POLAR EFFECTS OF MODERATE CURRENTS 215 



a minimal value of current is reached at which the effect 

 characteristic of Type I. begins to be evident. On continuing 

 to increase the e.m.f., the same characteristic effect is still 

 for a time obtained, until a critical value of the current is 

 reached above which the effect observed is transformed into 

 that which is characteristic of Type 11. There is thus a 

 given range within which we obtain the characteristic effect 

 of a particular type. 



The minimally effective current for the induction of any 

 given type of excitatory effect is modified by the condition 

 of the specimen, being relatively low when the plant is 

 highly excitable. Thus age, season, and temperature are 

 all modifying factors. Different species of plants, again, 

 exhibit different susceptibilities to excitation by a constant 

 current. Thus the leaflets of Mimosa pudica and Biophytum 

 sensitivum are highly susceptible, whereas the leaflets of 

 Averrhoa are much less so. 



It must also be borne in mind that under the same e.m.f. 

 the intensity of the exciting current will depend on the 

 electrical resistance interposed between the two contacts 

 by the intervening tissue. This resistance, it is obvious, 

 will vary with the distance between the two contacts and the 

 character of the specimen. Thus the resistance offered by 

 a specimen of Mimosa, where one contact is at or near the 

 pulvinus and the other on the stem, 7 cm. below, will be 

 of the order of 0*5 million ohm ; whereas that interposed 

 by the same length of a thin petiole of Biophytum will be 

 something like 15 million ohms. Hence it will be seen that 

 with different plants the value of the e.m.f. applied does 

 not by itself give a correct idea of the intensity of the exciting 

 current. In a series of experiments with the same plant, 

 where the resistance is constant, an increasing e.m.f. does 

 actually connote an increasing current. But in different 

 specimens it should not be assumed that the higher e.m.f. 

 necessarily means a higher value of the exciting current. 

 With a high e.m.f. we may have a feeble current and vice 



