LAWS OF RESPONSIVE GROWTH-CURVATURES 



531 



to serve as a writer. This writing-point is, by the elasticity 

 of the feather, pressed lightly against the smoked-paper 

 surface of a vertical revolving drum (fig. 223). 



When the leaf is under no stimulation, either direct or 

 transmitted, the record is a horizontal line. But true excita- 

 tion produces a depression of the leaf, causing an up curve. 

 The erectile response, on the 

 other hand, which is due to 

 indirect stimulation, produces 

 a down curve. The records 

 given in the following figures 

 are accurate reproductions of 

 some which were taken in this 

 way (fig. 224). 



(rt) Direct stimulation. — 

 Stimulus is applied by the close 

 proximity of a V-shaped plati- 

 num wire, heated electrically. 

 Its intensity is varied by varying 

 either the distance of the stimu- 

 lating wire or the intensity of the 

 heating current. When stimula- 

 tion is now applied directly at S, 

 that is to say, near the respond- 

 ing organ, response takes place 

 by a negative turgidity-varia- 

 tion, producing a fall of the leaf 

 This is seen in the up curve 

 (fig. 224, a). 



(b) Indirect stimulation, lon- 

 gitudinal transmission. — Sti- 

 mulus of moderate intensity is 



now applied lower down on the same side of the stem, at 

 S,,. This is observed first to induce the positive turgidity- 

 variation, causing erectile response, which is due to the indirect 

 effect of stimulus, and is shown in the preliminary down 

 twitch of the curve (fig. 224, b). Later, the direct effect is trans- 



Fig. 224. Mechanical Responses 

 of Leaf of Mimosa 



(a) record of responsive fall when 

 stimulus applied near the re- 

 sponding organ (cf. fig. 221); 

 (b) response when stimulus is 

 applied on same side, but at 

 greater distance, s^,. A pre- 

 liminary erectile response is here 

 followed by the true excitatory 

 depression. This is due to the 

 indirect effect Hrst transmitted 

 being succeeded by the direct. 

 Had the stimulus applied been 

 feebler, or more distant, there 

 would have been only the first, 

 or indirect erectile effect, similar 

 to fig. 225 (cf. fig. 220). 



