146 THE CARBOHYDRATES 



(C5H804)n according as they give rise to hexoses or pentoses 

 on hydrolysis. The value of " w " has not been determined as 

 yet for any particular case, but there is reason to believe that 

 it is fairly high. The various methods adopted for the eluci- 

 dation of this point have led to such widely different results 

 that a description of them here would not serve any useful 

 purpose. 



HEXOSANS. 



The general formula for all substances belonging to this 

 group is (CfiHioOajn, which indicates that on hydrolysis they 

 yield hexoses ; for this reason they may be termed hexosans. 



GLUCOSANS. 



Starch or Amyluni. 



Starch is one of the most widely distributed substances in 

 the vegetable kingdom ; it may be found in green leaves as a 

 temporary reserve of the photosynthetic products ; as a more 

 or less permanent reserve food-material it occurs in seeds and 

 fruits, where it is not infrequently accompanied by other 

 reserves, for instance proteins ; in the vegetative parts, such 

 as tubers, the living cells of the pith, medullary rays, and 

 cortex of roots and stems ; and also in the latex of certain 

 plants, e.g. Euphorbia. When especially stored, the amount 

 of starch may be considerable ; thus in cereals it may form 

 from 50 to 70 per cent of the dry weight of the grains, and 

 in potatoes from 15 to 30 per cent of the dry weight of 

 the tubers. As is well known, starch grains from different 

 sources show much variety in size and shape, and occur in 

 association with plastids, in which, as Schimper demonstrated, 

 they have their origin. Not only are the microscopic charac- 

 ters of starch grains of diagnostic value, but the different 

 varieties of starch can be grouped into generic, specific, and 

 varietal classes which correspond with the classification of 

 plants based on the ordinary morphological features.* 



Brief mention may be made of the ideas held regarding the 

 physical nature of starch grains. As is well known, the gran- 



* Reichert : " Amer. Journ. Bot.," 1916, 3, 91. 



