its abundant supply. 



The active agent in tanning substances, itself known as 

 TANNIN, occurs to somc extent in almost all plants. Its action on 

 animal hides makes them flexible, strong, waterproof, resistant 

 to decay and to abrasion. It is exploited in a limited number of 

 plant species, mostly in the form of bark, wood, fruit, and leaves. 

 Our native American forests supply about one-half of the tanning 



