CHEMICAL COMPOSITION OF PLANTS 



43 



of molecules per unit volume, and consequently their osmotic 

 pressure is only half that of the corresponding monosaccharides. 

 This is probably the reason why monosaccharides, as compared 

 with di- and polysaccharides, serve rarely as storage reserves. 



Of all the polysaccharides, the most widespread in plants is 

 starch. It is insoluble in water. Its empirical formula is 

 CeHioOs. It may be found in various tissues. Its presence in 

 leaves is only temporary during the day and is the result of intense 



f^. KUBAN 



Fig. 10. — Starch grains in the cells- of a potato tuber. 



assimilation and consequent accumulation of carbohydrates 

 that have not had the time to be translocated. Starch is like- 

 wise found in permanent storage tissues, e.g., in seeds (Fig. 9), 

 wood, and underground storage organs (Fig. 10), which are 

 sometimes closely packed with starch. Being insoluble in water, 

 starch accumulates in cells in the form of grains of various shape 

 and size, characteristic of certain kinds of plants. Examination 

 of the structure of the starch grains under the microscope will 

 determine to what kind of plant they belong. This serves as 

 one of the essential methods of determining the identity of flour 



