DIGESTION 255 



general appearance ; the other, diastase of secretion, dis- 

 integrates it by a process of corrosion before dissolving it 

 (fig. 123). The first of the varieties has a very wide distribu- 

 tion in plants, being present almost 

 everywhere. The second is the body fi\ /s\ 

 formed by the glandular covering (=/ \S 

 or epithelium of the scutellum of & ^ 



the grasses. ) ^ 



The great function of diastase in FlG - 123 STARCH GRAINS 



. IN PROCESS OF DIGES- 



the plant is to transform starch (and HON. THE SUCCESSIVE 

 probably glycogen where it occurs) FIGURES SHOW THE PRO- 



J J GRESS OF DISSOLUTION. 



into maltose or malt-sugar. Wher- 

 ever starch is formed, whether in the living leaf or in the 

 reservoir set apart for storage, it must be regarded as a 

 reserve material, and its removal from the seats of deposi- 

 tion is preceded by its conversion into this sugar. The 

 details of the transformation are not fully known at present, 

 but a good deal of information has been obtained through 

 the labours of many observers. Starch has a rather large 

 molecule, but its exact formula is not thoroughly known. 

 For a long time it was taken to be approximately w(C 6 H 10 5 ), 

 and the value of n was thought to be 5. More recently the 

 suggestion has been made that the molecule is much larger, 

 and may be more truly represented by 5[(Ci 2 H 20 10 )2o]j the 

 view being based upon the formation of several complex 

 substances during its decomposition. The starch molecule 

 is possibly composed of four dextrin-like groups, each 

 (C 12 H 2 o0 10 )oo arranged about a fifth. It has been suggested 

 that the first action of the diastase is the liberation of these 

 from one another ; and that four of them by successive 

 incorporations of water are converted, through a series of 

 complex substances called malto-dextrins, into maltose, 

 while the fifth withstands the action of the enzyme for a 

 considerable time. After the action of the diastase has 

 been proceeding for some time the resulting product is 

 found to be four parts maltose and one part dextrin. 



How far this series of decompositions represents what 



