210 RESEARCHES ON IRRITABILITY OF PLANTS 



series, the positive end of the first being connected with the 

 first projecting button. The negative end of each cell i, 2, 3, 

 and so on, is led to its own button. A key k carries a radial 

 arm, which during a rotation from left to right will make 

 successive connections with the projecting buttons, and 

 thus cause the e.m.f. between e and e' to rise by increments 

 of 2 volts at a time, the maximum e.m.f. thus obtained by 

 contact with the tenth button being 20 volts (fig. 112). A 

 second key, k', not shown in the diagram, may in a similar 



Fig. 112. — The Potential keyboard. 



manner make contact with other buttons, which are con- 

 nected with terminals of cells in groups of ten. Thus the 

 number of cells which may be included between the two 

 electrodes may be varied from o to 50, and the derived e.m.f. 

 may thus be raised from o to 100 volts by steps of 2 volts 

 at a time. 



Having thus found suitable means of applying currents 

 of increasing intensity, we shall in the course of subsequent 

 chapters study in detail the effects of feeble, moderate, and 

 strong currents on various sensitive plants. 



Summary 



In Mimosa, under feeble intensity of current, excitation 

 takes place only at the make of the kathode. In this the 

 polar reaction in plant is the same as in animal tissues. 



