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TEXTBOOK OF BOTANY 



Fig. 59. Photographs of white and green variegated leaves of geranium after 

 the leaves had been subjected to the iodine test for starch: A, leaves tested 

 directly after removal from the plant; B, comparable severed leaves tested after the 

 cut ends of their petioles had been immersed in a sugar solution for 15 hours. 

 Photo by A. G. Chapman and W. H. Camp. 



starch does not form in the non-green parts of such plants if plastids are 

 absent. 



Microscopic tests for starch are easily made, and the relations of 

 starch accumulation to certain conditions and processes in the plant are 

 fairly well known. As a consequence, starch synthesis, accumulation, 

 and digestion are often studied as an index to other processes in which 

 sugars are being made, consumed, or translocated. 



Starch may accumulate temporarily in chloroplasts when sugar is 

 being made, but in some plants, as in onions, oil accumulates in 

 chloroplasts. 



Commercial sources of starch. Conditions in certain tissues and organs 

 are especially suitable to starch accumulation. Certain roots, stems, fruits, 

 and seeds are of s;reat commercial value, mainlv because of the starch 

 they contain. Besides being valuable as a food, millions of tons are ex- 

 tracted from plants annually and used in the industries for making vari- 

 ous types of dusting powders, adhesives, pastes required in sizing cloth 

 and paper, and as a source of dextrin and glucose. In the manufacture of 

 alcohol a number of starch-rich cereals are used as a source of food for 

 yeast plants. 



