CONSTITUENTS OF PLANTS 3/1 



used for the construction of other plant substance, or stored up as 

 reserve material as such. Starch is thus found abundantly in 

 many seeds, roots, and tubers, the parts of the plant concerned 

 with new growth. 



The starch granules vary in size and structure according to 

 their origin. Potato starch appears mostly as oval granules with 

 an average diameter of 0.07 mm., but it contains large granules. 

 Wheat starch contains circular granules of two sizes, smaller than 

 0.007 mm - diameter and larger than 0.2 mm. with few granules 

 of intermediate size. The structure of the granules, and their be- 

 havior towards polarized light is also different, so that one 

 familiar with their appearance can easily identify starches of 

 different origin by means of a microscopic examination. 



The elementary composition of starch is represented by the 

 formula C 6 H 10 O 5 , but its molecular formula is not yet known. 

 Many chemists hold that the starch molecule may contain more 

 than loo carbon atoms. 



Properties of Starch. Air-dry starch contains 10 to 20 per cent 

 water. By carefully drying at 102-110 C., it may be obtained 

 water-free. In cold water it is insoluble. With hot water the 

 granules swell, break, and form starch paste, a pasty solution, 

 from which a clear filtrate can be secured. By treating starch 

 for several days with cold dilute mineral acids, it may be changed 

 into "soluble starch," which dissolves in hot water without forma- 

 tion of a paste. Starch is tasteless and colorless. 



Starch is especially characterized by the blue color it gives with 

 iodine. This is a very delicate test for both starch and iodine. 

 Starch is used as an indicator with volumetric solutions contain- 

 ing iodine. 



When heated with dilute mineral acids under proper conditions, 

 starch is converted almost quantitatively into glucose. As we 

 have already seen, this property is utilized in the manufacture of 

 glucose and glucose syrup from starch. 



Under other conditions, dilute acids change starch into a 

 gummy substance termed "dextrin." This occurs in some 

 mucilages made from starch. 



