THE ELECTRO-MOTIVE RESPONSE OF PLANTS 23 



which to test the excitability of a plant, independently of 

 any motile indication. On applying this test, I have found 

 that not the so-called sensitive plants alone, but all plants 

 and all organs of all plants, respond to stimulation. And 

 from this I was led to the discovery that ordinary plants also, 

 in spite of current misconceptions, exhibit motile response by 

 mechanical contraction. The common error of regarding 

 these plants as insensitive has arisen from the fact that in a 

 radial organ, diffuse stimulation induces equal contractions 

 on all sides. Hence those lateral movements, dependent on 

 differential contraction, which are seen so conspicuously dis- 

 played in Mimosa^ cannot take place here. But that the 

 organ as a whole undergoes a responsive contraction has 

 been demonstrated by recording the consequent induced 

 shortening of its length. Such longitudinal contraction is 

 sometimes very considerable ; for instance, in the filamentous 

 corona of Passiflora it may sometimes be as much as 20 per 

 cent, of the original length. 



Having thus shown that all plants are excitable, I shall 

 proceed to demonstrate the fact by means of electrical 

 response. In studying the excitatory effect on ordinary 

 plants, we must bear in mind that there are two different 

 ways of stimulating a given point : that is to say, locally or 

 directly, and by transmission of excitation from a distance. 

 Conducting tissues are capable of stimulation in either of 

 these ways, but the feebly conducting must be subjected to 

 local excitation, since the effect of stimulus applied at a 

 distance cannot, in their case, reach the responding point. 

 Organs containing fibro-vascular elements are fairly good 

 conductors, and stimulus applied on them at one or two 

 centimetres from the point to be stimulated will thus easily 

 reach it. It must, however, be remembered that stimulus 

 becomes enfeebled by transmission through a long tract, its 

 effect at a great distance being negligible. Parenchymatous 

 tissues are bad conductors of excitation, and in order to 

 excite them, stimulus must therefore be applied directly. 



Turning first to the transmitted mode of stimulation, 



