GEOTROPISM 66i 



and grow toward the direction of the force {i.e. outward) while the stems 

 grow away from the direction of the force {i.e. inward toward the hub of 

 the wheel). If the rotation of the wheel is stopped or slowed down to a 

 point where the centrifugal force is less than the pull of gravity, the usual 

 geotropic curvatures of both root and stem soon appear. 



Decapitated primary roots usually fail to exhibit geotropic curvatures 

 when placed in a horizontal position even though the amputation of the root 

 tip does not prevent the enlargement of cells below the tip. If tips from 

 vertically oriented roots are placed upon such decapitated horizontal roots 

 positive geotropic curvatures appear. Such experiments indicate that the 

 tip of the root exerts a predominating influence upon its geotropic movements, 

 a relation comparable to that of the coleoptile tip in phototropic reactions of 

 the coleoptile. 



In stems, however, geotropic reactions are not prevented by amputation 

 of the stem tip. In general the stem tip is more "sensitive" to the force of 

 gravity than zones at some distance below the tip but this "sensitivity" to 

 gravity is usually present throughout the growing region. In many grasses 

 geotropic reactions occur in mature nodes independently of the stem tip. 



The quantitative measurements of auxins made possible by the oat coleop- 

 tile technique have been utilized successfully in studying the role of auxins 

 in geotropic curvatures. Coleoptiles which are slowly rotated around their 

 own vertical axis while in a horizontal position do not differ from vertically 

 oriented coleoptiles in their rate of growth. Tips of the horizontally placed 

 coleoptiles produce the same amount of auxin as vertically oriented tips. 

 The negative (upward) curvatures of the coleoptiles induced by gravity are 

 not the result, therefore, of any total increase in the amount of hormones 

 produced on the lower side of the coleoptile tip. When the amount of auxin 

 diffusing out of the upper and lower halves of horizontally placed coleoptile 

 tips is determined by the agar block method it is found that more than half 

 of the auxin diffuses out of the lower half of the tip (Navez and Robinson, 

 1932). Apparently, gravity, like light, influences the distribution of the 

 auxin in the coleoptile and the upward curvatures obtained as a consequence 

 of the force of gravity are due to the greater concentrations of the hormone 

 on the lower side of a horizontally placed coleoptile. Unlike the effect of 

 light, however, the influence of gravity upon the distribution of the auxin 

 within the coleoptile does not persist for very long after the organ has been 

 returned to its original position. The effect of gravity upon curvature is 

 lost within forty minutes after the removal of the "stimulus" but the effect 

 of light upon the distribution of auxin in the cells of the coleoptile may per- 

 sist as long as six hours after removal of the light. 



