68 ORGANIC COMPOUNDS IN PLANTS. 



forms the substance of all wood, giving it strength, 

 hardness and elasticity. 



243. Vegetable Jelly is so called, because, while moist, 

 it looks and feels like common jelly. When dry, it 

 becomes horny or cartilaginous. Quince jelly and apple 

 jelly are forms of it, but mixed with the acids and other 

 compounds which give them their peculiar taste. 



244. Every-body is familiar with the appearance of Starch. 

 When dry, it is somewhat hard, and crumbles between 

 the fingers. When moist, it is somewhat like jelly. It is 

 completely soluble in warm water, and, when perfectly 

 pure, is clear and transparent. As it dries, it is at first 

 a trembling jelly, but at last becomes brittle as glass. 



Starch is found, already formed, in almost every plant 

 that has been examined, particularly in the grains of all 

 the cerealia, in beans and pease, and almost all seeds, in 

 potatoes and all other esculent roots, and in the pith of 

 many plants, as in the sago palm. In arrow-root it seems 

 to be purest. Starch, variously compounded, but never 

 absolutely pure, constitutes the most important, and often 

 the only food of two thirds of all mankind. It occiirs 

 in si null quantities in the bark and newly formed wood 

 of many trees, in winter, whence the inhabitants of the 

 Polar regions are able to use the bark of trees, when 

 baked, as bread. It is extracted, for use in the arts, from 

 potatoes, wheat, and some other substances. 



245. Gum is the substance which we often find hardened 

 in roundish masses on the bark of cherry and peach trees. 

 It is in all plants; in plants belonging to some families, 

 it is round very abundantly. Gum Arabic is a well-known 

 form of it. When pure, it is clear and transparent; 

 when dry, very brittle. It easily dissolves in water and 

 in weak acids, but not in alcohol. It is very nourishing, 



