GROWTH, IRRITABILITY AND MOVEMENT 155 



These geotropic curvatures appear in the zones of cell-elongation 

 (p. 143). They are due to the setting up of different rates of growth 

 on the upper and lower sides of the horizontal or oblique organ. 

 In a horizontally-placed stem growth becomes greater on the under 

 side of the organ than on the upper, while the opposite obtains in 

 the root. But it is only the apical part of the root or shoot that is 

 restored to its normal orientation : generally speaking the older 

 and basal part is incapable of longitudinal growth, and shows no 

 change of orientation. Geotropic 

 curvatures may also occur in plagio- 

 geotropic organs when they are dis- 

 placed. 



The length of time required for the 

 development of a visible geotropic 

 curvature in a horizontally-placed 

 root or shoot (known as the reaction Seedling of Be ^\ 94 after having been 



timp\ varies PTPaflv according to the germinated, and the radicle had grown 



n?ne) varies greauy accurumg lu uic downwards and p i umu ie upwards, its posi- 



material and the conditions. In some ^Tn^SL" ^ SZZSfSi 



cases a reaction time as low as fifteen X^^^^ST^i^X 



minutes is obtained, though values ft 8 latter negatively geotropic. (Dr. j. m. 



' ° Thompson.) 



of one hour or longer are more usual. 



The organ need not, however, be in the position of stimulation for 

 the whole of this period, but only for a part of it ; the necessary 

 time for effective stimulation being known as the presentation time. 

 It is possible to prevent the usual geotropic curvatures of roots 

 or shoots fixed in a horizontal position, by arranging for them to 

 be slowly but continuously turned about their long axes. For this 

 purpose the rotating Klinostat is used, and finds frequent employ- 

 ment in experiments on geotropism. In a root or shoot that is thus 

 revolved round a horizontal axis, that region of the organ which at 

 one moment is lowermost will in a short time be uppermost. Any 

 tendency to react to gravity which developed in the first position 

 would be cancelled by an equal but opposite tendency in the second, 

 and so on all round the organ, which therefore continues to grow 

 horizontally unless some other factor intervenes. A forerunner of 

 the Klinostat was the device known as Knight's wheel, used in experi- 

 ments carried out at the beginning of last century. Knight fastened 

 seedlings to the rim of a wheel which was rapidly rotated in a vertical 

 plane, a considerable centrifugal force being in this way set up. 

 Gravity was eliminated for the same reason as in the Klinostat : 

 but it was found that the direction of growth of the organs was now 



