CURVATURES PRODUCED BY, CHEMICAL STIMULI 91 



for Darwin found that stimuli travelled more rapidly parallel to the long axes of the 

 parenchyma cells than transversely to them. It is for these reasons that stimuli 

 radiate mainly centripetally from the marginal tentacles, and centrifugally from those 

 near the centre, but are only propagated slowly and feebly tangentially. 



It appears probable that the conduction of stimuli, at least in Drosera, involves 

 a transference of stimulatory materials, either by the diffusion of an absorbed 

 substance or as the result of the formation of stimulatory materials in the secretory 

 cells which diffuse to neighbouring ones and excite aggregation in them. In this 

 case the transference of the stimulus would be merely a matter of translocation, aided 

 possibly by the fact that the secondarily excited cells themselves begin to produce 

 stimulatory materials. The transference of these might take place if they are 

 diffusible, without the aid of any interprotoplasmic connexions, and, in fact, aggrega- 

 tion and granulation may be produced in the cells of the tentacle of Drosera by the 

 direct application of ammonium carbonate. 



Comparative investigations on other plants will, without doubt, aid in the 

 elucidation of these problems, but so far it is only known that the effects of 

 mechanical stimuli are propagated through the parenchyma of the leaf of Dionaea, 

 and more rapidly along the vascular bundles *. No aggregation is produced by 

 mechanical stimuli in Dionaea, but this change and the chemonastic excitation due 

 to absorbed proteids appear to follow the same path but to travel more slowly. The 

 rapid transference of mechanical stimuli in the leaf of Aldrovanda must, however, 

 take place through the parenchyma of the leaf, since in the leaf-lobes no vascular 

 bundles are present. 



SECTION 20. The Propagation of Mechanical and Chemical Stimuli. 



The influence of mechanical and chemical stimuli is often restricted to 

 the region immediately surrounding the point of application, or to the 

 pulvinus when this is the only irritable portion. On the other hand, 

 Mimosa pudica affords a well-known and striking instance of the trans- 

 mission of stimuli, for under favourable conditions burning or cutting off 

 the terminal leaflets of one of the segments of the leaf may cause all 

 the leaves and leaflets to be stimulated in succession. The stimulus 

 is conducted somewhat less readily in Biophytum sensitivum 2 , while in 

 the trifoliate leaves of Oxalis acetosella the reaction is restricted to the 

 leaflet directly stimulated 3 . 



In the case of the highly irritable stamens of Berberis and Centaurea 

 the stimulus is not transmitted from an excited stamen to neighbouring 



1 Darwin, 1. c., p. 313 ; Batalin, 1. c., p. 147. 



8 G. Haberlandt, Ann. du Jard. bot. de Buitenzorg, 1898, Suppl. II, p. 33; Sinnesorgane im 

 Pflanzenreich, 1901, p. 88. On Oxalis dendroides cf. Macfarlane, Biological Lectures, 1894, p. 194. 



3 Cohn, Verhdlg. d. schles. Ges. f. vaterl. Cultur, 1859, p. 56; Pfeffer, Physiol. Unters, 1873, 

 p. 74. 



