92 MOVEMENTS OF CURVATURE 



ones, and no response is aroused when the corolla is cut through above or 

 below the insertion of the stamens I . Stimulation of one stigma-lobe of 

 Martynia Intca, M. proboscidea, and Mimulus cardinalis causes the other to 

 move, but not in the case of the stigmas of Mimulus lutcus 2 . An excita- 

 tion is propagated with extreme rapidity from one leaf-lobe of Dionaca to 

 the other, while in the case of the fairly sensitive stamens of Sparmannia 

 africana 3 irritation of one stamen spreads to a limited extent to the 

 neighbouring ones. In the case of Phycomyces^ on the other hand, no 

 conduction of stimuli appears to take place, whereas a contact-stimulus is 

 rapidly propagated to the outer side of a tendril and also to some extent 

 longitudinally. The leaf of Drosera again affords a specially good instance 

 of the conduction of stimuli, for as the result of stimulating the head of 

 a single tentacle all the tentacle-stalks on the leaf may be caused to curve 

 inwards. Here the receptive and motory zones are distinct, and, according 

 to Oliver, the same is the case in the labellum of Masdcvallia mitscosa, 

 which appears to possess seismonic irritability. In all the other cases that 

 have been investigated the motory zone seems also to be capable of 

 perception, for one can hardly ascribe a vital power of perceiving stimuli 

 to the leaf-laminas or stems of Mimosa simply because the movement of 

 water produced when they are cut, crushed, or burned acts as a stimulus 

 to the motile pulvini 4 . 



The above instances suffice to show that a high sensitivity to contact 

 or seismonic stimuli does not necessarily involve a pronounced power of 

 conducting stimuli, and that the transference of the stimuli may either be 

 vital or purely mechanical. The latter is the case in all organs which 

 respond to seismonic stimuli, for the collapse of one stimulated cell excites 

 the next, this the next, and so on. In the case of Mimosa pudica the 

 stimulus is propagated by means of a movement of water or hydrostatic 

 pulsation which is able to travel through dead portions of the stem and 

 leaf, and which excites the pulvini on which it impinges. Since this can 

 only occur when a proper connexion is maintained between the conducting 

 channels and the responding organ, it is not surprising that the stamens of 

 Berberis and of Cynareae cannot be excited in this way. 



1 Pfeffer, Jahrb. f. wiss. Bot., 1873-4, Bd. ix, p. 317. 



2 Oliver, Ber. d. bot. Ges., 1887, p. 167 ; Hansgirg, Physiol. u. Phycophytol. Unters., 1893, 



P-47- 



3 Morren, Rech. s. 1. mouvement d'etamines du Sparmannia, 1841, p. 23 (reprint from Mem. 

 de 1'Acad. de Bmxelles, T. XIV). 



4 The movements observed by Darwin (The Power of Movement in Plants, 1880, p. 127) when 

 the laminas of the cotyledons of Oxalis sensitiva, and of a few species of Cassia, were strongly rubbed 

 were probably the result of the ensuing movements of water stimulating the motile pulvini. Goebel 

 (Pflanzenbiol. Schilderungen, 1893, Bd. II, p. 201, footnote) observed incidentally that in the case of 

 a feebly irritable leaf of Dionaea one leaf-lobe could be excited by stimulation of the other leaf-lobe, 

 but not directly. 



