292 PLANT RESPONSE 



brought to a state of standstill, care should be taken to select 

 those which have not permanently lost their motility, but in 

 which automatic movements have simply come to a stop for 

 want of a sufficient reserve of latent energy. 



Polar excitation of Desmodium leaflet in a state of 

 standstill. — We have seen how the leaflets of Biophytum 

 showed kathodic excitation at make. I shall now show that 

 a Desmodium leaflet, which has been brought to a state of 

 standstill, either naturally or artificially, will exhibit similar 

 effects of polar excitation ; and first, I shall take a case in 

 which arrest was produced by cautious application of cold. 

 I placed one electrode on the petiolule of the leaflet, and the 

 other on the main petiole of the leaf, and applied the stimu- 

 lus of condenser discharge, the petiole being made kathode. 

 In this way I obtained from the Desmodium leaflet a series 

 of single responses to single moderate stimuli. 



I next selected specimens which, in the winter season, had 

 come to a state of natural standstill, and tried on them the 

 polar effects of a constant current. Electrical connections 

 were now made at the bases of the petiolules of lateral 

 leaflets in two neighbouring leaves. In completing the elec- 

 trical circuit, one of these leaflets would be under anodic, and 

 the other under kathodic, action. I found that a consider- 

 able electromotive force was necessary to initiate the excita- 

 tory reaction. Thus, using thirty volts, I found that the 

 kathodic leaflet responded at make, and the anodic at break. 

 This shows that, as regards polar effects, Desmodium in a 

 state of standstill gives exactly the same responses as does 

 Biophytum. 



In the case of Biophytum, moreover, we found that with 

 high E.M.F., we arrived at a phase of response, the A stage, 

 in which both the anode and the kathode caused excitation 

 at make ; and in Desmodium in a state of standstill 

 I obtained an exactly similar result, for, on now using a 

 higher E.M.F. of forty-eight volts, with the same specimens 

 as in the last experiment, I obtained excitation at make, at 

 both anode and kathode. 



