CHAPTER X 



Rooti 



ns 



Before the discovery of auxins, many chemical compounds had been 

 reported to increase the rooting of cuttings, including such diverse 

 substances as permanganate (Curtis, 1918) and carbon monoxide (Zim- 

 merman et al, 1933). A much higher order of effectiveness was found 

 for auxin, however (Went, 1934), and in fact it was quickly discovered 

 that auxin exerts a primary control over root formation in general 

 (Thimann and Went, 1934). In the next year it was found that utiliz- 

 ing this property of auxins could be highly useful in the propagation 

 of many horticultural species (Cooper, 1935; Laibach and Fishnick, 

 1935; Thimann and Koepfli, 1935; Zimmerman and Wilcoxon, 1935). 

 Since that time, there have been well over 300 reports in the literature 

 describing the uses of auxins for stimulating rooting. 



It is interesting to note that Dutch gardeners have a centuries-old 

 practice of embedding grain seeds into cuttings to promote root for- 

 mation, and since we now know that germinating grains produce 

 large amounts of auxins, it appears that this age-old custom has a 

 sound basis in auxin physiology. 



THE PHYSIOLOGICAL BASIS FOR ROOTING 



An understanding of the physiological mechanisms responsible 

 for root formation has gradually emerged during the last decade. The 

 initial step in the formation of roots is the differentiation of a meri- 

 stem into root primordia. The researches of Skoog (1944, 1948) have 

 demonstrated that the type of differentiation that will occur in a 

 meristem is dependent upon the proportion of auxins to certain other 

 substances. He demonstrated, as has been described in chapter V, that 

 when the ratio of auxin to some plant constituents (particularly 

 purines such as adenine) is low, the meristem of tobacco stem sections 

 will tend to form bud and leaf primordia. When the ratio is inter- 

 mediate, simple callus will be formed. When the ratio is high (that 

 is, when the auxin content is relatively high), root primordia will be 

 evolved. The balance between auxins and other plant constituents as 

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