THE POLYSACCHARIDES 



377 



concentration of sugars to develop to permit the occurrence of starch 

 synthesis. 



A temporary accumulation of starch occurs in the leaves of most species 

 during photosynthesis (Chap. XX). Starch synthesis also takes place in the 

 cells of many other tissues. Starch accumulates in the xylem rays, xylem 

 parenchyma, phloem parenchyma, cortex and pith of the stems of many 

 species, as well as in tubers, bulbs, corms, underground rhizomes, fruits and 

 seeds. 



Starch is insoluble in water, but upon boiling is converted into a colloidal 

 sol. Under suitable conditions it may also form gels. Upon hydrolysis 

 with acids, or digestion with "diastase" (a mixture of the enzymes amylase 

 and maltase) starch is converted, first into dextrins, then into maltose, and 

 finally into glucose. It is generally considered that there are several dextrins, 

 which are produced step-wise in the hydrolysis of starch as follows: 



Starch — > Amylodextrin -^ Erythrodextrin —> Achroodextrin -^ Maltose 

 — > Glucose 

 Amylodextrin, erythrodextrin, and achroodextrin are differentiated by the 

 colors which they give with I2KI solution, these being blue, reddish brqA^n,^ 



■-2? 





Fig. 94. Starch grains. {A) from potato tuber, (B) from wheat grain, (C) from 



corn (maize) grain. 



and colorless respectively. The presence of starch itself is usually demon- 

 strated by the purplish-blue color which it gives with this same reagent. 



With very few exceptions starch is present in the cells of plants in the 

 form of definite grains or granules. These grains vary greatly in size, as 

 well as in the number which may be present in a single cell. The shape of 

 starch grains, however, is so nearly constant for a given species that the origin 

 of the starch in starch-containing products such as flour can often be deter- 

 mined by examining a small sample of the product under the microscope 

 (Fig. 94). 



Starch grains are usually built up of a number of laj'ers or lamellae 



