TRANSMISSION IN MIMOSA 641 



speed and without appreciable loss of intensity, even when both the 

 vascular bundles are severed. The cases of Minn/Ins and Martynia, 

 described by Oliver, 343 on the other hand, do not form real exceptions to 

 the rule. Here the very rapid transference of contact-stimuli from one 

 stigmatic lobe to the other takes place, not in the vascular bundles, 

 but in the parenchyma. This so-called parenchyma, however, which 

 consists of elongated cells, is really the motor-tissue ; but every motor- 

 tissue which executes rapid movements must be capable of transmitting 

 stimuli with a corresponding velocity by means of ultra-microscopic, 

 specialised, protoplasmic structures. 



III. TRANSMISSION OF STIMULI IN MIMOSA PUBIC A. 



At the present time we only know of a single instance, in the 

 entire vegetable kingdom, in which a special tissue is set apart for the 

 transmission of stimuli. This stimulus-transmitting system, which 

 occurs in Mimosa puclica, was discovered and has been fully investigated 

 from the anatomico-physiological standpoint by the author. 341 It should 

 be pointed out at once, that the tissue in question is in no sense 

 comparable to the nervous system of an animal, but merely serves as 

 the pathway for a purely mechanical propagation of stimuli. 



If a single pinnule of a sufficiently irritable plant of Mimosa 

 pudiea is stimulated by means of a gentle blow, the responsive 

 movement takes place almost as soon in the opposite pinnule as in 

 that which was directly stimulated, while the remaining pinnules of 

 the same pinna fold up very shortly afterwards. A more intense local 

 stimulus, such as that produced by wounding a pinnule, is transmitted 

 to a greater distance. In this case, it is the rule for the main pulvinus 

 to collapse first of all, whereupon the pinnules on each of the other 

 leaflets fold up in acropetal succession. If the plant is in a highly 

 irritable state, the pulvini of the sub-petioles may also become affected, 

 in which case the stimulus is propagated through the stem to the 

 neighbouring leaves. A very violent stimulus, such as results if a 

 portion of a leaf is scalded or singed, may even be transmitted through 

 the whole shoot-system of a moderate-sized plant. 



Dutrochet long ago proved by experiment, that the transmission of 

 stimuli through the stem of Mimosa pudiea takes place neither in the 

 cortical parenchyma nor in the pith, but only in the fibro- vascular 

 system (according to Dutrochet the woody cylinder) ; the same observer 

 further suggested that the transmission might depend upon movements 

 of liquids contained in the conducting elements. As a matter of fact, 

 if a sufficiently deep incision is made in the stem, a drop of liquid 

 does exude from the wound, just before the neighbouring leaves perform 



2s 



