72 PLANT GROWTH SUBSTANCES 



discussed among staff and students. It was partially to settle an argument 

 that I did my first experiment with auxin. Paal had pointed out that 

 there were three possible explanations for a positive phototropic curva- 

 ture in grass seedlings: i) decreased production of the growth regulator 

 in the front side of the coleoptile tip; 2) decreased translocation along 

 the front side; or 3) destruction by light along the front side of the 

 coleoptile. Some of my fellow students were in favor of this third possi- 

 bility, which seemed to be incompatible with many facts as I saw them. 

 If the growth regulator could be handled outside the plant its light 

 stability could be investigated. Therefore, I devoted most of the free 

 evenings and nights which my miHtary training left me to experiment 

 with Avena seedHngs in the laboratory darkroom. Those were exciting 

 nights when the effects of decapitation and regrafting of the tip were 

 studied and it was gradually revealed that the wounding as such did not 

 have such a severe effect as had been supposed. And then on the night 

 of April 16, 1926, the first coleoptiles made their bow to the tip diffusate 

 which had been collected in gelatin. By 3:00 a.m. the negative auxin 

 curvatures were clearly visible, and it was hard for me to realize that this 

 momentous experiment could not move my father to get up in the 

 middle of the night and accompany me to the laboratory! This work 

 was reported for the first time by my father at the International Botan- 

 ical Congress in Ithaca (26). I then worked out a quantitative method for 

 the assay of this growth-promoting substance or auxin (28), as it was 

 later named by Kogl and Haagen-Smit. In this way a good basis was 

 laid for a further physiological and chemical analysis of the auxin and its 

 effects. 



In a short time some important principles were established: without 

 auxin there is no growth; and as a corollary of this, the Cholodny-Went 

 theory of tropisms, saying that tropistic curvatures are due to differential 

 distribution of auxin within the responding stem or organ. 



It is typical of the timeHness of the subject, that first in 1926 for 

 geotropism, and then in 1927 for phototropism, Cholodny (5,6) pub- 

 lished two papers assuming, on theoretical grounds, that gravity or 

 unilateral light deflects the normal symmetrical downward stream of 

 auxin. And each time a paper of mine (27,28) was in press giving experi- 

 mental proof of this assumption. 



Concerning the role of auxin in the plant, Dolk found that after de- 

 capitation of a coleoptile the reduction in growth rate was due to 



