MECHANICAL MOVEMENTS IN CELLS. 71 



vegetable kingdom, such as the turning of leaves toward or away from 

 the light, the regular motion of certain algae, such as diatoms, desmidia, 

 oscillatoria, as well as the irritative motions of certain plants, such as the 

 sensitive plant (Mimosa pudica), or the Venus' Fty-Trap (Dionxa mus- 

 cipula); all of these motions depend upon a change in the physical state 

 of imbibition of certain cells. In the Mimosa pudica, the plant in which 

 motion is most marked, and apparently most closely analogous to that 

 occurring in the animal kingdom, motion of three different parts may be 

 recognized. 



While at rest during the day-time the leaf-stems of the sensitive 

 plant form an acute angle with the main stem, the secondary leaf-stems 

 diverge, and the leaves are opened out so that they form a plane surface. 

 When evening comes the leaf-stem sinks downward, the leaves approach 

 each other, as when the fingers of 'the open hand are adducted to the 

 middle finger, and the leaflets themselves close up so that the sur- 

 faces which during the da3 r -time are the uppermost now come in contact 

 with each other. If the entire plant is shaken the same changes occur 

 as have been just described to take place during the night ; or if the 

 under part of any one of the leaf-stems is gently touched, the closing 

 motion is localized in that part of the plant. If, however, the upper 

 portion of the leaf is touched, no change is produced in the position of 

 the leaves or of the stem. The under part of the leaf-stem is seen to be 

 cylindrical in shape, and this represents the sensitive portion of the 

 plant. 



Briicke, to whom we are indebted for the explanation of the 

 mechanism of this movement, has found that this cylindrical structure 

 which underlies the leaf-stem is composed of a bundle of vessels running 

 through the centre, and between it and the outer green bark there is a 

 layer of very succulent cells, which on the upper and non-sensitive side 

 of the stem are comparatively thick walled, while on the under side the 

 cells are provided with very delicate membranes. If a portion is cut 

 out of this cylindrical stem, the ends immediately become retracted so 

 that each extremity takes on a funnel-like form. If such a cylindrical 

 piece is then divided in the direction of its long axis, each part becomes 

 bent in the form of a bow, so that the external epidermal side is longer 

 than that bounded by the vascular bundle. This change in tension of 

 the cells is due to a change in distribution of the cell-juice. When the 

 membrane of the under portion of the leaf-stem is touched the cell-juice 

 flows from the lower to the upper cells and into the intercellular spaces; 

 the tension of the upper cells therefore becomes increased, while that of 

 the lower cells becomes reduced. The stem, therefore, sinks and the 

 leaves close. Movement occurring in the mimosa as a consequence of 

 mechanical irritation, therefore, depends upon differences in degree of 



