PHOTOTROPISM 



659 



low range of light intensities the degree of their curvature is proportional 

 to the quantity of illumination. With further increase in the quantity of 

 light, however, this relationship changes and the degree of phototropic cur- 

 vature decreases until negative curvatures are evoked. Still greater quantities 

 of light induce a second series of positive curvatures. On increasing the 

 quantity of light still more the coleoptiles react with decreasing curvatures 

 until a point is reached at which the light induces no visible end-reaction. 

 Further increases in light quantity beyond this point evoke a third set of 

 positive curvatures. 



A correlation exists between these variations in the magnitude of photo- 

 tropic curvatures and the quantity of auxin diffusing from the illuminated 

 and shaded sides of coleoptile tips (Table 60). 



TABLE 60 THE RELATION BETWEEN THE AUXIN CONTENT OF COLEOPTILE TIPS SUBJECTED TO 



UNILATERAL ILLUMINATION OF DIFFERENT INTENSITIES AND THE DEGREES OF PHOTO- 

 TROPIC CURVATURES (fROM DATA COMPILED BV WENT AND THIMANN, 1 937) 



Auxin concentration is not the only factor involved in phototropic move- 

 ments. The quantity of growth hormone can only control the extent of cell 

 elongation when it acts as a limiting factor. We have seen (Chap. XXXII) 

 that auxins are produced in the coleoptile tip and move basipetally into other 

 parts of the structure. It is not probable, therefore, that curvatures of the 

 upper portions of the coleoptile are due to differences in auxin content for 

 the hormones are present in excess in these regions. Photographic measure- 

 ments have shown that in the upper zones of the coleoptile, curvatures are 

 not produced by increases in the rate of elongation of cells on the shaded 

 side but by a decrease in the amount of cell elongation on the illuminated 

 side. Likewise uniformly illuminated coleoptiles grow more slowly than 

 coleoptiles kept in the dark. Numerous other experiments also indicate that 



