MOVEMENTS RESULTING FROM SHOCK 



519 



unable to observe any pressure worthy of the name in the intercellular spaces, 

 when these were injected during stimulation. Bonnier (1892) even established 

 a slight decrease in pressure after the stimulation by inserting a manometer 

 into the pulvinus, arranged so as to show pressure of the air in the inter- 

 cellular spaces. It is not easy to understand how, under such circumstances, 

 the extruded liquid can enter the vessels under pressure. Haberlandt's 

 views, on the other hand, are quite intelligible. On direct stimulation of 

 an articulation, flaccidity ensues in the sensitive parenchyma and, owing to 

 the deformation of the cells, a pressure will be induced on the conductive tissues 

 which is propagated along them and which, wherever it reaches a new pulvinus, 

 is capable of stimulating it just as if a blow had been inflicted on it from without. 

 In that case we should have to consider this really as a genuine instance of 

 transmission of a stimulus and not of an excitation as in other cases. 



Other parts of the plant as well as the pulvini may be affected by a 

 primary stimulus ; for example, a pinna may be stimulated without touching 

 its articulation (Bert, 1867), and yet it, too, is 

 able to appreciate the stimulus. In this case, also, 

 the cells conducting the stimulus must primarily 

 become deformed, and an increase of pressure 

 effected which is transmitted. Stimulus conduct- 

 ing cells are, as a matter of fact, also found in 

 the pinnae, following the course of the larger 

 vascular bundles. By employing the method of 

 stimulation by wounding, Bert(i867, 17) showed 

 that the leaf parenchyma was insensitive and that 

 response took place only when the stimulus 

 affected the veins. In the case of all stimuli, 

 whether of the nature of an incision, scorch- 

 ing, or corrosion by acid, there was always a 

 decrease in pressure in the conductive tissues, 

 never an increase, but which was propagated 

 just like increased pressure, and which led to 

 mechanical stimulation in the pulvini. It would 

 appear that increase and decrease of pressure 

 might take place more readily in Haberlandt's 

 thin-walled but turgescent tubular cells than 

 in the rigid-walled vessels. [Fitting has (1903) 

 carried out some new experiments on the trans- 

 mission of stimuli in Mimosa, but this author 

 has not been successful in solving the question 

 in dispute.] 



As far as regards sensitivity and the 

 mechanics of movement in response to stimulus, 



a perfect comparison may be instituted between those phenomena as exhibited 

 by Mimosa and those manifested by certain stamens, although the move- 

 ments in the latter have an entirely different biological significance. Let 

 us study the stamens of the Cynareae, more especially those of Centaurea 

 (Fig. 160) which have been minutely investigated by Pfeffer. The five 

 syngenesious anthers form a tube round the style ; from these arise live free 

 filaments, bent slightly outwards, the basal ends being inserted on the corolla 

 lower down. When the filaments are touched [A ) they contract and at the same 

 time straighten themselves {B) ; in this way the anther tube is pulled down- 

 wards and the pollen is thus swept out by the hairs on the style ; the movement 

 is thus obviously an adaptation to pollination by insects. The filaments react 

 only directly to the contact ; there is no transmission of the stimulus. The 

 experiment may also be performed on a single isolated filament, and it may be 



160 



Stamens of Centaurea 



jacea^ after removal of corolla. A, before, 

 B, after stimulation. <-, base of the 

 corolla ; ,?, filaments ; a, anthers ; g^ style ; 

 P, pollen. Enlarged. From tfie Bonn 

 Textbook. 



