80 SECRETIONS. 



tree, cannot circulate, except through the lateral orifi- 

 ces of its vessels ; for the small tubes through which 

 the sap is elevated, and the simple ones through which 

 the Cambium descends, have no direct communication 

 with each other, except on the leaf's expanded surface. 

 It is obvious, therefore, that the vascular system, at 

 least that portion of it through which the fluids circu- 

 late, must in every case be co-existent with the leaf, 

 annual in deciduous trees, and perennial in all others. 

 But the^renovated sap as it descends from the leaf, 

 contains not merely woody fibre for the increase of the 

 vegetable : it has acquired other substances on which 

 its flavour and medical virtues depend. We shall enu- 

 merate those only which are most important. 



STARCH, whose properties are well known, is a very 

 common product of vegetable secretion, being partic- 

 ularly abundant in every kind of grain, and existing in 

 large quantities in Potatoes and other nutritive roots. 

 In the form of Sago, it is obtained from the pith of some 

 of the East Indian Palms, and Salop which also is a pe- 

 culiar modification of starch, is procured from the roots 

 of various species of Orchis. 



GLUTEN. — After the Starch is extracted from flour, 

 there remains a darkish adhesive substance, named 

 gluten, or vegetable glue. Exposed to a moderate de- 

 gree of heat, it yields large quantities of fixed air, which 

 renders the pastry of flour porous and light ; and hence 

 its utility in the manufacture of bread . Yeast, in what- 

 ever mode it is formed, contains gluten, whose decom- 

 position affords fixed air and whose tenacity retains it. 

 GUM. — Another product of vegetable secretion is 

 mucilage or gum. It is commonly procured from those 

 trees and shrubs which, in their uncultivated state, 

 are armed with thorns. These organs are their only 



