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 is 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 

 I g ' was 90-100 min., when the centri-acceleration was equal to 35-38 g, the reaction 

 time was scarcely more than hal ved (45 min.) ; and when it was reduced to 0.00 1 g, the 

 reaction time was barely quadrupled (6 hr.). In some late experiments, however, 

 a root of Vicia, which reacted in 8 min. at i g, reacted in 0.25 min. with 27^. 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 layer 

 of cells around the vascular cylinder. 



' I ^ = the normal acceleration due to gravity. 



