66 HOW CROPS GROW. 



Next to water and cellulose, starch is the most abundant 

 ingredient of agricultural plants. 



In the subjoined table are given the proportions contained 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 are also 

 given, the quantity of which being changeable must be taken into ac- 

 count iu making any strict comparisons. 



AMOUNT OF STARCH IN PLAKTTS. 



Starch is quantitatively estimated by various methods. 



1. In case of potatoes or cereal grains, it may be determined roughly 

 by direct mechanical separation. For this purpose 5 to 20 grams of the 

 substance are reduced to fine division by grating (potatoes) or by soften- 

 ing in warm water, and crushing in a mortar (grains). The pulp thus 

 obtained is washed either upon a fine hair-sieve or in a bag of muslin, 

 until the water runs off clear. The starch is allowed to settle, dried, and 

 weighed. The value of this method depends upon the care employed 

 in the operations. The amount of starch falls out too low, because it is 

 impossible to break open all the minute cells of the substance analyzed. 



2. In many cases starch may be estimated with more precision by con- 

 version into sugar ; see p. 76. 



3. Dr. Dragendorff, of the Rostock Laboratory, proceeds with starch de- 

 terminations as follows : The pulverized substance, after drying out 

 all hygroscopic moisture at 212, is digested for 18 to 30 hours, at a tem- 

 perature of 212, in 10 to 12 times its weight of a solution of 5 to 6 parts 

 of hydrate of potash in 94 to 95 parts of anhydrous alcohol. The 

 digestion must take place in sealed glass tubes, or in a silver 

 vessel which admits of closing perfectly. By this treatment the 



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



