638 5. Housley and B. J. Dcvcrall 



Before elaborating on the above scheme, it is desirable first to 

 examine how well the whole inhibitor hypothesis accounts for the 

 results of Figures 2, 3, and 4. During the course of experimentation 

 an impression was obtained that large sparing actions were associated 

 with rapid rates of lAA inactivation and conversely smaller sparing 

 actions with less rapid rates. Data were analyzed and the results are 

 shown in Figure 6. Average rates of lAA destruction (no GA present) 

 over the first 3 hr. period arc plotted against differences between aver- 

 age rates when GA is present and absent. This period was chosen as 

 rates of lAA destruction in many experiments were fairly uniform 

 over this interval. Rates are expressed in absorption units to permit 

 data from the 1957 season to be included, while points are joined 

 merely to show trends more clearly. Figure 6 shows that the spar- 

 ing action with GA tends to increase as the rate of lAA destruction 

 increases. Expressing this in terms of Figure 5, as the rate of lAA 

 destruction increases (i.e., as E increases relative to / and EI) so the 

 effectiveness of GA increases shoiving the same effect as an increase in 

 1. Expressed alternatively, if EI is high and E is low, there wnll be a 

 low rate of lAA destruction and only a small effect of GA provided 

 the effect of GA can be equated to /. Thus, our own data are con- 

 sistent with Galston's statement that GA increases the le\'el of / in 

 the plant. It may be noted that Lockhart (21) has observed a similar 

 effect: intact dark-grown plants which do not respond to applied GA 

 give the greatest GA-induced growth promotion when decapitated 

 (very high / in intact plants results in no growth with applied GA; 

 however, if / is markedly reduced by decapitation of these plants, the 

 GA-induced growth relative to the intact plants will possibly be at a 

 maximum). 



It is now possible to consider the results of Table 1 against the 

 background of the inhibitor scheme. It will be recalled that concen- 

 tration of inhibitor is high at the apex and a gradient exists down 

 the plant. The concentration in the sections of the second column will 

 be lower than that in the apices of the sixth column, and therefore 

 there will be a tendency for greater sparing actions to occur in the 

 former; this expectation is reflected in the table. Comparison of 

 light- and dark-grown materials (i.e., column 3 with 5, and column 4 

 with 6) cannot be made readily as inhibitor levels of light- and dark- 

 grown stems do not appear to have been compared. Comparison of 

 experiments carried out in light and in darkness (column 3 with 4, 

 and column 5 with 6) suggests that inhibitor level in the former was 

 less than the latter on some occasions. This variation may have been 

 due to several factors, the most important possibly being insufficient 

 control over the quality of light used during experimentation. A sec- 



