8l4 PLANT GROWTH 10 



by several workers in recent years (see Thimann, 1954b, for review). Another is to 

 implicate other parts of the plant. It has been suggested that the real inhibitor is 

 a lactone formed in the roots (Libbert, 1954b), as witness the occurrence of scopole- 

 tin and related lactones in roots. In favor of this view is the fact that isolated stem 

 tissue of Pisum can be partly inhibited from elongating by root tips or their ether 

 extracts (Howell, 1954). Strongly against it is the fairly clear evidence that the 

 concentration of lactones in roots is too low to exert an appreciable growth inhi- 

 bition therein (Pollock et al., 1954). A curious feature is that hydrolysis of the 

 (crude) inhibitor preparation from roots is claimed to yield an auxin (Libbert, 

 1955), which seems at variance with the idea that it is a lactone. Just how the 

 stream of auxin would control the inhibitor, or its hydrolysis, has not been made 

 clear. 



New evidence does make it possible to set aside, at least tentatively, one theory, 

 namely that the lateral buds are inhibited because the auxin concentration 

 reaching them is supra-optimal (Thimann, 1937). For when in a two-shoot plant 

 the apical bud of the first shoot is being inhibited by the apical bud of the second 

 (or by auxin applied to the second apex) it has been found that auxin paste 

 applied directly to the inhibited apical bud will cause it to elongate (Fig. 14A; 

 Libbert, 1954a). This means that its failure to elongate could not have been due 

 to any excess of auxin in it. This evidence is somewhat weakened by the fact that 

 similar application of auxin to inhibited lateral buds in situ does not cause their 

 growth (Fig. 14B; Thimann, 1937); even isolated lateral buds show inhibition 

 by auxin, and in relatively low concentration, though the case is not quite identi- 

 cal. This theory had, however, been weakened much earlier through the demon- 

 stration by several workers, especially Ferman and Van Overbeek, that when 

 lateral buds are released from inhibition their auxin content rises markedly and 

 rapidly. While the situation is hard to understand, the sum of the evidence now 

 makes it rather improbable that inhibition could be due to a simple excess of 

 auxin in the buds. 



Another new point, though unrelated to any of the proposed theories, is that 

 cell divisions in the inhibited lateral bud show a sensitive response to auxin which 

 could possibly be the initial effect. After decapitation of the terminal bud, the 

 nuclei become deeply-staining, and mitoses soon begin, while these changes are 

 prevented when auxin is applied (Naylor, cited by Skoog, 1953). 



Perhaps a more fruitful approach is concerned with the factors necessary for 

 stem growth. To those already discussed in section VId should be added gibberel- 

 lin, which clearly promotes the elongation of lateral buds in peas (Brian et al., 1955), 

 being unlike auxin in this respect. It promotes branching and frond formation 

 also in duckweeds {Spirodela), a process which is homologous with lateral bud 

 development (author's unpublished data). Shoots in culture or buds growing on 

 tissue cultures respond strongly (like roots) to adenine, particularly when they have 

 roots attached. The combination of adenine with something synthesized in roots 

 allowed isolated but rooted pea epicotyls to grow to 20 mm length in 46 days 

 (Howell and Skoog, 1955). Since coconut milk in addition brought the elonga- 

 tion up to 77 mm it is evident that many growth factors interact for normal 

 growth. The growth promotion due to adenine and roots was, however, great 



