1876.] 



on the Leaf of Dionsea muscipula. 



421 



The greater number of the observations were made at Kew during the 

 month of August of this year, the plants being obtained from the hot- 

 houses of the Eoyal Gardens, through the kindness of the Director, and 

 brought to a conservatory in the house in which one of us resided several 

 days before they were used. The parts to be compared were connected 

 with the electrometer by means of electrodes of the same construction as 

 those which we always employ for electro-physiological purposes. Each 

 consists of a U-tube supported by a convenient holder, and half filled 

 with saturated solution of zinc sulphate. Into one arm is plunged a 

 zinc rod, of which the immersed end is amalgamated ; into the other a 

 glass tube filled with kaolin made into a paste with 0*75 per cent, 

 solution of chloride of sodium. The upper end of this mass of clay 

 forms a cushion, which can be moulded to any required shape. Elec- 

 trodes of this form are convenient to work with, and possess manifold 

 practical advantages, the greatest of all being (1) the facility with which 

 they can be renewed in case they are found (by testing with the galva- 

 nometer) to exhibit polarity, and (2) the facility with which they can be 

 brought into any desired position. 



The arrangement most usually adopted is as follows : — The pot con- 

 taining the plant had been previously kept plunged in water. Three 

 electrodes are used ; by one of them (called the fixed electrode) the damp 

 surface of the pot is connected with a gas-pipe. The other two (called 

 movable electrodes) are in contact with any two surfaces of the leaf 

 which it is desired to investigate. By means of a switch either can be 

 brought into connexion with the larger mercurial surface of the electro- 

 meter. The capillary surface is connected directly with the earth. In 

 many of the experiments the capillary surface was connected directly with 

 the fixed electrode. 



Section 3. — General Characters of the Electrical Disturbance. 



"When the capillary mercurial surface of the electrometer is connected 

 by the fixed electrode with the surface of the pot or of the petiole, and 

 the movable one is in contact with any part of the surface of the leaf, 

 whether inside or out, the effect of touching a sensitive hair is (with 

 certain exceptions to be mentioned hereafter) to produce a transitory 

 advance of the mercurial column in the capillary tube towards its orifice. 

 Such a movement will hereafter be referred to as a negative excursion. 

 Its extent may be readily measured with the aid of the vertical hair-line 

 which crosses the field of the microscope, and the result recorded in a 

 number which may express either the change of position of the mercurial 

 meniscus in millimerets, or the change of potential as compared with 

 some unit of electromotive force. 



Considering that the value of the direct measurement varies according 

 to the part of the capillary measured, it is clearly more convenient to 



