STIMULI AND THEIR ACTIONS 



381 



undisturbed holds its primary petioles, which spring from the trunk, 

 directed obliquely upward. The secondary petioles, which bear 

 the rows of leaflets, are spread out wide apart, and the leaflets them- 

 selves stand horizontal and widely extended (Fig. 172,^). But as 

 soon as the pot in which the plant is growiug is shaken, the picture 

 changes almost immediatel}^ The primary petioles fall down 

 as a result of the decrease of the turgor of the cells of their 

 pulvini, the secondarj^ petioles turn toward each other, while the leaf- 

 lets are raised and lie with their upper surfaces together (Fig. 172, B). 

 The plant, when left at rest, remains for some time in this position, 

 and then very gradually returns to its original condition, the cell- 

 turgor again increasing at the corresponding portions of the pulvini. 

 In the single leaflet the position of stimulation can be called out 



Fio. 172. — Mimosa piuUca. A. A bi-auch unstimulated and extended; S, a bi*ancli stimulated 

 and fallen, with its leaves folded. (After Detmer.) 



also by a very gentle local touch. If the touch be stronger, the 

 leaflets may be seen to move in succession like a row of dominoes, 

 thus affording a very striking demonstration of the transmission 

 of the stimulus. 



Among the contraction-movements resulting from the mechanical 

 stimuli the contraction-phase only is thus far recognised with 

 certainty, although it is not improbable that in many cases very 

 delicate tactile stimuli may produce expansion. Thus, the contact 

 of an amceboid protoplasmic mass with a smooth support might 

 influence by cohesion the extension of the pseudopodia. 



In the naked protoplasmic bodies of Bhizopoda a single shock upon 

 the extended pseudopodia, such as can be produced by a vigorous 

 rap of the slide under the microscope, produces more or less pro- 



