PLANT HORMONES AND GROWTH 



in young shoots, thr ramhium is stimulated to divide by a diffusible 

 substance and that pure indoleacetic acid liad the same effect. Like- 

 wise, the substance in orchid pollen, shown by Fitting to be responsible 

 for the swelling of the ovary on fertilization, was identified with auxin. 

 Finally the elongation of roots, unlike that of shoots, was shown to be 

 inhibited by auxins. Wherever synthetic substances have been tested, 

 their influence on these other growth processes has been to some 

 extent quantitatively comparable with their effectiveness in producing 

 simple growth by elongation (5,10). 



Thus, unlike many animal hormones of specialized activity, 

 the functions of auxins are manifold. The stream of auxin coming 

 from the apical bud and the young leaves in it promotes the elongation 

 of the shoot, stimulates the cambium to produce thickening, inhibits 

 development of the lateral buds lower down, and promotes the forma- 

 tion of root initials toward the base. In the flower, the auxin con- 

 tributed by the pollen, or released from the ovary tissue by fertilization, 

 promotes the swelling of the ovary into a "fruit" and also prevents the 

 fruit stalk from becoming separated (inhibition of abscission) . 



Morphologically, at least, these functions all appear to be very 

 different. Not only do they comprise effects of opposite sign (namely, 

 growth and inhibition) but even where the effect is positive the types 

 of growth controlled by auxin differ widely. The stimulation of 

 cambium or root initials to divide bears little resemblance to the 

 promotion of simple elongation without cell division, as in the oat 

 coleoptile. It is hard, therefore, to avoid the conclusion that the 

 primary effect of auxin is in all tissues the same — a fundamental 

 reaction whose morphological sequelae depend on the kind and age of 

 the tissue, the availability of other interacting substances, and the ex- 

 ternal and internal conditions. This concept and the experiments 

 bearing on it will be taken up in more detail below. 



The general position described above has been established for 

 some time. During the last few years the field has developed mainly 

 along the following three lines. 



(a) Practical Applications. Originally auxin was most gen- 

 erally used to induce the formation of roots on cuttings. Auxin 

 treatment both accelerates rooting and increases the number of roots 

 formed in cuttings of a wide variety of herbs, shrubs, and trees. It 

 does not, of course, solve all the problems of vegetative propagation. 



