

THE LEAF CONSIDERED AS AN ELECTRIC ORGAN 245 



reaction of each of these two types is governed entirely by 

 the question of the relative excitabilities of the two surfaces. 

 The physiological anisotropy on which the distinctive effect 

 of the type depends is very pronounced in the representa- 

 tive cases of the vegetal analogues which have been named. In 

 many other cases, however, though the results under normal 

 conditions are fairly definite, and approach one or other of 

 the two types, yet the characteristic responses are liable to be 

 reversed under the physiological modifications induced by 

 age and surrounding conditions. In this way it may be said 

 of the leaf of water-lily (Nymphaa alba\ of Bryophyllum 

 calcineum, and of Coleus aromaticus that when vigorous, and 

 in their proper season, their responses are of the first of these 

 two types, while those of the bulb-scale of Uriclis lily, with 

 its glandular inner surface, are of the second type. 



The electrical organ of the fish may be excited indirectly 

 by means of stimulus transmitted through the nerve ; or 

 direct stimulation may be applied, as by means of induction- 

 shocks. Under either of these conditions the excitatory 

 discharge is definite in its direction. In the case of Torpedo, 

 as already mentioned, this is always from the ventral and 

 anterior to the dorsal or posterior surface. Turning then to 

 the corresponding vegetable organ of the first type, I shall 

 show that transmitted stimulus induces an effect exactly 

 similar ; and I shall demonstrate this experimentally by means 

 of the leaf of Nymphaa alba. Suitable galvanometric connec- 

 tions were made with the ventral anterior and with the dorsal 

 posterior surfaces of the lamina. Thermal shocks, by means 

 of the electro-thermic stimulator, were applied on the petiole, 

 close to the lamina, at intervals of one minute, records being 

 taken photographically of the resulting responses. It should 

 be remembered here that excitation is transmitted to the 

 lamina by the conducting nerve-like elements present in the 

 petiole. The records (fig. 156) show that the effect of this 

 periodically transmitted stimulation was a series of respon- 

 sive currents, whose direction was like that of the discharge 

 in Torpedo, from the anterior surface to the posterior. 





