TROPISMS 



155 



be inserted into the cavity (see Figure 47). He thus arrived 

 (1926) at a number of important conclusions. In logical 

 order these are: (1) "growth hormones play an essential 

 role in the mechanism of the geotropic reaction"; (2) ''in 

 vertically placed stems and roots the growth-regulating 

 substances are equally distributed on all sides"; (3) "as 

 soon as these organs are placed in a horizontal position, 



Fig. 47. Geotropism of hypocotyl cylinders of Lupinus angustifolius, 

 from which the central cylinder has been removed. Upper left, four bored- 

 out controls, no geotropic response; lower left, three intact controls, nor- 

 mal response. Right, six plants in the hollow of each of which a coleoptile 

 tip of Zea Mays has been placed, as in diagram; geotropic sensitivity re- 

 stored. (From Cholodny, Jahrb. vnss. Bot. 65: 447-459, 1926.) 



the normal diffusion of the growth hormones is disturbed; 

 the upper and lower cortical cells now obtain different 

 amounts of these substances." This unequal distribution 

 is ascribed to a physiological polarity induced by gravity, 

 a polarity which he had tentatively postulated in 1918; 

 (4) the opposite signs of the reactions of roots and shoots 

 fit in with the fact that they react in opposite ways to the 

 growth hormones coming from their tips; (5) he concludes 

 that "the problem of geotropism can be traced back to the 

 much simpler problem of the chemical control of growth." 

 This passage recalls Blaauw's (1918) statement that "the 



