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 cliiwstat, 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 a po 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 b). 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 



1 Otherwise any exact experiments may be vitiated by errors due to unequal stimula- 

 tion, a common fault with makeshift clock clinostats, which suffice, however, for elemen 

 tary demonstrations. 



