III. PLANT GROWTH HORMONES 



BY KENNETH V. THIMANN^ 



THE field of plant growth hormones, or auxins, has broadened 

 out so enormously in recent years that I shall be able here to 

 deal only with certain parts of it. The various applications to 

 agriculture and horticulture, for instance, I will scarcely touch on, not 

 because they are unimportant but because it seems preferable to center 

 attention on the basic phenomenon of growth itself and how it is 

 influenced by the hormones in plants. 



One of the most striking features of the auxins is the multiplicity of 

 their actions. Although they were discovered through their growth 

 promoting effect on the Avena coleoptile, especially when applied to one 

 side, and have been assayed in this manner ever since, it was soon made 

 clear that they promote straight growth in many elongating stems and 

 organs, i.e. enlargement along the longitudinal axis. Also, when such 

 organs are slit and placed in a solution of an auxin they undergo dif- 

 ferential growth due to greater elongation of the outer than the inner 

 tissues. The auxins cause formation of roots on stems, they cause cell 

 division in the cambium and the formation of callus at cut surfaces; 

 on the other hand they inhibit the growth of buds, and they also inhibit 

 both the elongation of roots and the formation of the abscission layer 

 on leafstalks and fruitstalks. Further, they trigger in some way the 

 swelling of the ovary into a fruit, as pollination does, so that by applying 

 auxin in the absence of pollination a normal but seedless fruit is formed. 

 Finally, in pineapple but not (so far as known) in other plants, they 

 even cause flowering. 



This multiple action is in strong contrast with that of some of the 

 animal hormones which typically cause specific effects on specifically 

 reactive tissues. The multiple action has led to the view, expressed 

 especially by Went (1938) and more recently by Gautheret (1944) - that 

 auxin acts as a mobilizing agent, bringing specific hormones to the 

 tissue at which it is applied. According to this view, root formation when 

 auxin is applied to the base of a cutting would be due to the mobiliza- 

 tion there of a specific root-forming hormone. Experiments of Went, of 

 Bouillenne, and of Cooper have given indirect support to this view, 

 but the support is not strong and there is evidence against it. For in- 

 stance, the rooting of small cubes of potato tubers when given high 



1 Department of Biology, Harvard University. 



