For the Layman 



everybody who has grown plants inside a window-space. For 

 some years, plant physiologists sought to explain the effect 

 of the light, before they suspected the existence of growth- 

 substances. As soon as it became reasonably clear that there 

 was a connexion between light and the elaboration of some 

 growth-material by the plant, efforts were made to isolate in 

 a pure state the substance causing the cell-elongation. 

 Microscopic studies had shown that the observed bending 

 was an expression of greater growth in length of some of the 

 cells on one side of the stem. 



The story of the isolation of highly-active growth-sub- 

 stances is an interesting one. The amount of growth- 

 substances in the tips of seedlings was too small to offer any 

 hope of obtaining much of it from that source. After it was 

 noticed that urine had a marked effect in making young 

 plant-cells become longer, efforts were made to obtain the 

 active substance (whatever it was) from urine, which could be 

 obtained easily in quantity. The effort was successful, and 

 enough of a substance, at first called auxin, was obtained 

 from urine, to enable its chemical structure to be ascertained. 

 It was found that there were in urine two rather similar 

 growth-substances ; the one first discovered is now called auxin- 

 a, the other is known as auxin-b. 



In undertaking this chemical investigation of urine, the 

 researchers did not know whether the result of their labours 

 would after all be the finding of a substance identical with 

 or different from the growth-hormone of oat seedlings. Even 

 now it is not known with certainty — only with a very high 

 degree of probabiHty — that an auxin is in fact the natural 

 growth-substance of the seedling oat tip. The presence of 

 auxins in urine propounded the problem as to what they were 

 doing there. It was found that both auxins could be isolated 

 from some vegetable oils, and from malt, so their presence in 

 urine can be accounted for by their being included in our 

 vegetable foods. 



Although the formulae of both auxins are known, they have 

 not yet been prepared by purely artificial methods in the 



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