CHAPTER V 



THE RELATION BETWEEN AUXIN 

 AND GROWTH 



In order to be able to explain the normal growth of the 

 plant in terms of auxin, we must first be able to demon- 

 strate that in every case auxin is formed above the region 

 where growth in length occurs. That this is true was shown 

 in Chapter IV. The second prerequisite is that there shall 

 be a direct relation between the quantity of auxin applied 

 and the growth obtained. Such direct proportionality has 

 been shown to hold in whatever plants have been studied 

 in sufficient detail. The best cases are those of the curvature 

 and straight growth of Avena resulting from the apphcation 

 of auxin in agar (see III C 5). Similar proof was given for 

 Raphanus hypocotyls by van Overbeek (1933) and for 

 Lupinus hypocotyls by Dijkman (1934); in these, as in 

 Avena, both curvature and straight growth appear to be 

 strictly proportional to the applied auxin up to a clearly 

 defined limit. It goes without saying that such proportion- 

 ality holds only if all other conditions are constant, 

 especially the physiological age of the reacting cells. The 

 phenomenon of aging will be further discussed below. 



A. Auxin and the Growth of Coleoptiles 

 In itself the direct proportionality between auxin applied 

 and growth resulting is not rigid proof that auxin is indis- 

 pensable for growth. The proof is only complete if it can 

 be shown, firstly, that when the plant is freed from its 

 auxin its growth ceases, and, secondly, that on subsequent 

 application of auxin growth is resumed. Since auxin is a 

 hormone and is produced only in certain regions, which 

 may be separate from the reacting zones, we should be able 

 to free the growing zones from auxin by removal of the 



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