542 PLANT RESPONSE 



of the tip, proves that, as pointed out by Darwin, the 

 tip is, with regard to gravity, the perceptive organ of 

 the root. 



The tip of the root the organ of gravi-perception. — The 

 inference that it is the root-tip which is the perceptive organ 

 has also been arrived at by Pfefifer and Czapek, pursuing 

 an independent method of investigation which discarded 

 amputation. 



There is again the very suggestive fact that the starch 

 grains whose weight, according to the statolithic theory, 

 brings about stimulation, occur generally at the root-tips 

 alone. Thus three different lines of research lead to the 

 conclusion that it is in that region that the gravi-perceptive 

 power resides, the power of response by curvature being 

 behind, in the region of growth. 



The perceptive versus the responding organ. — In view 

 of much existing vagueness on this subject, it is desirable 

 here to obtain clear and well-defined conceptions as to how 

 far the differences commonly insisted on between perceptive 

 and responding organs are justified. To take the familiar 

 instance of Mimosa, we know that when stimulus is applied, 

 for example, on the stem, there is no perceptible motile effect 

 in the stem itself, but excitation is markedly manifested at 

 the pulvinus. From this it has been erroneously supposed 

 that the stem merely perceived, and did not respond to, 

 stimulus. It has, however, been shown in previous chapters 

 that every part of the plant-tissue responds to stimulus by 

 contraction, the visible and striking manifestation seen at the 

 pulvinus being simply the result of certain accessory ana- 

 tomical facilities which happen to exist at that point. 



Thus, every part of the plant-tissue is both perceptive 

 and responsive to external stimulus, to a greater or less 

 extent. When we come to the growing organ, responsive 

 curvature is induced by the concavity of the excited side. 

 And as the zone of growth is, generally speaking, 

 the region of the greatest excitability, the greatest cur- 

 vature usually takes place there, that region functioning 



