594 PLANT RESPONSE 



pulvinated and growing organs. We have also seen that in 

 both, on the cessation of stimulus, there is a tendency towards 

 recovery. In the case of the growing organ, when stimulus 

 is moderate, recovery is fairly complete ; but when the 

 stimulus is very strong and long-continued, some part of the 

 induced curvature is rendered permanent through fixation by 

 growth. 



An attempt has been made in the case of Vochting's 

 Theory of Recti-petality to account for this recovery, by 

 assuming the action of an unknown regulating power which 

 would tend always to bring the organ back to a straight line ; 

 but, beyond the assumption of an unknown specific power, 

 this theory affords no explanation of the mechanism by 

 which recovery is brought about, and I am able to adduce 

 considerations which obviate the necessity for thus assuming 

 the existence of any such specific agency as that of recti- 

 petality. 



In a growing organ which is radial, the tip of the growing 

 region being free, the vertical direction is that in which there 

 is least obstruction to growth, and as long as all the lateral 

 tensions are the same in all directions, there is no reason why 

 the organ in the course of its upward growth should bend 

 permanently on any one side more than on another. We 

 have therefore the normal growth of radial organs in a 

 straight line ; but when stimulus acts unilaterally on the 

 growing region, a sequence of events ensues, which has already 

 been fully explained : 



(i) Active contraction is induced during the continuance 

 of stimulus, on the proximal or excited side, with concomitant 

 diminished turgidity, and retardation of growth. 



(2) The water thus expelled is forced, against tension, 

 into the growing cells of the distal side, raising their 

 turgescence and consequent rate of growth above par. 



(3) The curvature thus induced is maintained as long as 

 the difference of hydrostatic pressure on the two sides is 

 continued, by the persistent contraction of the proximal, 

 under the action of stimulus. 



