GROWTH HORMONES IN PLANTS 



By Kenneth V. Thimann 

 Harvard Biological Laboratories, Cambridge, Mass. 



[With 2 plates] 



The development of our knowledge of the plant hormones is a 

 very interesting example of how a piece of research which seems 

 purely academic may lead to results of considerable practical impor- 

 tance. It also demonstrates rather well the reason for the fascination 

 of scientific work, because one never knows quite where one is going 

 to be led next. Almost any research problem becomes a kind of chase, 

 with all the excitement of an old-time comedian's chase, which may 

 embroil him in all kinds of difficulties and may finally land in the 

 most unexpected places. 



No scientific story, of course, has a true beginning, for they all 

 grow out of some earlier one, but this may be regarded for the present 

 as beginning in 1919, when Professor Paal, in Hungary, was studying 

 the response of certain seedlings to light. For this work he used the 

 coleoptiles of the cereals, especially oats. 



In a field of oats or wheat, when the crop is still young, one may 

 often see a thin, papery sheath at the base of the stalk. It is soon 

 torn open by the leaves which grow up through it, and withers early. 

 This delicate shoot first attracted the attention of Charles Darwin by 

 its extreme sensitivity to light, and since Darwin many others have 

 studied it. Now Paal was interested in the effect of the extreme tip 

 of the coleoptile on the sensitivity of the part below it. He was anx- 

 ious to confirm the earlier finding of Boysen-Jensen (1913), which 

 was that if the tip is removed, the sensitivity to light — as shown by 

 the curving of the coleoptile toward the source of light — was lost, 

 and that when the tip was replaced (not grafted but just glued on) 

 this sensitivity returned. He not only did confirm this, but found 

 something even more important. If the tip which had been cut off 



^ Presented at a meeting at the Franltlin Institute March 9, 1939. Reprinted by permis- 

 sion from the Journal of the Franklin Institute, vol. 229, No. 3, March 1940. 



393 



