378 CARBOHYDRATE AIETABOLISAI 



which have been deposited around a central locus. In "simple" starch grains 

 there is only one locus of deposition; in the "compound" type there are a 

 number of such loci. 



As far as is known, starch grains are formed only in chloroplasts and 

 leucoplasts. The grains formed in the chloroplasts are usually small, and 

 seem to be produced within the interior of the chloroplast. ]\Iost of the 

 larger and more prominent starch grains produced in plant cells are synthe- 

 sized by the leucoplasts. Apparently each leucoplast produces only a single 

 starch grain. It is not certain whether the starch grain in its growth by 

 the accretion of new layers ruptures the outer membrane of the leucoplast, 

 or whether the outermost layer of the leucoplast stretches as the starch grain 

 enlarges and thus remains as a thin membrane around the grain. 



Starch grains are hydrophilic, usually containing about 13-15 per cent of 

 water. The material present cannot be converted quantitatively into glucose, 

 indicating that some other substances are present in the grain. The amounts 

 of these other compounds are small, however, seldom equalling even i per 

 cent of the total dry matter present. They include such compounds as fats, 

 proteins, tannins, phosphates, and other mineral compounds. 



The dextrins are a group of compounds formed as intermediate products 

 in the hydrolysis of starch, and undoubtedly in its synthesis also. The dex- 

 trins do not often accumulate in plants, usually being present only as tran- 

 sitory products. They form colloidal sols in water. Although their constitu- 

 tion is represented by the same formula as that for starch, the number of 

 unit CgHioOs groups composing a dextrin molecule is smaller than in starch. 



Glycogen (sometimes called "animal starch") is a hexosan which serves 

 as a storage carbohydrate in animal tissues, being especially abundant in 

 the muscles and liver. It is unicnown in green plants, but found in many of 

 the fungi, being especially abundant in yeasts. 



The only well-known levulosan is iniilin which is accumulated and serves 

 as a storage product in a number of plants, especially members of the com- 

 posite family. Some species in which inulin is found are the dahlia, chickory, 

 salsify, dandelion, Jerusalem artichoke, and goldenrod. Inulin is seldom if 

 ever found in the aerial organs of plants, but may constitute as much as 15 

 per cent of the dry weight of some underground parts. Some species (Jeru- 

 salem artichoke) accumulate starch in the aerial parts, but inulin in the 

 underground portions. Inulin is a white powder-like compound which forms 

 colloidal sols in water. It is dispersed in the cell sap of those cells in which 

 it accumulates and can be precipitated as crystals in the cells by immersing 

 them in alcohol. Inulin is hydrolyzed in plants to fructose by the enzyme 

 inulase. 



