HAPTOTROPISM 



493 



Convex side 



layers behave passively in relation to the peripheral layers where vigorous 

 growth takes place. Unfortunately space will not permit of our discussing the 

 question at greater length (FITTING, 1903 a, p. 615). 



Some time after the completion of the curving a cessation of growth takes 

 place and thereafter begins, as we have already seen, a straightening of the tendril. 

 Measurements show that the hitherto convex side remains quite unaltered while 

 the movement of the indices on the concave side indicate that growth is taking 

 place, less vigorously it is true than on the convex side during the period of 

 curvature, but still so as to show that a marked acceleration in growth is occur- 

 ring as contrasted with that exhibited by the unstimulated tendril. This 

 acceleration can be traced beyond the middle line. A graphic representation 

 of these phenomena is given at Fig. 154. 



The recorded facts show in the clearest manner that the whole movement 

 following on stimulation is exceedingly complicated, since the curvature is not 

 effected by a simple contraction of the stimulated side but by an acceleration of 

 growth which is most marked at the spot on the convex side directly opposite 

 to that on the concave side where the stimulus has been applied. Apparently 

 therefore perception is followed by a transmission of the stimulus. Perception, 

 transmission and re- 

 action follow each 

 other in this case with 

 a rapidity which is 

 very much greater 

 than we have seen to 

 be the case in any of 

 the tropistic move- 

 ments previously dis- 

 cussed. 



If we now trace 

 the growth in a tendril 

 which has been stimu- 

 lated equally and 

 simultaneously on 

 both sides, we shall 

 find that we always 

 obtain the same result, 

 whether it be a tendril 

 which reacts on one 

 side only or on all sides 

 equally. The tendril 



grows on as if nothing had happened to it ; increased growth, more especially, is 

 absent altogether. We must therefore conclude that the absence of curvature 

 when both sides are stimulated is not, as might be imagined, more especially 

 in the case of tendrils which react equally all round, to be explained by assuming 

 that the two stimuli induce two similar reactions on opposite sides. On the 

 contrary one stimulus inhibits the effect of the other, and this is the case even 

 when curving has already commenced. This fact shows that stimulation of the 

 upper side does not by any means render perception on the under side im- 

 possible, but whether the inhibition affects the transmission or the reaction we 

 cannot say. 



There remains for consideration the significance of the backward curvature 

 which takes place after each incurving. This is due doubtless to internal causes, 

 for it is not a consequent of the contact stimulus, but primarily of the reaction 

 for it follows, in virtue of growth acceleration, after every curvature induced 

 by purely mechanical means. That it is a case of autotropism naturally suggests 

 itself, such as we have already become acquainted with in considering the counter 



20' 



JO' 



JO' 



Minutes 



60' 



Fig. 154. Graphic representation of the growth of a tendril of Sicyos 

 angulatus. *, beginning of the stimulation ; f, completion of the curvature ; 

 !, beginning of the readjustment. 



