THE TRANSFERENCE OF STIMULI 185 



potential energy by the conversion of some labile condition of equilibrium into a 

 stable one. 



A stimulus which acts in an orienting manner must do so owing to its mode 

 of application and the properties of the organ affected. It is, however, impossible 

 to say how a geotropic excitation or one due to wounding is able to produce 

 a definite curvature in the responding zone, or how correlative stimuli can produce 

 different results at the same point according to their character. 



SECTION 53. The Transference of Stimuli (continued}. 



The general importance of irritability and irritable responses has 

 hitherto been hardly sufficiently recognized, largely owing to the fact that 

 undue attention has been paid to the more obvious movements resulting 

 from special stimuli, while the complex and interacting stimuli involved in 

 correlation and co-ordination have been for the most part overlooked 1 . 

 Irritable movements usually take place close to the point of application of 

 the stimulus, but in many cases the percipient and responding organs may 

 be some distance apart. 



In thesensitive plant (Mimosa pudica), for example, a cutmade in the stem, 

 or direct stimulation of the main pulvinus of the compound leaf, produces 

 a change of hydrostatic pressure which is propagated to some distance and 

 stimulates successive pulvini. Stimuli are propagated in a similar manner 

 along the staminal filaments of Cynareae and Berberideae by the tensions 

 exerted by the contracting stimulated cells affecting neighbouring ones, 

 these the next, and so on. In this case, however, the resulting movements 

 of water are insufficient to propagate the stimulus to neighbouring stamens. 



Still more important are the stimuli transmitted by vital means from 

 a stimulated root-apex, or from the heliotropically sensitive apex of the 

 first seedling leaf of certain grasses, to the curving zones. Similarly it is by 

 a physiological transmission that stimuli pass from an irritated head of a 

 Drosera tentacle to the stalk, where a curvature is produced, and to the other 

 glands, whose secretory activity is excited. 



Owing to the variable duration of the latent period of response, it is 

 frequently difficult to exactly determine the velocity of transmission of 

 a stimulus. Transmission by vital means seems, however, to take place but 

 slowly in plants, for geotropic and heliotropic stimuli appear to travel at 

 a rate of not more than one or two millimetres in five minutes in passing from 

 a sensitive apex to the responding region. Ewart has, however, observed 

 that a stimulus exciting streaming had an actual rate of propagation of 

 i -a to 0-5 mm. per min. at 18 C., and of 2 to 0-8 mm. per min. at 30 C. 

 through the long axes of leaf-cells of Vallisneria, whereas within the long 



1 Cf. Pfeffer, Die Reizbarkeit der Pflanzen, 1893 (repr. from Verb. d. Ges. deutsch. Naturf. u. 

 Aerzte). 



