232 TLANT RESPONSE 



be decreased, below the normal, by the action of an external 

 magnet which reduces the earth's directive force. 



On now obtaining responses to a uniform disturbing 

 force, under these three conditions of normal, increased, and 

 diminished stability, we shall find that while in the first case 

 we get moderate response, in the second the response is 

 very much diminished (and may even disappear entirely, when 

 the stability is very great), and in the third it becomes exalted. 



An-electrotonus and kat-electrotonus. — I shall now 

 proceed to show the opposite effects of the anode and 

 kathode on molecular responsiveness, during the passage of 

 an electrical current through a plant-tissue. This change, 

 induced by an electrical current, is known as electrotonus, 

 and the effect due to the kathode is distinguished as kat- 

 electrotonus, while that due to the anode is known as an- 

 electrotonus. It is probable that here, also, the variation of 

 sensibility is brought about by the variation of molecular 

 mobility, and that this is induced by an increase or diminu- 

 tion in the conditions of stability, as in the model. These 

 opposite variations of the susceptibility to excitation, due to 

 the anode and kathode respectively, will be demonstrated by 

 the changes which they induce in the conductivity and 

 excitability of the tissue. 



In the chapter on the Excitatory Polar Effects of Currents, 

 the intensity of the E.M.F. used was such that the excitation 

 caused by the kathode was visibly manifested in the motile 

 effects to which it gave rise. In the present chapter, how- 

 ever, we shall have to deal with latent excitatory effects, the 

 E.M.F. used not being sufficient to give rise to any imme- 

 diate external reaction. In the cases referred to, again, the 

 distinctive action of the anode could not be demonstrated, 

 inasmuch as under ordinary conditions it could not give 

 rise to any motile indication. It will now, however, be 

 shown that the effect of the anode is one of depression, or 

 the opposite of that of the kathode. In studying variations 

 of conductivity we have to remember that when the conduc- 

 tivity of a tissue is great, the state of excitation is transmitted 



