532 



PLANT RESPONSE 



mitted, causing the fall of the leaf, as shown in the up curve. 

 When the stimulus is feebler, or applied at a still greater 

 distance, the indirect effect alone reaches the organ, and only 



the erectile response, due to posi- 

 tive turgidity-variation, results, being 

 similar to that shown in the next 

 record (fig. 225). 



(V) Indirect stimulation, transverse 

 transmission. — We next obtain the 

 very interesting case in which feeble 

 stimulus is applied at the transverse 

 point s,. The record (fig. 225) shows 

 that we have here an erectile response 

 due to the positive turgidity-variation 

 of indirect stimulation. But if this 

 transverse stimulus be made strong 

 or be long continued, the direct effect 

 is transmitted somewhat later, and, 

 in that case, we obtain a fall of the 

 leaf, preceded by the positive erectile 

 twitch, which is similar to that shown 

 in the previous record (fig. 224, b). 



The curious response of an 

 Arisaema. — This fact will explain a 

 very remarkable phenomenon which I 

 have noticed in certain species of Arisatma, that grow on the 

 mountains round Darjeeling, at a height of about 7,000 feet. 

 This plant, before flowering, consists of a long petiole bearing 

 a terminal whorl of leaflets, which are arranged like rays in a 

 strictly horizontal plane. Later, however, the inflorescence, 

 borne on a peduncle enclosed within its spathe, breaks out 

 from one side of this petiole. Unilateral mechanical stimu- 

 lation is thus undoubtedly brought about, and gives rise to 

 indirect stimulation on the distal side, which, as we have just 

 seen, causes an erectile mechanical response. In the case of 

 this Arisicma, it is a striking fact that immediately after flower- 

 ing, the most distal leaflet of the whorl — that is to say, the 



Fig. 225. Erectile Re- 

 sponse of Leaf of Mimosa 



due to Transmission of 

 Indirect Effect to Distal 

 Side, when Proximal, S„ 

 is Stimulated 



If stimulus were stronger, 

 this would be followed 

 by the fall of the leaf 

 due to the later trans- 

 mission of true excitation. 

 The response would then 

 become like that of fig. 

 224 (b) (cf. fig. 219). 



