CHEMICAL COMPOSITION OF PLANTS 45 



Starch is not soluble in cold water, but it swells considerably 

 by absorbing as much as 40 per cent of water. This water is 

 retained with high imbibitional force. Even in an air-dry condi- 

 tion, starch will hold as much as 15 per cent of water. Starch 

 has a high specific gravity (1.5 to 1.6). It precipitates easily 

 and hence may be separated by setthng, which is used in its 

 purification. The starch grain seems to have a complex struc- 

 ture. The principal constituent of the grain, called ''granulose," 

 becomes more soluble upon heating in water and is easily hydro- 

 lyzed. The rest of the grain is less easily hydrolyzed. It remains 

 in the form of a fine skeleton when the granulose fraction is dis- 

 solved. Both give the iodine reaction typical for starch, granu- 

 lose, however, showing more of a blue, the skeletal part a violet 

 shade. 



Cellulose is a carbohydrate closely related to starch. It forms 

 the basic substance of cell walls and has the empirical formula 

 C6H10O5. Cellulose differs from starch in being considerably 

 more stable chemically. It is not soluble in hot water and can 

 be 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. 



The arrangement of the molecules of cellobiose in the cellu- 

 lose, like that of maltose molecules in starch, is chainlike, con- 

 sisting of rows of constituent units arranged in a definite space 

 lattice. 



In elongated plant cells, e.g., in bast fibers, all these chains are 

 situated parallel to each other and at the same time somewhat 

 inclined in relation to the longitudinal axis of the cell. In 

 consequence, the fibrils are disposed in spirals, which may be 

 shown by means of an ultramicroscope or X rays. Sometimes 

 the so-called ''streakiness" is perceptible under an ordinary 

 microscope. 



Owing to its chemical stabihty, cellulose forms the basic 

 skeleton of the cell wall and does not represent a reserve carbo- 

 hydrate. The role of a storage substance belongs to certain 

 secondary deposits on the wall designated '^ reserve cellulose" 

 or ''hemicellulose." This is stored in the endosperm and 

 cotyledons of many seeds in the form of substantial thickenings 

 of the cell wall, which lend to the entire tissue considerable 



