52 HOW CROPS GROW. 



Gelatinous Starch. When starch is heated to near boiling with 12 to 

 15 times its weight of water, the grains swell and burst, or exfoliate, 

 the water is xibsorbed, and the whole forms a jelly. This is the starch- 

 paste used by the laundress for stiffening muslin. The starch is but 

 very slightly dissolved by this treatment. On freezing gelatinous 

 starch, the water belonging to it Is separated as ice and on melting 

 remains for the most part distinct. 



Exp. 27.— Place a bit of starch as large as a grain of wheat in 30 c. c. 

 of cold water and heat to boiling. The starch is converted into thin, 

 translucent paste. That a portion is dissolved Is shown by filtering 

 through paper and adding to one-half of the filtrate a few drops of 

 iodine solution, when a perfectly clear blue liquid is obtained. The 

 delicacy of the reaction is shown by adding to 30 c. c. of water a little 

 solution of iodine, and noting that a, few drops of the solution of starch 

 suffice to make the large mass of liquid perceptibly blue. 



"When starch-paste is dried, it forms a hard, horn-like mass. 



Tapioca and Sago are starch, which, from being heated while still 

 moist, is partially converted into starch-paste, and, on drying, acquires 

 a more or less translucent aspect. Tapioca is obtained from the roots 

 of various kinds of Manihot, cultivated in the West Indies and South 

 America. Cassava is a preparation of the same stcrch, roasted. Sago 

 is made in the islands of the East Indian Archipelago, from the pith of 

 palms (Sagus). It is granulated by forcing the paste through metallic 

 sieves. Both tapioca and sago are now imitated from maize starch. 



Next to water and cellulose, starch, is the most abund- 

 ant ingredient of agricultural plants. 



In the subjoined table are given the proportions of starch in certain 

 vegetable products, as determined by Dr. Dragendorff. The quantities 

 are, however, somewhat variable. Since the figures below mostly 

 refer to air-dry substances, the proportions of hygroscopic water found 

 In the plants by Dragendorff are also given, the quantity of which, 

 being changeable, must be taken into account in making any strict 

 comparisons. 



AMOUNT OF BTABCH IN PLANTS. 



Water. Starch. 



Per cent. Per cent. 



Wheat 13.2 69.6 



Wlieat flour 15.8 68.7 



Rye 11.0 59.7 



Oats 11.9 46.6 



Barley 11.5 67.5 



Timothy-seed 12.6 45.0 



Rice (hulled) 13.3 61.7 



Peas 5.0 37.3 



Beans (white) 16.7 33.0 



Clover-seed 10.8 10.8 



Flaxseed 7.6 23.4 



Mustard-seed 8.6 9.9 



Colza-seed 6.8 8.6 



Teltow turnips* dry substance 9.8 



Potatoes dry substance 62.5 



* A Sweet and mealy turnip, grown on light soils, for table use. 



