vi ELECTROMOTIVE ACTIOX IN VEGETABLE CELLS 17 



that the equilibrium is balanced as follows : on the lower side the 

 compressed tissue endeavours to extend itself and to increase its 

 length ; on the upper there is a marked turgescence, counteracted 

 by the elasticity of the cell-membranes which tend to contract 

 upon themselves. When, in consequence of excitation, water is 

 discharged from the cells of the upper surface, the equilibrium is 

 disturbed (Batalin, 1 6), and a new state induced, characterised by 

 an actual relaxation and shortening of the upper layers. This 

 cannot, even in the closed leaf, amount to perfect equilibrium, 

 since it is hindered by the contact of the lobes in juxtaposition. 



This is plainly seen from the fact that, after cutting off one 

 lobe near the mid-rib, the other, in its excitatory movements, 

 is jerked far beyond the position it occupied in the closed and 

 uninjured leaf. Hand in hand with the shortening of the upper 

 layers at closure, there is a corresponding lengthening of the 

 lower, so that each lobe passes from downward to upward con- 

 cavity. On opening the leaf, the reverse processes occur. The 

 previously relaxed and diminished cells of the upper parenchyma 

 swell out as turgor increases, and recover their former tension. 

 According to Darwin's measurements, the shortening of the upper 

 layers in a lobe 10 mm. broad reduces it to - 6 mm. only. 

 This is most obvious when, after taking away one half of a leaf, 

 two points are marked on both upper and lower surface of the 

 other half, and a movement excited ; the distance of the marks 

 alters in an opposite direction. 



Along with these excitatory movements of the leaf of Dionwa 

 (as also of Mimosa) there are very striking .electromotive effects, 

 which, as was said above, were first recognised by Burdon- 

 Sandersou to be a " negative variation." As the result of his 

 first observations he communicated the following propositions : 



(a) If the leaf is laid on the electrodes, so that the normal 

 current is manifested on leading off from both ends of the leaf 

 by a deflection of the magnet to the left, and a fly is then made 

 to creep on to it, the needle will swing to the right at the 

 moment when the fly reaches the interior of the leaf, and touches 

 the sensitive hairs of the upper surface, the leaf closing over 

 the fly at the same moment. 



(&) After the fly has been captured, the needle swings to the 

 right each time that it makes a movement. 



(c) The same series of manifestations occurs if the sensitive 



c 



