206 TEXTBOOK OF PLANT PHYSIOLOGY 



and industry (example, soap root). The toxicity and the sharp 

 taste of many glucosides indicate that they are special protective 

 substances which save the seeds and other parts of plants from 

 being eaten or injured by animals. There are a-glucosides, right- 

 rotating and split by the enzymes of yeast cells, and /3-glucosides, 

 left-rotating and split by emulsin, which has no effect whatever on 

 a-glucosides. The difference between these two types of gluco- 

 sides is explained by the fact that the a-glucoside contains a 

 more strongly rotating modification of glucose, the so-called 

 a-d-glucose, while in /3-glucosides the more feebly rotating /3-d-glu- 

 cose occurs. 



The glucosides are connected with a number of plant pigments. 

 The yellow pigments of many flowers are glucosides in which the 

 carbohydrate groups are combined with the derivatives of the 

 complex aromatic flavone nucleus. The anthocyans, giving a blue 

 or red color to the cell sap of plants, according to the investigations 

 of Willstatter and his collaborators, are combinations of sugars 

 with anthocyanidines — colored substances related to flavones but 

 of somewhat different structure. The anthocyanidines are ampho- 

 lytes, i.e., they show simultaneously basic and acid properties. 

 Their combinations with bases are blue; with acids, red. This 

 explains the change in their color with change in acidity. 



67. Enzymatic Splitting of Polysaccharides. Amylase (Dia- 

 stase) and Cytase. Hydrolysis of Disaccharides and Glucosides. — 

 The germination of seeds and other food-storage organs is always 

 accompanied by rapid hydrolysis of the polysaccharides present. 

 Let us see what happens to the most important of these reserve 

 substances, starch. Microscopic observations have shown that 

 from the first days of germination, starch becomes subject to an 

 intensive chemical transformation. On the surface of the grains 

 of starch first appear small depressions, which gradually deepen 

 into the grain. By the uniting of these depressions and the forma- 

 tion of cavities, the starch grain finally becomes so tunneled that it 

 falls apart into minute granules, which in the end are completely 

 dissolved (Fig. 90). In place of starch there now appear sugars, 

 namely maltose and glucose. It is easily possible to detect this 

 accumulation of sugar by special reagents, such as Fehling's solu- 

 tion, or simply by tasting. The ungerminated grain of barley has a 

 mealy taste, while the germinating one is sweet, hence, in a dry 

 state, it is called malt. 



