596 LECTURE XXXIV. 



the pulvini of Mimos» are put into a condition in which they do not respond 

 to stimuh, although they rise during the continued stimulation, and assume a 

 position of rest as if they had been left alone after the first shock. The irritability 

 for new shocks — i. e. descent under their influence — does not begin to be restored 

 until 5-10 minutes after the cessation of the shocks. 



(7) Temporary rigor due to electric influence was found by Kabsch to occur 

 in the gynostemium of Stylidium. A feeble current acted as a stimulus like 

 vibrations; a stronger one caused a loss of irritability, which returned again, 

 however, after half-an-hour. In Hedysarum gyrans, on the other hand, the leaflets 

 which had become immovable in cold-rigor (at 22° C.) were set in movement by 

 the action of induction currents. 



There are two other observations regarding stimulation which have led to 

 particular remark, and even astonishment; these are the propagation of a stimulus, 

 and the after-eff"ects of it. Careful consideration, especially based on the phenomena 

 described at the beginning of the present lecture, shows, however, that both the 

 propagation and the after-effect of a stimulus can be by no means accidental 

 peculiarities, but that both must be necessary characters of all phenomena of 

 irritability. 



The propagation of a stimulus, regarded in the first place merely as matter 

 of fact, is particularly well seen in the Mimosoe and in tendrils. If, for example, 

 any one of the small leaflets of a leaf, on a shoot of Mimosa with five or six leaves, 

 is stimulated by means of the hot focus of a burning glass, all the other leaflets of 

 the same leaf gradually fold together, and after a short time the large motile 

 organ at the base of the main petiole also becomes bent, and again after a few 

 seconds the stimulation extends to the nearest neighbouring leaf, then to the succeed- 

 ing one, and so on, till at last all the leaves of the shoot have made the movement : 

 this of course only happens if the Mimosa is in a condition of the highest irritability. 

 In the case of long tendrils the propagation of the stimulus is effected in such a way 

 that a curvature results first at that point of the tendril which is immediately in 

 contact with a solid body ; the curving action proceeds from this point, however, 

 both upwards and downwards in the tendril. The latter then forms, in the course of 

 a few minutes or hours, a wide loop which narrows more and more, and closes on 

 the thin support : then follows the free end of the tendril, which winds itself fast 

 round the support, and finally, after several hours, the stimulation makes itself 

 evident also in that part of the tendril which lies between the clasped support 

 and the base of the tendril. This portion becomes rolled up spirally like a 

 corkscrew. 



These examples illustrate particularly clearly both the propagation in space and 

 the after-effect in time of a stimulus. As another example, especially concerning the 

 after-effect, I may refer in anticipation to the heliotropic and geotropic curvatures. 

 If, for instance, an erect shoot-axis growing vigorously in length (e.g. seedling 

 stems of Buckwheat, Indian Cress, Mustard, the flowering scape of the Crown 

 Imperial, &c.) is illuminated from one side, even for only one or two hours, when 

 there is at first no very noticeable curvature, and if the plants are then left for 

 a time in profound darkness, the curvature towards the previous source of light 

 already introduced continues for some hours, and may become very considerable. 



