248 PHYTOHORMONES 



become incorporated in the plant to such an extent that they 

 may be regarded as internal factors. There are a number 

 of diseases in which growth correlations are destroyed or 

 new ones established, and in these auxin must play a part. 

 Thus, in ''curly-top," one of the virus diseases, the growth 

 of the shoot is greatly reduced, and Grieve (1936) has ob- 

 tained evidence for a corresponding reduction in auxin con- 

 tent in the infected plant. Galls may be due to the opposite 

 effect, namely production of growth hormones by the in- 

 fective agent. Indeed, both Sachs (1882) and Beijerinck 

 (1897) based their views on growth correlations partly on such 

 pathological growths (c/. Figure 2). In a study of Erineum 

 outgrowths on leaves (intumescences) La Rue (1935, 1936a) 

 has shown that auxin will produce similar effects, while 

 Brown and Gardner (1936) have shown the same thing for 

 crown gall. In the case of root nodules there is good evidence 

 that they are due to auxin produced by the infective agent, 

 Rhizobium leguminosarum (see IX C). Comparable evidence 

 for the production of auxin by the crown-gall organism has 

 been given in XIII C. The galls due to gall-wasps {Cynipi- 

 dae) and gall-midges (Cecidomyidae), which show elaborate 

 differentiation, can probably not be considered in such 

 simple terms. 



Another suggestive observation for the interpretation of 

 abnormal growth is that of Laibach and Mai (1936), who 

 have described malformations of leaves and buds caused 

 by repeated application of concentrated auxin pastes to the 

 growing point. 



H. Outlook 



This survey of the rapidly developing field of phyto- 

 hormones shows that many problems have been solved and 

 few really important points are still subjects of disagree- 

 ment. This result has not been achieved without effort. 

 In the first place emphasis has been laid from the very 

 beginning on quantitative work. In the second place the 

 development of the chemistry has made it possible to check 



