Growth Substances 411 



Leaf growth is affected by gibberellin and is often, though not always, 

 increased. Radish leaves floated on a gibberellin solution in the dark 

 grew larger than the controls. It is perhaps significant that kinetin has 

 much the same effect and that when both substances are applied the 

 increase in growth is equal to the sum of their separate effects. 



Although gibberellin and auxin are similar in some respects, notably 

 in stimulating cell elongation, they differ chemically and in other im- 

 portant ways. Gibberellin fails to produce typical epinasty nor does 

 it induce callus formation, both of which usually result from auxin ac- 

 tion. It also fails to show the polar transport within the plant so character- 

 istic of auxin. It does not inhibit the growth of lateral buds but tends 

 instead to stimulate their development. It does not check leaf abscission. 

 It inhibits rather than promotes root initiation but does not inhibit root 

 growth. 



Gibberellin evidently has some relation in its effects on development 

 to those produced by light, though this relation is not clear. Lang ( 1957 ) 

 found that it induced biennial Hyoscyamus to flower the first season, re- 

 gardless of day-length. The usual inhibition of growth produced by red 

 light is removed by treatment with gibberellin. The effects of this sub- 

 stance much resemble etiolation but are independent of light. This 

 is unlike the effect of auxin. 



In its influence on dwarf plants, which seems to be its diagnostic 

 feature, gibberellin perhaps substitutes for some essential factor that is 

 normally present and which may have been lost by mutation. No effects 

 of gibberellin have as yet been found in any of the lower forms. Sub- 

 stances essentially like it have now been extracted from several higher 

 plants (Radley, 1958), and it is probably widely spread in the plant king- 

 dom. 



The literature in this field has been reviewed by Stowe and Yamaki 

 (1957). Further study of gibberellin should yield important information 

 on the factors governing plant development. 



MECHANISM OF ACTION 



Relatively little is known chemically about most of the growth sub- 

 stances. Some of them, like the calines, are little more than inferences. 

 The existence of others, such as florigen, can be proved by experiment, 

 though they have not been isolated. Others can be isolated, at least 

 partially, but their chemical nature is not well known. As to auxin, 

 gibberellin, and traumatic acid, fairly complete chemical information is 

 now available. 



Many attempts have been made to find some common features of 

 chemical structure among these substances which have formative effects 



