460 PHYSIOLOGY 
tial to arrange the plants so that the only factor in their environment that is altered 
is the direction from which the stimulus acts. The clumsy wheel has been replaced 
by the modern clinostat, a disk to which potted plants can be conveniently attached 
and capable of rotation in any plane, continuously or intermittently, at a very even 
speed * by strong clockwork or by a water or electric motor. The centrifuge is a modi- 
fication whose disk is driven at a high speed when centrifugal acceleration is to 
be compared with gravitational. 
Parallelotropic organs. The behavior of parallelotropic and plagio- 
tropic organs differs in certain particulars. The former will first be 
considered. Parallelotropic stems in responding to gravity curve so 
as to erect their apices when displaced. Primary roots, which are usu- 
ally directed straight downwards, when displaced respond by turning the 
tip toward the earth. These responses, in quite opposite directions, arise 
from an identical original stimulus. By some mechanism within the 
plant body the end reaction is made different. It is convenient to dis- 
tinguish the difference by assuming some difference in the sensitiveness. 
So the special term positive geotropism or progeotropism is used to desig- 
nate the property by which the growing point is directed toward the 
center of the earth, and negative geotropism or apo geotropism that by 
which the tip is turned away from it. The curvature might be due (a) 
to unequal retardation of growth along both sides; or (b) to unequal 
acceleration of growth along both sides; or (c} to an unchanged rate of 
growth on one side with either acceleration or retardation on the other ; 
or finally (d) to simultaneous retardation on one side and acceleration 
on the other. It has been determined that usually, in both stems and 
roots, gravity accelerates growth, but the segments are unequally affected 
according to position (case ft). In the one case (apogeotropism), the 
lower side is caused to grow more rapidly than the upper ; in the other 
(progeotropism), the upper side grows more rapidly than the lower. 
How this difference in action is brought about is quite unknown. 
Course of curvature. The course of curvature in a parallelotropic 
stem continuously stimulated by being laid horizontal shows an interest- 
ing example of " after-effects." The reaction time is usually some hours 
in length. When the apex has reached the erect posture again, it might 
be supposed that it would go no further. On the contrary, it is carried 
past the vertical, responding to the excitation set up some hours before. 
Being thus carried beyond the position of equilibrium, it is stimulated 
'Otherwise any exact experiments may be vitiated by errors due to unequal stimula- 
tion, a common fault with makeshift dock clinostats, which suffice, however, for elemen- 
tary demonstrations. 
