364 LIFE MOVEMENTS IN PLANTS 



accentuate the urgent necessity o£ revision of our existing 

 terminology. 



I have shown that the effects of other forms of sti- 

 muli are also transmitted from the perceptive to the res- 

 ponding region along the intervening path of conduction. 

 Thus the petiole of Mimosa perceive any form of stimu- 

 lus applied to it, and the induced excitation is conducted 

 to the distant pulvinus to evoke the familiar respon- 

 sive fall of the leaf. The pulvinus, moreover, perceives and 

 responds to direct stimulation. In a nerve-and-muscle 

 preparation the responding muscle is alike perceptive and 

 responsive. 



But in Setaria we meet with certain characteristics of 

 reaction which are quite inexplicable. Thus if 



"the seedling be illuminated on one side, a sharp 

 heliotropic curving takes place at the apex of hypocotyl. 

 The curvature makes itself apparent only if the cotyledon 

 be illuminated from one side whether the hypocotyl be 

 exposed to light or not. If the cotyledon be shaded and 

 the light be permicted to fall on one side of the hypo- 

 cotyl, no heliotropic curving takes place. Hence we may 

 conclude that it is only the cotyledon that is sensitive to 

 the light stimulus, and it is only the hypocotyl which can 

 carry out the movement. The excitation which the light 

 effects in the cotyledon must be transmitted to the hypo- 

 cotyl and curvature takes place only from such a trans- 

 mitted excitation. We have thus in this case a definite 

 organ for the perception of the stimulus of light, viz., 

 the cotyledon, and as Rothert has shown, it is more 

 specially the apex of that organ that is the sensitive 

 part ; on the other hand, the motile organ, the hypocotyl, 

 is some distance away from the sensitive organ, and in it 

 the power of perception is entirely absent. From the behavi- 

 our of these organs we may draw the further conclusion that 



