210 ANTHOCYANINS AND GENETICS 



case of other species which give rise to varieties with soluble yellow sap 

 pigments, such as Dahlia variabilis and Althaea rosea, we must suppose 

 that some relationship exists similar to that in Antirrhinum, that is 

 that the plant loses an inhibitor which suppresses the formation of a 

 more highly coloured flavone. Research upon these points is much 

 needed. 



While we are attempting to identify the functions of the various 

 Mendelian factors, it may not be out of place, at this point, to 

 emphasise certain facts concerning the nature of many reactions in 

 plant metabolism. It is known that a large number of the chemical 

 reactions in the plant are what are termed reversible reactions. An 

 example of such a reaction (outside the plant) is the formation of acetic 

 acid and alcohol from ethvl acetate and water: 



*/ 







ethyl acetate + water ^: ethyl alcohol + acetic acid. 



The characteristic of a reaction 'of this kind is that it progresses 

 in one direction or the other until a certain equilibrium, specific to the 

 reaction, is established. Any alteration in concentration of the reacting 

 substances is immediately followed by reaction towards equilibrium 

 again. The velocity of the reaction, moreover, may be accelerated 

 by the addition of a little hydrochloric acid, which, though it does not 

 affect the final products, accelerates the process. Such a substance has 

 been termed a catalyst. In the plant many reactions are known to 

 be of the above type, as for instance the hydrolysis of starch into sugar, 

 and the synthesis of starch again from sugar. Outside the plant, 

 hydrolysis of starch, by purely chemical methods, would take place 

 only at a high temperature. Within the plant, it occurs at ordinary 

 temperatures with considerable rapidity, and is controlled by the enzyme 

 diastase ; diastase is therefore a catalyst produced by the living plant. 

 The extraction of diastase and the demonstration of its hydrolytic action 

 in vitro is of course a perfectly simple experiment. But the synthesis 

 of starch from sugar by the same enzyme has not been carried out in 

 vitro. It is reasonable, however, to assume that diastase controls 

 the process in both directions, for there are other cases where 

 the synthetic power of enzymes has been demonstrated in vitro 

 under special conditions. It is nevertheless possible that in some 

 cases one enzyme may control the synthetic, and another the hydrolytic 

 process. Diastase is moreover only one example; numbers of plant 

 reactions concerned with oxidation, pigment formation, hydrolysis of 

 proteins, glucosides, fats, etc., are known to be controlled by enzymes. 



