236 



BOTANY 



PART I 



possible that diastase may only form a relatively small proportion of 

 such substances, and may itself not be an albuminous substance. 



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. 203.) 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 at 



first changes to a wine-red 

 tint, and ultimately a yellow 

 colour is given. Dextrin is 

 an intermediate product be- 

 tween the starch and the 

 dextrose. 



Cellulose is also of frequent 

 occurrence as a reserve sub- 

 stance. In the endosperm of 

 many seeds the cell walls are 

 very strongly thickened and 

 the thickening layers are dis- 

 solved in the process of ger- 

 mination. 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. 76) is not attacked by it, 

 but only reserve cellulose, which difters in its chemical structure 

 (p. 78). Corresponding to this difference, reserve cellulose yields on 

 hydrolysis other products than does typical cellulose, viz. besides 

 glucose the hexoses, mannose and galactose. 



Inulin, which is found esjiecially in Compositae and Campanulaceae, 

 is related according to its empirical formula (CjjHjQOg),^ 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 accoiuit of the size of its molecule, and 

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

 formula C^HjoOg. The sugar in this case is, however, levulose. 



Cane Sug'ar which occurs for example in the sugar-cane and sugar- 

 beet may be connected with inulin. It is converted by the widely 

 spread enzyme " invertin " into equal parts of dextrose and levulose. 





Fifi. 203. — Different stages of the corrosion shown 

 by the starch grains of germinating Barley. 



