MOVEMENTS OF IRRITATION. 



537 



pulviims from the fibrovascular system, the former vigorously 

 carves downwards, the latter feebly upwards. If the organ of 

 movement so prepared, and still remaining in connection with its 

 leaf-stalk, is laid in water, the curvature of the upper parenchyma, 

 and especially the upward curvature of the lower, since its cells 

 have again become turgescent, is intensified. The isolated lamellae 

 of parenchyma, moreover, exceed the fibrovascular bodies of the 

 pulvinus in length, and everything points to the conclusion that 

 in uninjured motile organs considerable tensions must exist be- 



FIG. 133. Portion of a shoot of Mimosa pudica. The leaf represented on the left isun- 

 stimulated, that on the right hand stimulated* 



tween the axile strand on the one hand and the parenchyma on 

 the other. 



The following simple experiment is very instructive. We care- 

 fully, and so as not to agitate the plant, cut off with a pair of 

 scissors one of the small leaflets at the end of a secondary leaf- 

 stalk of a very sensitive Mimosa, or we stimulate a leaflet of a 

 Mimosa by focussing on it convergent rays of light from the sun 

 by means of a lens. There is now exhibited a transmission of 

 stimulus, due to movement of water in the plant. Advancing 

 from the tip towards the base of the secondary leaf-stalk, the 

 transmission of stimulus causes more and more remote pairs of 

 leaves to lay themselves together, and then the leaflets of neigh- 

 bouring secondary leaf-stalks lay themselves together (first the 

 lower and later the higher ones), and, indeed, even the main leaf- 



