GROWTH AND MOVEMENT 463 
of its rotation and c into A, any residual excitation from the former posi- 
tion is balanced by excitation that would lead to a contrary reaction. 
All the while, therefore, the plant is under excitation, which is the greater 
the more opportunity there is for summation; and if the responses were 
not contrary the one to the other, curvature would show itself. The 
net result upon the rotated plant is that growth is at first accelerated as 
compared with a control plant rotated in the vertical plane; but long- 
continued rotation leads to fatigue and no response. 
Position of equilibrium. In order that a parallelotropic axis be in 
a position of stable equilibrium with respect to gravity, it must not only 
be parallel to its direction, but the stem must be erect and the root pointed 
downward. There i? a polarity which must be conserved. Though 
the strictly inverted position for either roots or stems is one of little 
stimulation or possibly of none, it is a position of such instability that 
the slightest deviation leads to stimulation, which results in decided cur- 
vatures and recovery of the normal position. Much study has been 
given also to the position of maximum stimulation. The general results 
are most strongly in favor of a 90 deviation from the normal, as against 
135 or any intermediate angle. 
Variable intensity. By comparing centrifugal acceleration with 
that due to gravity, it has been shown that it produces the same curva- 
tures. So while it is not possible to alter appreciably the intensity of 
gravity, it is possible to vary this corresponding stimulus. Experiments 
in this line show that as the centrifugal acceleration is increased or di- 
minished, the reaction time is shortened or lengthened, but whether pro- 
portionately or not is uncertain. 
Thus, in earlier experiments with a root of Vicia, whose usual reaction time at 
l g 1 was 90-100 min., when the centri-acceleration was equal to 35-38 g, the reaction 
time was scarcely more than halved (45 min.); and when it was reduced to o.ooi g, the 
reaction time was barely quadrupled (6 hr.). In some late experiments, however, 
a root of Vicia, which reacted in 8 min. at tg, reacted in 0.25 min. with 27 g. Here 
the ratio is 32 : 27, a change in reaction time nearly proportionate to the change in 
Stimulus. 
Perceptive region. It is extremely difficult to locate beyond question 
the exact region where the geotropic stimulus is perceived. In stems 
perception does not seem to be localized. If the statolith theory of geo- 
perception be true, it takes place probably in the starch sheath, a layei 
of cells around the vascular cylinder. 
1 i % the normal acceleration due to gravity. 
