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 parallelolropic and plagiolropic, 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 t 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, v 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 wer-2 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 sucn experiments it is essen- 
