POSITIVE AND NEGATIVE TURGIDITY-VARIATIONS 55 



end is connected alternately with a vacuum and a force*- 

 pump, by means of which a diminution or increase of hydro- 

 static pressure may be induced at will. In this way it is 

 possible to suck water away from, or force it into, the plant 

 and its organs, thus producing negative and positive turgidity - 

 variations at will. When turgidity is thus diminished the 

 indicating leaf is seen to fall. This is what happens also 

 under the ordinary negative turgidity-variation induced by 

 excitation. When turgidity is increased, on the other hand, 

 the leaf is erected (fig. 41). In the case of the negative 

 variation of turgidity, the pulvinus as a whole loses water, 



FIG. 41. Artificial Hydraulic Response of Mimosa 



The plant was subjected to diminished pressure up to a, and to normal 

 pressure to 6, after which the pressure was increased. The effect 

 of diminished pressure, in the depression of the leaf, continues for a 

 while. The ordinate represents movement of 'tip of leaf in cm., 

 abscissa represents time. 



but more from the lower and more excitable half than 

 from the upper. In the case of the positive turgidity- 

 variation, also, it is again the more excitable lower half 

 which absorbs the greater quantity of water. Thus, in Mimosa, 

 and in Biophytum, the mechanical indication of increased 

 turgidity is an erection of the leaf or leaflet. The 

 characteristic electrical indication of this will be observed 

 presently. 



It has already been mentioned that the direct application 

 of stimulus at a point causes a negative turgidity-variation 

 of that point. We shall now see whether, under any 



