PLANT GROWTH HORMONES 65 



nodules and coleoptiles is very largely in the free state while that in the 

 green tissue is overwhelmingly bound. It is difficult to resist the con- 

 clusion that this protein-bound auxin of leaves is not a reserve and is 

 (mainly at least) unavailable to the plant for growth. 



Seeds, however, show a very different picture. When dry seeds of 

 Avena or of corn are dampened, auxin is very rapidly set free. Heating 

 with alkali liberates auxin very rapidly in this material, as shown by 

 Avery and coworkers (1941). It is now clear that this auxin, too, is 

 protein-bound, and Gordon (1946) has prepared pure proteins from 

 wheat which liberate auxin on hydrolysis with alkali or acid. We know 

 that seeds deliver to the seedling a precursor which the seedling tip con- 

 verts to auxin. While this mobile precursor, partially purified by Berger 

 and Avery (1944), does not itself seem to be a protein, it is clear that 

 the auxin-protein of seeds is a true reserve or auxin-source, available 

 to the plant for growth. 



A word of warning is in place here. Tryptophane on heating with 

 alkali forms a small amount of indoleacetic acid; casein and other 

 proteins do the same. This phenomenon probably cannot account quan- 

 titatively for the auxin liberated by alkali from these seed proteins, but 

 it certainly enters in. The fact that some seedlings are able to convert 

 both tryptophane and tryptamine to free auxin should be remembered 

 also. The demonstration by Haagen Smit, and later by Avery et al., and 

 by Larsen and van Overbeek, that indoleacetic acid is no "heteroauxin" 

 but a true plant auxin is of importance here. 



Besides the above instances, there is still another type of behavior. 

 When grass-stalks are laid horizontal, the nodes, which have long 

 ceased growing, begin to grow on the lower side and this produces geo- 

 tropic curvature of the stalks. Many years ago Schmitz showed by 

 diffusion that this is due to the new production of auxin in these nodes ; 

 here gravity, instead of merely redistributing the auxin, as in the coleop- 

 tile, causes renewed production of auxin. Now van Overbeek et al. 

 (1945) have shown in sugar cane that this new production is at the 

 expense of bound auxin in the node ; gravity in some way initiates the 

 conversion of the bound to the free form. This phenomenon of gravi- 

 tational release of auxin may be connected with another curious phenom- 

 enon. In pineapple (but not in other plants) application of auxin to 

 the growing tip causes formation of flowers. Van Overbeek and his 

 coworkers (unpublished) have now found that placing the pineapple 

 plant horizontal has the same effect. This remarkable result may be 

 due to liberation of bound auxin, but could of course be merely the result 



