D1V. II 



PHYSIOLOGY - 



265 



quantity of the substance acted on, if the products of the reaction 

 are continually removed. 



Diastase is found in many parts of the plant, especially in those 

 which contain much starch, such as foliage leaves and germinating 

 seeds. The amount of diastase in an organ is not constant, but is 

 regulated according to the needs of the plant ; further, its action 

 can be arrested by the formation of other enzymes (anti-enzymes). 

 This is one of the many regulatory processes so characteristic of the 

 organism. 



In the plant diastase acts on the starch grains. These are corroded 

 under its influence; they are dissolved away from without inwards, but 

 this proceeds as a rule irregularly, so that the shape of the grain changes. 

 At particular spots the diastase 

 eats more quickly into the 

 grain and, using pre-existing 

 splits and canals, breaks it up 

 into smaller portions which 

 then dissolve further (Fig. 

 253). Outside the plant the 

 action of diastase can best 

 be shown on thin starch 

 paste : on adding diastase to 

 this the characteristic iodine 

 reaction is lost after a few 

 minutes or a quarter of an 

 hour. The blue colour given 



. n . , FIG. 253. Different stages of the corrosion shown by 



at first, Changes tO a Wine-red the starch grains of germinating Barley. (After NOLL.) 



tint, and ultimately a yellow 



colour is given. Dextrin is an intermediate product between the starch 



and the maltose. 



Cellulose is also of frequent occurrence as a reserve substance. In 

 the endosperm of many seeds the cell walls are very strongly thickened 

 and the thickening layers are dissolved in the process of germination. 

 Such thickened walls are beautifully shown in many palm seeds, e.g. in 

 the Vegetable Ivory Palm. The solution of the thickening is due to 

 an enzyme, the so-called cytase, which, however, does not act on every 

 variety of cellulose. Typical cellulose (p. 38) is not attacked by it, 

 but only reserve cellulose, which differs in its chemical structure. 



Inulin, which is found especially in Compositae and Campanulaceae, 

 is related according to its empirical formula (C 6 H 10 5 ) n with cellulose 

 and starch, but is distinguished from these substances by always 

 occurring in plants in the dissolved form. In spite of this it is 

 incapable of translocation on account of the size of its molecule, and 

 is broken down on germination by an enzyme into a sugar of the 

 formula C 6 H 12 6 . The sugar in this case is, however, levulose. 



Cane Sugar, which occurs for example in the sugar-cane and sugar- 



