244 



POPULAR ECONOMIC BOTANY. 



it is quite white and free from impurities. The ordinary 

 kinds, which we receive from Turkey, are produced in Asia 

 Minor and Northern Persia by the A. vems ; and the 

 inferior kinds, which are less flaky, being often in worm- 

 shaped fragments and irregular tears, and generally of a 

 ferruginous colour, are produced by A, strohiliferus. 



These plants are small shrubs, with clusters of small 

 yellow pea-shaped flowers ; they are either terminal (that is, 

 growing at the ends of the branches), or produced in the 

 axils of the leaves (axillary), which are pinnate, with five or 

 six leaflets ; the branches are covered with the remains of 

 the petioles of the leaves, which gives them a rough spiny 

 appearance. 



The singular appearance the flakes of this gum present, 

 has been explained by De Candolle as the result of a 

 peculiar property, possessed by some portions of the stem 

 more than others, of absorbing moisture from the atmo- 

 sphere ; this property is termed hygrometric. It acts thus : — 

 The tragacanth is a natural secretion of the plant, which is 

 formed by the bark and albumen ; it consists of arabin, the 

 soluble principle of gum, and a large quantity of the in- 

 soluble principle, called bassorin. In the damp state of the 

 atmosphere, which is often produced by the heavy mists 



