220 RESEARCHES ON IRRITABILITY OF PLANTS 



In another series the e.m.f. was gradually raised in 

 successive experiments. The specimen in this case was 

 older and less sensitive than the former. The minimally 

 effective e.m.f. was here found to be 8 volts. Successive 

 observations were taken with an e.m.f. of 8 volts, 12 volts, 

 and 14 volts. In all these the excitatory effect only took 

 place at kathode-make. The sole difference was in the 

 increased extent of transmission of excitation with higher 

 voltage. With 8 volts only one pair of leaflets underwent 

 closure ; with 12 volts this was increased to 7 pairs ; with 

 14 volts to 9 pairs. With still higher voltage the responsive 

 effects became those of Type II., which will be described 

 presently. 



I carried out twenty-five sets of experiments with different 

 specimens of Mimosa on the effect of feeble current. The 

 minimally effective e.m.f. was found to vary from 4 to 12 

 volts, according to the age of the specimen. The young 

 leaflets were excited under a lower e.m.f. than the old. In 

 all these, without a single exception, excitation was found 

 to take place only at the kathode at make. 



Hence as the result of these direct and reverse experi- 

 ments on the effect of feeble current, undertaken with various 

 specimens of leaflets of Mimosa, we find one invariable 

 result — namely, that kathode excites at make and that no 

 excitation is occasioned by the break of kathode or by the 

 make or break of anode. The excitation formula of the first 

 type is, to follow the parallel nomenclature in the case of 

 animal tissues, KCC, or, as I have preferred for the sake 

 of convenience, the simpler formula Km, which is a short 

 way of saying that excitation took place at the make of 

 kathode. The excitation formula will always be represented 

 by letters in thick type. 



Effect of moderate current. — The characteristic effect of 

 the first type is obtained, normally speaking, from an 

 E.M.F. of 4 volts, and continues till the e.m.f. is raised to 

 about 8 volts. In the case of young and excitable specimens 

 we obtain here a transition from the effect of the first to 



