GROWTH AND MOVEMENT 459 



parts, such as the branches or leaves, set themselves at a definite angle to 

 the line of the stimulus. Other plants may place even the main axes 

 at an angle to the stimulus. This difference of behavior is expressed by 

 the terms parallelotropic and plagiotropic, applied to the organ concerned. 



Because responses to tropic stimuli lead so often to the erect position of axes, 

 such axes were first called orthotropic organs, and their correlates were called 

 plagiotropic, with reference merely to position. No confusion can arise from the 

 substitution of the more specific term parallelotropic, and the use of plagiotropic in 

 a somewhat modified sense. 



(i) Geotropism 



The stimulus. No force acts so constantly and so equally in all parts 

 of the earth and in all situations as gravity. It might be expected, there- 

 fore, that it would have some influence upon the position that the parts 

 of plants assume. If there were nothing more to be observed than that 

 the main stems of so many plants in all countries are directed away 

 from the center of the earth, this would suggest the agency of some 

 general stimulus. But it is easy to observe that as soon as a plant stem 

 which usually grows erect is overthrown, curvatures occur in the younger 

 parts that again direct the apex upward, though the older parts are 

 unable to erect themselves. Fallen trees, and corn or other cereals 

 beaten down by wind and rain, offer many examples, and the simplest 

 experiments suffice to demonstrate the main facts; namely, that gravity 

 is the stimulus, and unequal growth the end reaction. 



The first demonstrative experiments were conducted at the beginning 

 of the last century, by affixing boxes to the rim of a wheel, which could 

 be rotated either in the vertical or the horizontal plane, and planting 

 seeds in these boxes. When the seedlings appeared on the vertically 

 placed wheel, they seemed to have quite lost their way, growing in any 

 direction in which they happened to be pointed when they broke through 

 the soil; and some did not even emerge. On the horizontal wheel, 

 however, no difference was apparent when it was rotated slowly; but 

 when it was turned rapidly enough to introduce a considerable centrifu- 

 gal acceleration (" centrifugal force "), the usual position of the axes 

 was changed, the stems which would normally grow erect tending to 

 direct themselves toward the center of the wheel, and the primary roots, 

 which usually grow downwards, growing toward the periphery ; and these 

 tendencies were the more pronounced the more rapid the rotation. 



This mode of experimentation is universally used when one wishes to equalize 

 or modify the action of any one-sided stimulus. In all such experiments it is essen- 



