PLANT HORMONES AND GROWTH 



hormone may consist of typical rapidly dividing cells or may involve 

 principally enlarged (elongated) and undivided cells. No doubt the 

 underlying response is the same in both cases, but the external visible 

 effect may differ. It does not seem at present that there is any specific 

 hormonal stimulus for cell division per se. However, the control of 

 cell division is one of the problems which on further study may yield 

 information about the underlying mechanism of the wound reaction 

 and its relation to other forms of growth. 



Lastly, there are a series of problems involving the interaction 

 of auxin with other special substances. Biotin and auxin act co- 

 operatively in the formation of roots by certain types of cuttings. 

 Thiamin and pyridoxine are necessary for the elongation of roots, 

 and auxin inhibits this elongation, but whether the availability of 

 these vitamins is inhibited is not known. In fact, the growth physi- 

 ology of the root is still largely unexplored, despite a great many in- 

 genious experiments (10). Thus it may be concluded that no more 

 than a beginning has been made in the study of plant growth 

 hormones. 



References 



(1) Burk, D., and Winsler, R. J., Vitamins and Hormones, 2, 306 (1944). 



(2) Commoner, B., and Thimann, K. V., J. Gen. Physiol., 24, 279 (1941). 

 (2a) Gustafson, F. G., Botan. Rev., 8, 599 (1942). 



(3) Kogl, F., Ber., A68, 16 (1935). 



(4) Sweeney, B. M., and Thimann, K. V., J. Gen. Physiol., 25, 841 (1942). 



(5) Thimann, K. V., Plant Physiol., 13, 437 (1938). 



(6) Thimann, K. V., Biol. Rev. Cambridge Phil. Soc, 14, 314 (1939). 



(7) Thimann, K. V., and Bonner, J., Physiol. Revs., 18, 524 (1938). 



(8) van Overbeek, J., Ann. Rev. Biochem., 13, 631 (1944). 



(9) Went, F. W., Am. J. Botany, 25, 44 (1938); Plant Physiol., 13, 55 (1938). 

 (10) Went, F. W., and Thimann, K. V., Phytohormones. Macmillan, New 



York, 1937. 



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