



RESPONSE OF LEAVES 225 



current a positive, variation. No explanation has as yet been 

 offered, regarding either the existence of these opposite- 

 directioned currents of rest, or the apparently anomalous 

 result, that an identical stimulus would induce, in one case 

 a negative, and in the other a positive, variation of them. 



As regards these peculiar currents of rest we have 

 seen (p. 176), that if an intermediate point be physio- 

 logically less excitable than either of the two terminal 

 points, then a resting current will flow from the less to the 

 more excitable. This is the particular current-distribution 

 in the leaf of Dioncea. It is not a unique phenomenon, for I 

 have noticed other such instances in ordinary leaves. The 

 point of junction of the petiole with the lamina of Ficus 

 religiosa, for example, is galvariometrically the most negative 

 point in that petiole-and-midrib. The currents here also, 

 then, as in the case of Dioncea^ flow outwards from the point 



junction the leaf-current towards the tip of the leaf, and 



e stalk-current in the opposite direction (fig. 147 (b) ). 

 We also saw, however, in the same place, that there may 



instances in which an intermediate point is more excitable 

 n either of the two terminal. When this is so, the 



rrents of rest will be reversed in direction,, and flow 

 inwards. This I find to be the case in the leaf of Citrus 

 decumana (fig. 147 (c] ). 



Next with regard to the excitatory variation of these 



sting-currents in leaf and stalk, we must remember 

 that the effect of stimulation is to give rise to a true 

 excitatory current, flowing away from the excited. If then 

 there be already a resting-current, the responsive current will 

 be added to this algebraically. When the lamina to the 

 right is excited, the responsive current flows from right to 

 left. This would naturally, in the case of Dioncea, induce a 

 negative variation of the leaf-current, and a positive variation 

 of the stalk-current (fig. 147 (a) ). The same thing is seen 

 on stimulating the lamina of Ficus religiosa, where also the 

 excitatory current, being of opposite sign to the leaf-current, 

 and of the same sign as the stalk-current, induces a negative 



Q 



