DEVELOPMENT OF THE HORAIOXE CONCEPT 15 



auxin is a change in the properties of the cell wall, which 

 allows extension to take place. The intermediate stages 

 between the entry of auxin and its final effect have been 

 investigated by Thimann and Bonner. 



Following the discovery of the growth-promoting activity 

 of the auxins, it was foimd that many well-known correla- 

 tions in organ development are brought about by the same 

 substances. As mentioned above, Sachs assumed that root 

 formation is due to a special root-forming substance. Ex- 

 perimental e\4dence to support this \dew was brought forward 

 by several investigators, particularly by van der Lek. Proof 

 that a special substance is indeed concerned was obtained 

 by Went and Bouillenne, and the isolation of this active 

 substance by Thimann and Went led to its identification 

 with the auxins, an identity which was independently con- 

 firmed by Laibach and others. Another phenomenon long 

 known as a typical correlation is the inhibitory effect of the 

 terminal bud of a shoot on the development of lateral buds 

 (Goebel, Dostal, Reed, Snow); this effect was shown by 

 Thimann and Skoog to be due to auxins, produced in the 

 growing bud. Still other important correlative phenomena, 

 such as cambial growth, swelling of the ovary of orchids 

 after polHnation, petiole abscission, and callus formation 

 have since been shown to be brought about by auxins (Snow, 

 Laibach). 



The interrelations between the fields of organ formation, 

 correlation, tropisms, and growth have been summarized 

 in the chart. Table I, which shows how these four lines of 

 approach have gradually come together, and how each in- 

 vestigator's contribution has been merged into a more and 

 more complete system. Exceptions must be made for Bei- 

 jerinck and Loeb, whose work, although of importance, had 

 little influence on the development of this field. The sound- 

 ness of their views, however, must now be acknowledged. 

 The placing of an investigator's name between two vertical 

 lines indicates that his work linked up these two lines ; thus 

 Boysen Jensen showed that tropisms were a correlation 



