ON ELECTROMOTIVE PHENOMENA IN PLANTS. 289 



In studying the blaze current of Prunus laurocerasus simultaneously 

 with the evolution of prussic acid under the influence of chloroform 

 I immersed a laurel leaf in picrate of soda saturated with chloroform 

 for one mjnute and then tested the blaze current;. One minute's 

 immersion was not sufficient at the temperature of 40° 0. to start 

 the evolution of prussic acid nor to abolish the blaze current. A young 

 leaf which gave a blaze of 0'0500 volt was immersed ten times for 

 one minute and tested after each minute, it was then left in a 

 moist tube for four days and it still blazed to the amount of O'OOIO volt. 

 After four minutes in picrate of soda saturated with chloroform the leaf 

 gave no blaze and smelt of prussic acid. The blaze is not abolished 

 after two minutes' continuous immersion in picrate of soda saturated 

 with chloroform at 40° C, nor after three minutes' immersion, but it is 

 entirely abolished after four minutes, and the evolution of prussic acid 

 is now beginning. A blaze current of 0'0200 volt was reduced to 

 0"0005 volt after three minutes' immersion at 40° C. 



Blaze currents are more easily studied on young than on old leaves. 

 Their more delicate cuticle offers a comparatively small resistance — 

 100,000 to 200,000 ohms — whereas a full-grown laurel leaf, with its 

 hard shiny surface, has a resistance of many megohms, and is thus an 

 impossible subject to work with in its intact state. However, I found 

 that by gently scraping off the thin external epidermis without causing 

 injury the resistance was greatly diminished, and good blaze currents 

 were then obtained. 



There is a double process at work in all tissues, the polarisation 

 effect and the blaze. The blaze is the sign of the actively living tissue, 

 the polarisation goes on after its death. A perfectly fresh leaf will only 

 show the blaze, but a leaf that is beginning to be abnormal will respond 

 to excitation by a considerable polarisation current, which quickly 

 subsides and gives place to the blaze current, which lasts five to ten 

 minutes and subsides slowly. It is not easy to distinguish with cer- 

 tainty between an antidrome blaze current, which is a sign of life, 

 and an antidrome polarisation current, which is a general physical 

 effect. The difference in duration appears, however, to be charac- 

 teristic. A blaze current is abolished by an anaesthetic; a polarisation 

 current is not abolished. 



Yellow Leaves. — As a laurel leaf fades it changes colour and 

 falls. Prom dark green the colour is altered to light yellow. Is such 

 a leaf alive or dead? The answer to this question is afforded by its 

 blaze currents, which are unmistakably pronounced (0"005 volt); 

 although the voltage is smaller than that of the blaze of a normal leaf, 

 the galvanometer deflection is relatively large because a yellow leaf has 

 a lower elect»cal resistance than a green leaf. 



Presumably a yellow leaf is a dying leaf. It might, therefore, be 

 expected to give off hydrocyanic acid independently of the action of 

 anaesthetics. But, as a matter of fact, it does not do so; a yellow leaf, 

 like a green leaf, does not give off any HON in simple picrate solution, 

 and it gives off little or no HON in chloroform picrate, according as 

 the fading is more or less complete. Yet a leaf that would be called 

 completely faded, i.e., in which the green colour has disappeared even 



