THE CARBOHYDRATES 139 



The dextrosans or starches are the most common polysaccharides 

 found in plants. The sugar produced in the green leaves as the 

 result of photosynthesis is carried to all parts of the plant, where 

 it is stored up in the form of starch as a temporary or permanent 

 reserve material. Fleshy roots, the rays of the stem, and seeds 

 are especially rich in this substance. In the cereals about 60% 

 of the dry weight is starch, and in potatoes about 80%. The 

 latex of laticiferous plants also contains much starch. However 

 some plants among the monocots (Scilla nutans, Phleum pratense, 

 Allium) contain very little starch, the reserves being stored either 

 as fats or as sugars. 



The starch occurs in grains of characteristic size and shape 

 depending upon the species of the plant; and the starch granule 

 is therefore one of the common criteria for detecting adulterations. 

 The starch is often deposited in layers, the cause of which has not 

 been definitely determined. Meyer proposed that the layers were 

 due to differences in chemical composition, while Kramer has 

 stated that they are alternations of colloidal and crystalline starch. 

 Others have associated the layers with differences in water content 

 and the alternation of day and night. 



Starch is insoluble in water but when boiled yields a colloidal 

 solution. Under the action of enzymes such as diastase, the starch 

 is broken down through a series of simpler compounds until ulti- 

 mately glucose is produced (Chap. XXII). The most character- 

 istic reaction is the blue color which results when treated with 

 iodine. 



Dextrins may occur in plants as a form of stored food (Arum 

 italicum and sweet corn, according to Weatherwax), but in general 

 the dextrins are found as transition products between starch and 

 maltose. Three such intermediate steps have been distinguished, 

 known as amylo-, erythro-, and.achroo-dextrin. Amylodextrin 

 is the principal constituent of " soluble starch." It is readily 

 soluble in hot water, has a starchy taste, and turns blue with 

 iodine. Erythrodextrin is easily soluble in cold water, is taste- 

 less, and gives a reddish-brown color with iodine. Achroodextrin 

 is soluble in water, has a sweetish taste, and gives no color with 

 iodine. 



Commercial dextrin is made by heating starch to about 250° C. 

 or by hydrolysis with acids. It is a mixture of glucose and the 

 two simplest dextrins. It is used in library paste and on postage 



