GROWTH SUBSTANCES FOR GEOTROPISM 



181 



Table 14. — Comparison of the Amount of Growth Substance Recov- 

 ered Separately in Two Agar Blocks from the Upper and Lower 

 Halves op an Avena-coleoptile Cylinder Which Had Been 

 Supplied Artificially with the Hormone at the Distal 



End 

 See Fig. 2 (Dolk) 



the transverse transport of growth substance ordinarily induced 

 by the unilateral action of gravity. He concluded that trans- 

 verse transport is brought about by the increased resistance to 

 longitudinal transport on the upper side when the Avena coleop- 

 tile is placed in a horizontal position (see also du Buy, 1933). On 

 the other hand, Dolk's work indicates that the unilateral effect of 

 gravity brings about displacement of growth substance in the 

 geotropically stimulated tip. 



With recognition of the displacement of growth substance 

 in the coleoptile tip as a link in the geotropic stimulus chain, it 

 becomes necessary to examine the separate steps in the process, 

 i.e., how gravity can produce this displacement and how the 

 negatively geotropic curvature arises in the Avena coleoptile as a 

 result of the unequal distribution of growth substance. 



The Statolith Theory. — Contemporaneously with certain 

 theoretical observations by Noll (1900), two investigators, Nemec 

 and Haberlandt, suggested that movable grains of starch function 

 as statoliths. If the organ is in a position of geotropic equilib- 

 rium, the pressure of the starch grains cannot produce a stimulus. 

 If the organ is taken out of its position of equilibrium, the starch 

 grains accumulate in layers on one side of the cell and exert a 

 pressure upon the plasma membrane of the tangential (and 

 perhaps also radial) walls. This pressure is supposed to produce 



