ORGANIC ACIDS 



287 



system. The chief use of the alkaloids to man is in the medicines 

 where they serve many purposes. 



Organic acids. — The vegetable acids occur in all parts of 

 plants. They are best known in fruits and vegetables. Malic 

 acid, citric acid, and tartaric acid are the chief ones. They 

 contain only carbon, hydrogen, and oxygen. In fruits the per- 

 centage of acids ranges from a fraction of a percent in Bananas 

 to almost 5 percent in Lemons. Apples and Pears contain be- 

 tween one and two percent. Oxalic acid is common in a number 

 of plants of which the Sheep Sorrel and Horse Sorrel are examples. 



Commonly the vegetable 

 acids are in solution in the 

 cell sap, but often they form 

 salts with potassium, calcium 

 or some other mineral and 

 then take the form of crystals 

 {Fig. 253). 



Tannin. — Tannin occurs 

 abimdantly in the bark of 

 trees; in the leaves and stems 

 of some of the Sumachs; in 

 the fruits of a number of plants, especially in persimmons; and in 

 the insect galls common on leaves and_^stems of trees. The bark 

 of some Oaks yields 25 to 30 percent of tannin. Tannin is very 

 bitter and is antiseptic. It is supposed to protect plants against 

 the attacks of Bacteria and other destructive organisms. 



Tannin is used- in making ink. Tannin has the property of 

 combining with substances in hides which are thereby changed 

 into leather and made resistant to decaying organisms, and in 

 this connection tannin from the bark of oaks and coniferous trees 

 has been of invaluable service. 



Fig. 253 



. — Cells containing crystals 

 of calcium oxalate. 



