216 RESEARCHES ON IRRITABILITY OF PLANTS 



versa, on account of the very different resistances offered 

 by different specimens. 



To give some idea of the order of magnitude of 

 the current found efficient in causing excitation, I shall, in 

 typical cases, specify its value as read by an interposed micro- 

 ammeter. The unit of current found convenient for these 

 investigations is, as I have stated elsewhere, one-millionth 

 part of an ampere, designated as a micro-ampere or sym- 

 bolised as io- 6 ampere. 



Having given a general description of the method 

 employed, we will now study the effects of feeble and 

 moderate currents in inducing excitation. I shall begin by de- 

 scribing the responsive effects seen in the leaflets of Mimosa. 

 The bi-polar method has the advantage, as already stated, 

 that the effects at anode and kathode are simultaneously 

 displayed ; moreover, the selection of the leaflets instead 

 of the primary pulvinus as indicators of the excitatory 

 effects enables us not only to observe the occurrence of 

 local excitation but also to watch its propagation, as shown 

 in a very striking manner by their serial closure. 



Leaflets of Mimosa 



Effect of feeble current. The sensibility of leaflets of this 

 plant is considerable, being even greater than that of the 

 leaf. The conducting power of the sub-petiole is of the same 

 order as that of the main petiole. Selecting a young leaf of 

 Mimosa, I made appropriate connections with two neigh- 

 bouring sub-petioles, the length interposed between the 

 two contacts being 4 cm. I shall distinguish the right of 

 these as R and the left as L. 



In making repeated experiments on the secondary petioles 

 of Mimosa bearing the leaflets, much trouble is occasioned 

 by the travelling of the excitation to the primary pulvinus, 

 in consequence of which the leaf is apt to fall and thereby 

 interrupt the electric circuit. This is obviated by means of 

 a special electrode-holder. A lateral rod, carrying a piece 



