228 ENZYMES AND 



in the dark ; exposure to bright light suspends and gradually 

 destroys it. 



2. The following are the most important kinds of enzymes 

 occurring in plants : 



(i) Those which transform the different insoluble carbohydrates 

 into sugars. 



{a) To this class belong diastase which attacks starch and by 

 a gradual and continuous process of decomposition converts 

 it ultimately into maltose and a small proportion of a gum- 

 like substance termed dextrin. Other forms of dextrin arise 

 during the intermediate stages of the process but are soon 

 split up into maltose : some of them give a reddish-brown 

 colour with iodine. 



Two slightly different forms of diastase are met with in plants. 

 The one known as diastase of secretion is concerned with the 

 dissolution of starch in germinating seeds, and is especially 

 prevalent in the germinating grains of barley and other cereals 

 and grasses. This form of diastase which is the characteristic 

 enzyme in malt, corrodes and eats pit-like depressions in the 

 substance of starch-grains before finally dissolving them. 



In the seeds of the Graminese this enzyme is secreted by the 

 long cylindrical cells forming the surface-layer or epithelium of 

 that side of the scutellum of the embryo which adjoins the endo- 

 sperm. After its formation by the epithelium, the diastase 

 diffuses into the endosperm and transforms the starch into 

 maltose, which is ultimately absorbed by the scutellum and 

 transferred to the growing-points of the developing embryo. 



The other form of diastase is spoken of as diastase oj 

 translocation. It is more widely distributed than the diastase 

 of secretion, being found in the leaves, shoots and other 

 vegetative parts of plants. The amount present in leaves is 

 greatest during the night or when the plant is kept in darkness. 

 By its agency, the starch produced in the chloroplasts of green 

 leaves during the daytime is transformed into sugar at night, 



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