44 



PLANT PHYSIOLOGY 



and distinguishing various admixtures (Fig. 11). In their 

 internal structure, starch grains represent spherocrystals com- 

 posed of very fine crystaUine needles (trichites) closely packed 

 and radiating from the hilum in concentric layers. 



The formula CeHioOa does not express precisely the compo- 

 sition of starch. All the methods of determining the molecular 

 weight prove with certainty that its particle is considerably 

 larger and that its formula should be written (CeHioOs)^. The 

 cryoscopic method indicates an especially large starch molecule, 



and different authors give values 

 to the magnitude n of 30, 

 108, 149, or 638, for various 

 preparations. 



Starch is hydrolyzed by the 

 action of dilute acids and yields 

 glucose; so its particle may be 

 represented as a certain number 

 of glucose molecules joined 

 through the removal of water. 

 The formation and hydrolysis of 

 starch may thus be expressed by 

 the following reaction : 







// 





Q 



/->. 



w-^' 



'/ 





nCeHisOe ^{n - 1)H20 + 



(C6Hio05)nH20. 



Fig. 11. — Starch grains. Top, 

 potato; center, corn and bean; bottom, 

 rice and wheat. 



This formula shows that a 

 particle of starch contains 1 

 molecule of H2O more than is expressed by the ordinary formula 

 (CeHioOs)^. 



If starch is hydrolyzed chemically, e.g., by heating it with 

 HCl, the formation of glucose proceeds in several steps; first 

 appear the polysaccharides, soluble in water, which are termed 

 dextrines; these are followed by the disaccharide maltose, which 

 further disintegrates into 2 molecules of glucose. In hving 

 tissues of plants, however, hydrolysis- of starch leads to an 

 accumulation of sucrose, which involves the transformation of 

 half of the glucose to fructose. The mechanism of this reaction 

 taking place in living cells is as yet not clear, but it proves how 

 much more complicated vital changes are than the chemical 

 reactions that occur in the test tube. 



