158 



PHYSIOLOGY OF NUTRITION. 



which we may very suitably use small hair pencils, are laid on 

 the objects under examination. The glass tube of the electrodes 

 is filled with a solution of Zinc sulphate ; in this dips a zinc rod.* 

 To prevent the pencils from drying, we dip them, when not in 

 use, into water. Renewal of the clay is then only occasionally 

 necessary. The customary kneading of the clay with f per cent, 

 alt solution does not appear to be necessary for our experiments ; 

 according to my experience spring water is sufficient for the 

 purpose. 



FIG. 63. Unpolarisable electrodes. 



We connect up the electrodes with the galvanometer, one 

 of them directly, the other through, a key, by means of which 

 the circuit can be closed or opened as required, while we look 

 through the telescope used for observing the deflection of the 

 galvanometer. We now bring the threads or hair pencils of the 

 electrodes into contact with each other. If 110 deflection of the 

 needle of the galvanometer takes place when we close the circuit 

 by means of the key, the apparatus is ready for use. 



We now proceed to experiments with plants. We lay the elec- 

 trodes on two points of the surface of a shoot axis not far removed 

 from one another. In many cases, e.g. in the case of Aristolochia, 

 no current can be detected when the circuit is closed. It may be 

 just incidentally remarked here that a considerable deflection is 



* The zinc rods are, if necessary, to be cleaned by washing with acid, and 

 reamalgamated by dipping in mercury. 



