Natural auxins 



required for a geotropic response to become manifest. In Artemisia roots, the 

 geotropic reaction time is less than 8 minutes. Thus in order to test the 

 auxin theory of geotropism in roots we need information on the response of 

 the roots within, say, less than half an hour after the application of auxin. 

 There are but few studies on the effect of auxin on the elongation of roots 

 within such a short time, and all of these show retardation rather than 

 acceleration of growth. Some observations showed that a growth accelera- 

 tion, if found, was preceded by a retardation of elongation (e.g. Thimann 

 and Lane, 1938), but there are several cases in which retardation only, with 

 no acceleration of root growth, was found. Thus, for example, Seller (1951) 

 measured the elongation of maize roots at 20-minute intervals and found that 

 the application of lAA for 30 seconds resulted in a transitory retardation of 

 elongation, no acceleration of growth being observed up to 4 hours after 

 treatment with lAA. Lundegardh (1949) was able to measure root elonga- 

 tion at 4-minute intervals, and although he had previously reported accelera- 

 tion of the elongation of wheat roots 6 days after the addition of auxin 

 (Lundegardh, 1942), he found only retardation when the response was 

 recorded after a few hours (1942) or after less than one hour (1949). Similarly 

 Ashby (1951) showed that auxin increased the growth rate of Artemisia 

 roots when measured 24 hours after the application of auxin, but found no 

 effect, or even a slight retardation, after 4 hours (Table 2). 



Table 2 

 The tirtie factor in auxin-induced stimulation of root elongation 



t After application of auxin. 



According to Burstrom (1942, 1950, 1954), an auxin-induced acceleration 

 of elongation in individual root cells, when such acceleration occurs, is 

 caused by a stimulation of the first phase of cell stretching (characterized by 

 a dissolution of the cell-wall material). The second phase of cell elongation 

 (characterized by intussusception of new cell-wall material) can only be 

 retarded by auxin. Thus, an over-all acceleration of elongation can be brought 

 about by the addition of auxin only if its concentration is so low that the 

 acceleration of the first phase of growth is not completely masked by a 

 retardation of the second phase. However, since the rate of cell elongation 

 during the first phase is low, it is reasonable to assume that the immediate 



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