The Dia-Heliotropic Attitude of Leaves. 



165 



definite nervous link exists between the receptor and the effector, and that 

 there is a well-developed system of innervation, by which the " attitude " of 

 the plant-organ becomes adjusted to the incident stimulus. 



7. Localisation of Nervous Tissues in Plants. 



I have in my previous works shown that in Mimosa stimulus gives rise to 

 an excitatory impulse, which is transmitted with a definite velocity, that this 

 impulse has all the characteristics of the nervous impulse in animals. The 

 most important problem in connection with this subject is the localisation of 

 the conducting or nervous tissues. I succeeded in isolating a length of such 

 a tissue in ferns and was able to obtain with it many results which are 

 regarded as characteristic of nervous tissue in animals. In Mimosa, however, 

 it is impossible to isolate the nervous tissues without injury, and I have for 

 many years been confronted with the problem of localising in situ the 

 particular tissue which serves as the conductor of excitation. I have recently 

 been successful in my efforts, the method employed being that of the 

 electric probe (6), by which I was able to localise the geotropic sense organ 

 in plants. 



Limitation of space enables me only to give the essential details of the 

 method of localisation of nervous tissues and some typical results. A fuller 

 account will be given in the forthcoming number of the ' Transactions ' of my 

 Institute. 



The principle of the method will be understood if we take the somewhat 

 analogous case of a cable along which electric messages are being transmitted. 

 The conducting strand is here embedded in a non-conducting sheath. We 

 can localise the embedded conductor and pick up the transmitted message 

 by gradually thrusting in the electric probe, which is insulated except at the 

 extreme tip. A galvanometer included in the circuit of the probe will 

 begin to pick up messages that are being transmitted from the moment of 

 contact of the tip of the probe with the conducting strand. The depth of 

 insertion for contact can be read on a suitable scale and the position of the 

 conductor may thus be determined. 



We may similarly localise the exact position of the conducting nerve 

 embedded in the petiole of Mimosa (fig. 5). Excitation of the sub-petiole will 

 give rise to an excitatory impulse which travels in a centrifugal direction 

 towards the stem. This excitatory impulse is of galvanometric negativity. 

 The conducting nerve will be most intensely excited by the transmitted 

 impulse, and the induced electrical change of this particular tissue will be 

 maximum. Excitation will no doubt be irradiated to the adjoining tissue, 



