120 



BOTANY: PRINCIPLES AND PROBLEMS 



must be converted into sugar before they can be transported 

 from place to place, or before they can be assimilated into living 

 protoplasm. 



The starches are insoluble carbohydrates, derived from glucose 

 but much more complex in their chemical composition. Their 

 general formula is (CeHioOs)!!. Starch is produced from glucose 

 by the removal of a molecule of water, thus: 



nC6Hi206 — nH20 = (CeHioOs)]! 



The formation of starch is confined to certain plastids in the cell. 

 These are the chloroplasts, in cells where photosynthesis is going 



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^ 

 ^ 



Fig. 66. — Starch-grains from various plants. A, potato. B, wheat. C, bean. 

 D, corn. E, gloxinia. F, rice. 



on, and the leucoplasts in storage cells. In these plastids the 

 starch is laid down in small grains which increase in number and 

 size until in storage tissues the entire cell-cavity may become 

 filled with them. Starch-grains usually display very character- 

 istic shapes and markings distinctive of the species by which they 

 are produced (Fig. 66). Indeed, it is often possible by this means 

 to identify with certainty the source from which a particular 

 sample of starch has been derived. A definite core or hilum, 

 often cracked and shrunken, appears in most cases in the grain 

 and is usually surrounded by a series of more or less concentric 

 rings or striations. 



Cellulose is a carbohydrate even more complex chemically 

 and physically than starch but with the same basic formula. 



