GROWTH SUBSTANCES FOR GEOTROPISM 



185 



From what has been said it is clear that the first step in the 

 geotropic response in the Avena coleoptile is the displacement of 

 growth substance. The manner of its transport and the changes 

 in rate of growth in the basal region remain to be discussed. 



Comparison of Phototropic and Geotropic Curvatures. — As 

 soon as an unequal distribution of growth substance has taken 

 place in the tip of the Avena coleoptile as a result of the action of 

 gravity, a geotropic curvature in the basal region must of neces- 



_ + + jt 



+ + 



+ 



+ 



A B 



Fig. 57.— Diagram showing distribution of ions in geotropism and electro- 

 tropism. A, Avena coleoptile. B, root of Vicia. When the plant parts are 

 placed in an electric field, in a moist chamber, an internal polarity results by 

 induction. The coleoptile bends toward the positive pole, while the root bends 

 toward the negative. It is assumed that growth hormone accumulates in the 

 tissue regions which are electropositive, and there promotes growth (in the 

 coleoptDe) or inhibits it (in the root). Geotropic response of these organs has 

 been explained on the basis of electrical potentials induced by gravity, as shown 

 at the top of the figure. (From Brauner and Banning, 1930.) 



sity result (because of the longitudinal transport of growth sub- 

 stance which follows). From the time displacement of growth 

 substance occurs in phototropism and geotropism, the two types 

 of curvature are essentially the same. There is one difference 

 between the two which might be cited as an objection to the 

 growth-substance explanation, and it may be worth while to 

 examine it more closely (see du Buy and Nuernbergk, 1930). A 

 comparison of the curves (Figs. 46 and 54) showing the course of 

 phototropic and geotropic curvature in the Avena coleoptile 

 indicates that in both cases the curvatures begin in the tip and 

 proceed toward the base; the geotropic curvature sets in much 

 earUer than does the phototropic. Two different conditions may 



