Hormonal Mechanism of Growth Inhibition by Radiation 547 



not change the relative curvatures. Plasticity has also been measured 

 as the irreversible stretching of freeze-killed tissue under a longitudi- 

 nal force. After 2 hrs. irradiation a decrease in plasticity of about 75 

 per cent was found by this method also. 



Gibberellic acid applied to plants shortly before irradiation com- 

 pletely prevents the decrease in plasticity just as it prevents inhibi- 

 tion of growth. It has no effect on the plasticity of dark-grown plants 

 (Figure 1). The participation of gibberellin in radiation inhibition 

 of growth is discussed below. 



To demonstrate that the change in plasticity is a cause rather than 

 an effect of growth inhibition, the various other factors which could 

 affect growth rate were investigated. No change in any of these fac- 

 tors was found, even after growth inhibition was complete. The os- 

 motic pressure of elongating stem cells was measured by a turgor 

 method developed for the purpose (21) and also by determining the 

 osmotic pressure of expressed sap. No difference was found in the 

 osmotic pressure of dark-grown and irradiated plants by either 

 method. Water permeability of the cells was determined by measuring 

 the rate of opening of plasmolyzed, split stem sections when placed in 

 distilled water. No effect of irradiation was found. Rate of water 

 movement through growing tissue was measured as the rate of plas- 

 molysis of stem tissue sections. No effect of irradiation was found. A 

 possible water deficit in irradiated plants was investigated by deter- 

 mining the rate of water loss from stems of dark-grown and previously 

 irradiated plants. No difference was found. The possibility that a 

 water deficiency might cause growth inhibition of irradiated plants 

 was further tested by measuring growth inhibition in a water- 

 saturated atmosphere. No effect of relative humidity was found on 

 radiation-inhibition of growth. 



MORPHOLOGICAL REGION OF RADIATION SENSITIVITY 



Idle (14) has compared the radiation sensitivity of the stem tip 

 with that of the elongating region of Vicia faba. Either the tip or grow- 

 ing region was briefly irradiated and the time-course of the growth 

 response (of the elongating region only) was followed. Substantial 

 growth inhibition was observed in both cases but irradiation of the 

 tip was most effective. When the tip was irradiated, inhibition was 

 rapid and substantial, and growth of the elongating region never re- 

 covered. When the elongating region itself was irradiated, inhibition 

 was equally rapid but somewhat less complete. The most striking 

 effect, as a result of irradiation of the elongating region, was the ob- 

 servation that growth of the irradiated region recovered almost com- 

 pletely after 4 to 5 hrs. 



