148 PLANT GROWTH SUBSTANCES 



Argument has been leveled against the theory based on a rapid 

 increase in osmotic pressure. So far it has not been possible to demon- 

 strate experimentally such an increase in osmotic pressure following 

 application of a growth regulator. On the other hand, the theory based 

 on wall extensibility may not entirely explain the phenomenon since 

 it has been repeatedly demonstrated during the last three years that once 

 a cell has expanded and taken up water under the influence of the 

 regulating chemical, the cell has then changed in a way that makes it 

 better able to retain the water it holds against forces of evaporation, than 

 is the case with an untreated cell (32). Wall extensibility would hardly 

 account for this increased water-retaining capacity of treated tissues. 

 Increased osmotic pressure, on the other hand, might in part account 

 for this effect. 



There is an abundance of evidence indicating that growth regulators 

 accelerate either directly or indirectly the activity of some enzymes 

 in plants. However, direct proof of their effect on enzyme systems is 

 meager. Berger and Avery (3,4,5) obtained more highly active de- 

 hydrogenase from oat plants treated with indoleacetic acid than from 

 untreated ones. Eyster (14,15,16) believes that growth stimulation re- 

 sults from the release of diastase from an inactive to an active form. Gall 

 (18) demonstrated that starch in agar media was more readily digested 

 by enzymes that diffused from sections of bean plants treated with 2,4-D 

 than by enzymes from comparable untreated sections. His results are 

 inconclusive for he states that the growth regulator may either have 

 increased production of enzymes or increased their activity, or the 

 chemical may have affected the tissues so that, although they contained 

 the same amount of enzyme, more of it diffused from the treated than 

 from the untreated section. His work shows clearly, however, that 

 enzyme digestion of starch outside of the treated stems was greater than 

 it was outside the untreated ones. 



Within the plant the activity of the enzyme system involved in the 

 conversion of starch to sugar is also affected by some growth-regulating 

 chemicals. In leaves of bean and morning-glory plants, for instance, 

 starch hydrolysis has been accelerated by the application of relatively 

 large amounts of such substances as indoleacetic acid, indolebutyric acid, 

 naphthaleneacetic acid, naphthoxyacetic acid, and 2,4-D (34,38,43). In 

 treated leaves a marked increase in sugar content at first paralleled 

 starch degradation, but as the starch was depleted their sugar content 



