GROWTH HORMONES — THTMANN 399 



first curve outward, then after an hour or so the mward curvature 

 begins and is complete in about 20 hours. 



Now in this reaction the auxin is being supplied to all the tissue, 

 since the piece of stem is immersed in the solution, yet nevertheless 

 the curvature which results shows that the tissues on the outside 

 grow more than those on the inside. It is evident that this phenom- 

 enon is quite different from those described at the beginning, in 

 which curvature results because the auxin is only applied to one side. 

 We have accumulated a good deal of evidence to prove that here 

 there is truly a difference in the response of the outer and inner 

 sides to the same auxin concentration. 



Such subtle differences as this between closely appressed layers 

 of tissue, or those described above between the rooting response of 

 different parts, can now be investigated for the first time by the use 

 of growth hormones. These substances are a powerful tool for 

 studying all kinds of phenomena in plants and especially that most 

 obscure process of all, whose understanding is one of the most 

 fundamental things with which biologists are concerned, the 

 phenomenon of growth. 



BIBLIOGRAPHY 



BOTSEN-jENSEN, P. 



1913. tjber die Leitung des phototropischen Beizes in der Avenakoleoptile. 

 Ber. Deutsch. Bot. Ges., vol. 31, No. 10, pp. 559-566. 

 Cholodnt, N. 



1927. Wuchshormone und Tropismen bei den Pflanzen. Biol. Zentralbl., vol. 

 47, No. 10, pp. 604-626. 

 KoGL, Fkitz, Ebxxeben, Hanni, and Haagen Smit, A. J. 



1934. Uber die Isolierung der Auxine a und b aus pflanzlichen Materialen. 

 Hoppe-Seyler's Zeitschr. Phys> Chem., vol. 225, Nos. 5-6, pp. 215-229. 

 KoGL, Fbitz, and Haagen Smit, A. J. 



1931. tJber die Chemie des Wuchsstoffs. Kon. Akad. Wetensch. Amsterdam, 

 Proc. Sec. Sci., vol. 34, No. 10, pp. 1411-1416. 

 KoGL, Fbitz, Haagen Smit, A. J., and Eexleben, Hanni. 



1934. Uber ein neues Amxin ("Hetero-auxin") aus Ham. Hoppe-Seyler's 



Zeitschr. Phys. Chem., vol. 228, Nos. 1-2, pp. 90-103. 

 PaIx, a. 



1919. "fiber phototropische Reizleitung. Jahrb. Wiss. Bot., vol. 58, Na 1, 

 pp. 406-458. 

 TEaMANN, Kenneth V. 



1935. On the plant growth hormone produced by Bhizopus suinus. Journ. 



Biol. Chem., vol. 109, No. 2, pp. 279-291. 

 Thimann, Kenneth V., and Skoog, Folkk. 



1934. On the inhibition of bud development and other functions of growth 

 substance in Vicia Faba. Proc. Roy. Soc, ser. B, vol. 114, No. B 789, 

 pp. 317-339. 

 Thimann, Kenneth V., and Went, F. W. 



1934. On the chemical nature of the root-forming hormone. Kon. Akad. 

 Wetensch. Amsterdam, Proc. Sec. Sci., vol. 37, No. 7, pp. 456-459. 



