12 ELECTRO-PHYSIOLOGY CHAP. 



either side (as was pointed out by Briicke, 10); this is due to an 

 initial tension in the living plant between the succulent cells of 

 the pulvinus and the vascular bundle, " so that on cutting through 

 it the cellular portion tries to expand in the direction of the long- 

 axis, while the central vascular bundle cannot lengthen beyond 

 its original proportions. It may be compared with an inexten- 

 sible wire drawn through a piece of gutta-percha, and screwed 

 against the end of it with a nut " (Briicke). This may occur 

 when a leaf-stalk changes its position, tension being raised in 

 the upper half of the swelling, or diminished in the lower. 



It is easy to prove that the second alternative invariably 

 occurs in excitation. On comparing the colour of the excitable 

 under surface of a pulvinus before and after stimulation, a strik- 

 ing difference is apparent. Before excitation it is light green, 

 aftenvards it is a darker colour. There can be no doubt that 

 this change is due solely to the discharge of fluid from the cells 

 into the large intercellular spaces previously filled with air, and 

 obviously there must be extension and relaxation of the layers 

 of tissue involved in the process. The fact that any excitation 

 of the under side of the motor organ (cushion) of Mimosa, how- 

 ever scrupulously localised, discharges water from all cells of the 

 parenchyma, testifies to the propagation in all other cells of the 

 under surface, of certain alterations of protoplasm in the cells 

 directly excited, which result in the discharge of water. Pfeffer, 

 indeed, saw the darker colour spreading " like lightning " from the 

 point excited. Thus within the pulvinus itself there is con- 

 ductivity of excitation from cell to cell. 



Still more striking, however, is the fact that the stimulus can 

 be transmitted over large tracts, and even to the most remote 

 parts of the plant. The external features of this phenomenon 

 are so well known that it is superfluous to dilate on them, but 

 the correlative internal processes must be briefly referred to. 

 The account given by Haberlandt (11) in his treatise on the 

 " sensitive plant " will be followed substantially. 



Dutrochet (12) endeavoured to determine which parts of the 

 plant served to transmit the excitation. He showed that the 

 cortex was not involved, by paring away a ring of it, when the 

 conductivity of the twig remained unaffected. So, too, on re- 

 moving the pith. The wood alone was involved, without excep- 

 tion, or more correctly the fibre-vascular system (bast and vessels). 



