188 THE CONSTITUENTS OF PLANTS. IX. 



for the nourishment of the growing portions. It exists in the 

 form of granules, which vary greatly in size and form in 

 different plants. 



The starch from any one source may show considerable 

 differences in the size of its granules, but their form and 

 general appearance is always the same and may be readily 

 recognised under the microscope. 



Of the common starches that from potatoes has the largest 

 granules, varying from -07 to -03mm. in diameter, while wheat 

 starch varies from -045 to -003mm., and rice starch from -0075 

 to -0050mm. in diameter. 



The granule is made up of a cell wall of starch cellulose and 

 an interior mass of granulose. So long as the cell wall is 

 uninjured starch will not dissolve in cold water, but by 

 bruising the cell wall, or better, by causing the granules to 

 swell up and burst, the contents, granulose, escape and yield 

 with the water a kind of viscid solution known as starch paste. 



The temperature at which this swelling up of the granule 

 occurs varies with different starches ; it usually commences 

 about 50 and is completed about 70 C. 



Starch is converted by free iodine in the presence of water 

 into a blue or violet coloured substance. Starch paste is 

 coloured deep blue by iodine, the colour being deeper the low r er 

 the temperature ; even below the boiling point of water the 

 colour disappears entirely, but reappears on cooling The blue 

 substance is said to have the composition [(C ( . ( H 10 5 ) 4 I] 4 .HL 

 and the presence of hydriodic acid or an iodide is said to be 

 essential to its formation." 



Starch unites with the alkaline earths to form definite com- 

 pounds which are insoluble in dilute alcohol. In the case of 

 barium the precipitate has the composition BaO.(C r> H 10 5 ) 4 . 

 A volumetric method of estimating starch has been based upon 

 this reaction. f 



When starch is heated for half an hour in glycerine to 190 

 it is converted into soluble starch, which can be precipitated 

 from aqueous solution by the addition of alcohol. The white 

 powder so formed is soluble in water, and according to Brown 



* Mylius, Ber. 20, 688; Jour. Chem. Soc. 1887, abst. 568. 

 t Asboth, Chem. Zeit. 11, 785 ; Jour. Chem. Soc. 1887, abst. 868. 



