760 PLANT GROWTH lO 



culture work, especially that of Steward and coworkers on carrot tissue (Steward 

 and Caplin, 1954). 



For gibberellins, the original bioassay, used in all the earlier work in Japan, 

 rests on the excessive elongation of the leaves of rice {Oryza sativa). This was origi- 

 nally observed in plants infected with "bakanae" disease, caused by the fungus 

 Gibberella fiijikuroi, the commoner imperfect form of which is Fusarium moniliforme 

 (see reviews of Stowe and Yamaki, 1957, 1959). The assay was carried out by 

 adding to the soil the gibberellin, or even the culture filtrate of the fungus, and 

 observing the leaf elongations after varying numbers of days or weeks (Brian et al., 

 1954; C/"- P- 780). 



However, with the recent discovery of the powerful effects of gibberellins on the 

 elongation of dwarf plants of several species, the more favored methods of assay 

 make use of these. The gibberellin preparation is either added directly to the leaves, 

 by spraying or as measured droplets, or else added to a nutrient solution in which 

 the roots are immersed. The substance is rapidly transported through the plant 

 and the resulting stem elongation can be measured in as little as 24 h. The tests 

 can be carried out in the light with light-grown plants, for the outstanding charac- 

 teristic of gibberellin's action is that it is scarcely affected by light. In this it is 

 quite unlike the action of auxin which, at least for stem elongation, is very greatly 

 decreased in the light. In other respects it does have much in common with the 

 auxins, and certainly qualifies as an auxin by definition (Brian and Hemming, 

 1955). As test plants, dwarf varieties of peas (Brian and Hemming, 1955) or corn 

 (Phinney, 1956) are very satisfactory. 



Another possible assay would be based on the elongation of isolated sections of 

 leaves, particularly young leaves of oats or barley (Hayashi and Murakami, 1954). 

 The growth of this material is not promoted, and is even slightly inhibited, by 

 auxin so that the test would be very specific. Since it is only in the last four years 

 that the Western world has become aware of the extensive and thorovigh Japanese 

 work on the gibberellins it may be expected that the methods used will be rapidly 

 modified. 



IV. CHEMISTRY OF THE GROWTH SUBSTANCES 



The most widely distributed of the auxins is indoleacetic acid I, (usually abbre- 

 viated as lAA). This has been isolated in pure form from corn seeds as well as 

 from yeast, and from cultures of the fungus Rhizopus suinus. Its ethyl ester has been 

 isolated from immature corn kernels, though the conditions of extraction were 

 such that esterification with ethanol might have taken place. It has been identi- 

 fied (with varying degrees of certainty) by chromatographic and colorimetric 

 methods in a great many plants, including both mono- and dicotyledons in many 

 different families (Bentley, 1958). It has been shown also to be present in plant 

 tissue cultures (Table 3), where it is actually synthesized in good quantity (Kule- 

 scha, 1951 ; Stowe, Thimann and Kefford, 1956), and in pollen, where it is formed 

 in relatively large amounts from tryptophan (Lund, 1 956) . Some of these identifica- 

 tions rest on several properties, such as Rph in different solvents (in paper chro- 

 matography), the Salkowski or other color reagent, molecular weight by diffusion 



