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ANNUAL REPORT SMITHSONIAN INSTITUTION, 1941 



was stuck on again a little to one side, the side on which it rested 

 grew more than the opposite side, with the result that the plant 

 curved (fig. 1). No light was here involved; the curvature was due 

 to the one-sided influence of the tip. Paal deduced that the growth 

 of the shoot was controlled by a growth substance or hormone which 

 was produced by the tip. 



Now the idea of hormones was developed by zoologists to account 

 for those phenomena in which one organ influences tissues in other 

 parts of the body. The heroine of the dime novel, who is suddenly 

 confronted by the villain, or by the family ghost, turns as white as 

 a sheet, her hair stands on end, and her eyes widen with horror. 

 These effects result from her having received a dose of a hormone 

 (adrenalin) which is secreted in a special gland and travels about 



A a. 



FlQUBE 1.- 



-The experiment of PaS,l. Left, intact seedling; center, tip removed 

 and replaced to one side ; right, curvature resulting. 



in the blood stream, causing the capillaries to contract all over the 

 skin and scalp. Many other hormones are known. All of them are 

 secreted in some part of the animal body and travel about it to exert 

 their effects in other parts. 



In this case growth is controlled by a substance or hormone se- 

 creted by the tip and traveling down the side of the plant, which 

 responds by growing faster. In the normal plant, with the tip 

 symmetrically placed, all sides would receive the same amount of 

 the hormone and consequently would grow equally. 



It was 10 years before the next step forward was taken by Went, 

 in Holland (1928). He found that if the tips were cut off and 

 placed on a jelly of agar or gelatin, this jelly acquired the property 

 of hastening the growth of a coleoptile stump when applied to one 

 side of it. The growth-promoting hormone had diffused from the 

 tip into the agar. The curvatures which resulted were very regular, 

 and Went found that under constant conditions the reaction could 



