60 CHEMISTRY OF PLANT LIFE 



Starch occurs in plant tissues in the form of microscopic gran- 

 ules, composed of concentric layers, there being apparently alter- 

 nate layers of two types of carbohydrate material, which have 

 been distinguished from each other by several different pairs of 

 names used by different authors: thus, Nageli uses the terms 

 "granulose" and " amylocellulose" ; Meyer, "a and /3 amylose"; 

 Wolff, " amylo-cellulose " and " amylo-pectin" ; while Kramer 

 asserts that the layers are alternate lamella of crystalline and col- 

 loidal starch. Many theories as to the nature of these concentric 

 layers and their mode of deposition have been advanced, but it 

 would not be profitable to discuss them in detail here. 



For purposes of study, starch may be prepared from the ground 

 meal of cereals, potatoes, etc., by kneading the meal in a bag or 

 sieve of fine-meshed muslin or silk, under a slow stream of water. 

 The starch granules, being microscopic in size, readily pass through 

 the cloth with the water, and may be caught in any suitable con- 

 tainer. The starch is then allowed to settle to the bottom, the 

 water poured off and the starch collected and dried. 



Starch is insoluble in water; but if boiled in water, the granules 

 burst and a slimy opalescent mass, known as " starch paste," is 

 obtained. This is undoubtedly a colloidal suspension of the 

 starch in water. By various processes, such as boiling with very 

 dilute acids, treatment with acetone, etc., starch is converted into 

 " soluble starch " which dissolves in water to a clear solution. 

 Soluble starch is precipitated out of solution by alcohol, or by 

 lead subacetate solution. 



Air-dried starch contains from 15 to 20 per cent of water; but 

 this can be completely removed, without altering the starch in 

 any way, by heating for some time at 100 C. 



The starch granules from different sources vary considerably 

 in size and shape, and can generally be identified by observation 

 under the microscope. 



The most characteristic reaction of starch is the blue color 

 which it gives with iodine. The reaction is most marked with 

 starch paste or soluble starch, but even dry starch granules are 

 colored blue when moistened with a solution of iodine in water con- 

 taining potassium iodide, or with tincture of iodine. 



When hydrolyzed, either by boiling with dilute acids or under 

 the influence of enzymes, starch undergoes a series of decomposi- 

 tions, yielding first dextrins, then maltose, and finally glucose. 



