536 PHYSIOLOGY OF GROWTH. 



202. Movements of Variation in Mimosa pudica Evoked by 

 Shock or Contact. 



Mimosa pudica is only irritable to any considerable extent at a 

 fairly nigh temperature, and when the soil and the surrounding 

 air are sufficiently moist. If, however, under such conditions, 

 plants of Mimosa raised from seed in flower- pots are agitated 

 without the plants themselves being touched, a striking irritation- 

 effect is exhibited. The primary leaf-stalks sink, the secondary 

 leaf-stalks approach one another, and the leaflets lay themselves 

 together forward and upwards (see Fig. 183). These movements 

 are all effected by pulvini, which are situated at the base of the 

 leaf-stalks and leaflets, and have a structure similar to those of 

 Phaseolus leaves. We can, however, incite the Mimosa to move- 

 ments, not only by shocks but also by contact. If the upper side 

 of the large pulvinus at the base of the primary leaf-stalk is 

 carefully touched, no movement, it is true, follows ; but move- 

 ment at once appears if the lower side of it is so stimulated. 

 Accordingly only the under side of the pulvinus reacts to impact 

 stimulus [Stossreiz] ; its cells lose water, which may really pass 

 into the intercellular spaces occurring between the cells, the 

 hydrostatic equilibrium in the pulvinus is upset, so that finally, 

 according to Haberlandt, the stimulus-conducting elements in the 

 vascular bundle region also let fluid escape. Owing to the con- 

 traction of the underside of the pulvinus due to loss of water by 

 its cells, and the co-operation of the upper side of the pulvinus 

 which is in a state of high positive tension, an energetic down- 

 ward movement of the main leaf-stalk is brought about. The loss 

 of water by the pulvinus on stimulation, and the existence in the 

 pulvinus of conditions of tension, may be demonstrated as fol- 

 lows : 



The primary leaf-stalk is separated by a sharp cut from its 

 pulvinus, and the Mimosa is then left for some time under a bell- 

 glass in an atmosphere saturated with moisture. If the pulvinus, 

 after it has risen to some extent, is stimulated, it sinks, and water 

 issues from the cut surface. In an uninjured plant the water is 

 carried away from the pulvinus, chiefly into the stem or leaf-stalk. 

 If we cut away one of the large pulvini close to the shoot axis, 

 without removing it from its leaf -stalk, it bends naturally in 

 consequence of the stimulus in the usual way. If we now by two 

 longitudinal cuts isolate the upper and lower parenchyma of the 



