CARBOHYDRATES 59 



POLYSACCHARIDES 



The polysaccharides which, like the simpler saccharides, or 

 sugars, which have thus far been studied, undoubtedly serve as 

 reserve food for plants, are known under the general name of 

 " starches." They are substances of high molecular weight, whose 

 constitution is represented by the general formula (CeHioOs)^ 

 It should be noted that an exactly accurate formula should be 

 (C6)re(Hi206)-i ; but since the value of n is very high, the simpler 

 formula is approximately correct. The value of n has not been 

 accurately determined for any of the individual members of the 

 group, but is probably never less than 30 and may often be 200 or 

 more. The fact that these compounds are insoluble in most of the 

 solvents which can be used for molecular weight determinations 

 makes it difficult to determine their actual molecular constitution. 



When completely hydrolyzed, the polysaccharides yield only 

 hexoses. They are, therefore, technically known as " hexosans." 

 Each individual polysaccharide which has been studied thus far 

 yields only a single hexose, although the particular hexose obtained 

 varies in different cases. In fact, the polysaccharides are often 

 classified according to the hexoses which they yield on hydrolysis, 

 into the following groups : the dextrosans, which yield glucose, and 

 include starch, dextrin, glycogen, lichenin, etc.; the levulosans, 

 which yield fructose, and include inulin, graminin, triticin, etc.; 

 the mannans; and the galactans. The more common representa- 

 tives of each of these groups are discussed below. 



(A) THE DEXTROSANS 



These are by far the most common type of polysaccharides to 

 be found in plants. 



Starch. It is probable that no other single organic compound 

 is so widely distributed in plants as is ordinary starch. It is pro- 

 duced in large quantities in green leaves as the temporary storage 

 form of photosynthetic products. As a permanent reserve food 

 material, it occurs in seeds, in fruits, in tubers, in the pith, medul- 

 lary rays and cortex of the stems of perennials, etc. It constitutes 

 from 50 to 65 per cent of the dry weight of seeds of cereals, and as 

 high as 80 per cent of the dry matter of potato tubers. 



