Growth Substances 



377 



abscission, parthenocarpy, tumor formation, root production, dominance 

 relationships among buds, nastic responses, and tropisms generally. The 

 effect in each case must evidently be a function of the specificity of the 

 responding system, including other biologically active compounds, rather 

 than of a particular evocating substance. 



The developmental effects of growth substances are chiefly of three 

 sorts: on growth in general (defined as permanent increase in volume), 

 on the correlations of growth, and on development and differentiation of 

 specific structures. These may involve very different problems. 



GROWTH SUBSTANCES AND PLANT GROWTH 



Growth of the plant as a whole may be controlled by various growth 

 substances. Van Overbeek (1935) showed that in genetically dwarf races 



12 3 4 



STAGE OF DEVELOPMENT 



I 2 3 



STAGE OF DEVELOPMENT 



Fig. 18-1. Yield of diffusible auxin from long shoots (left) and short shoots (right) 

 at successive stages in development in Ginkgo. ( From Gunckel and Thirnann. ) 



of maize there was much less auxin than in plants of normal height, indi- 

 cating that auxin was associated with growth in stem length. The 

 dwarfing gene here effects a more rapid destruction of auxin rather than a 

 lesser production of it. Von Abrams (1953) sprayed IAA on etiolated 

 plants of dwarf and tall varieties of peas. The dwarf increased 30 per 

 cent in height but the tall one was slightly reduced, an instance of the 

 frequently observed fact that the effect of auxin is different with different 

 material. In a number of other cases the dwarfing of plants has been 

 shown to be related to a deficiency of auxin. 



Auxin particularly affects growth in length, especially of the stem, and 

 in many cases has been found to be the factor that determines length 

 growth of the internodes. Gunckel and Thirnann (1949) compared the 



