204 TEXTBOOK OF PLANT PHYSIOLOGY 



arated by settling, which is made use of in its purification. The 

 starch grain seems to have a double structure. The basic mass of 

 the grain, called "a-amylose" is more soluble upon heating in 

 water and is easily hydrolyzed. The other part, called "/3-amylose" 

 is less soluble. It remains in the form of a fine skeleton when the 

 a-amylose fraction is dissolved. Both give the iodine reaction 

 typical for starch, the a-amylose, however, showing more of a blue, 

 the /3-amylose a violet shade. 



Cellulose is a carbohydrate closely related to starch. It forms 

 the basic substance of cell walls and has the empirical formula 

 CoHioOy. Cellulose differs from starch in being considerably 

 more stable. It is not soluble in hot water and is hydrolyzed with 

 difficulty by dilute acids. The end product of the hydrolysis of 

 cellulose, as in the case of starch, is glucose. The intermediate 

 product is likewise a disaccharide but, instead of maltose, it is 

 cellobiose. It is possible that the structure of cellulose is similar 

 to that of starch. 



Owing to its chemical stability, cellulose forms the basic skele- 

 ton of cell membranes. In reality, therefore, it is not a reserve 

 carbohydrate. Analogous to starch are the hemicelluloses, also 

 known as reserve celluloses. Hemicelluloses are found in the 

 endosperm or in the cotyledons of many plants as thickenings of 

 the cell walls, thus giving to the tissues considerable solidity. 1 

 Such, for example, is the horny endosperm of some palms (Phytele- 

 phas and C oelococcus) , so-called vegetable ivory, from which but- 

 tons are manufactured. Of a similar structure are the stones of 

 dates and the seeds of coffee, peonies, lupine, nasturtium, and other 

 plants. The chemical composition of reserve cellulose is not 

 fully known. Apparently it is not the same in different plants. 

 Among the products of hydrolysis are usually found glucose, 

 maltose, and arabinose. 2 Hydrolysis as a rule proceeds very 

 easily, often faster than that of starch ; hence, at the germination of 

 the seeds rich in reserve celluloses, they dissolve very rapidly. 



In the underground storage organs, which differ from seeds by 

 having a high water content, the reserve carbohydrate most fre- 

 quently is starch, with which the cells are sometimes literally 

 gorged, as, for instance, in the potato tuber. However, other 



1 It is present often in lignified and non-lignified tissues in various vege- 

 tative organs. 



2 Also galactose and xylose. 



