158 CHEMICAL COMPOSITION OF PLANTS 



The substance forming the grain is termed starch or amylase, 

 of which there appears to be two sHghtly different modifications. 

 When treated with a solution of iodine it turns a characteristic 

 deep violet-blue colour. 



The enzyme diastase converts it into maltose and various 

 soluble gum-like carbohydrates termed dextrins. 



Formerly Nageli and others consideired that a starch-grain 

 consisted of two substances, namely, granulose, and a substance 

 starch-cellulose ox farinose which remains as an insoluble residue 

 when starch-grains are treated with saliva or weak acids : this 

 residue, however, does not pre-exist in the starch-grains but is 

 a product of the action of the solvents employed, and according 

 to A. Meyer is amylodextrin. 



On boiling with dilute acids starch is changed into glucose 

 and dextrin. 



Heated with water starch swells and forms an insoluble jelly- 

 like paste : subjected to dry heat or roasted to a temperature of 

 150° to 200° C. it turns brown and becomes altered into a form 

 of dextrin. 



In certain cases starch-grains contain amylose with a larger or 

 smaller proportion of amylodextrin : the latter is coloured wine- 

 red by a solution of iodine. 



Commercial starch is obtained chiefly by mechanical separation 

 with water from crushed potato tubers, or from maize and wheat 

 grains. 



Ex. 82. — Divide a grain of wheat, barley, oat, rye, maize and rice trans- 

 versely with a knife. Gently scrape off a very small portion of the endosperm 

 and mount in water. Examine the starch-grains with a low and a high 

 power, noting whether simple compound, their form and relative size, and 

 also the shape and po.sition of the hilum in each. 



Ex. 83. — Cut through the cotyledons of a bean and pea seed and also 

 through a potato tuber : gently scrape the cut surface with the point of a knife 

 and transfer the starch-grains obtained to a drop of water on a slide. Examine 

 and note the form, size and shape of the starch-grains. 



Ex. 84.— Cut thin sections from a piece of potato tuber and from a wheat 



