VELOCITY OF TRANSMITTED IMPULSE 133 



any tissue analogous to the nerve, then it is in the fibro- 

 vascular bundle that we must look for it. 



In a nerve-and-muscle preparation the transmitted 

 excitation is detected by the contraction of the terminal 

 muscle. Even in the absence of any terminal contractile 

 organ, we can detect the passage of excitation by an elec- 

 trical method. It is known that the excitation of a living 

 tissue is attended by a concomitant electrical change of 

 galvanometric negativity. If we make suitable galvano- 

 metric connections with two points on a nerve, and we 

 stimulate the nerve at a distant point, we shall find that 

 the arrival of excitation from the distant stimulated point 

 is at a proper moment signalised in the galvanometer by a 

 deflection of a definite sign. 



Similarly, I have found that the excitatory change of 

 galvanometric negativity is transmitted to a distance 

 through certain plant-organs. Tissues containing fibro- 

 vascular elements, such as stems and petioles, are found 

 to be good conductors of excitation. Indifferent tissues in 

 leaves and tubers possess little power of conduction ; in 

 such cases excitation remains more or less localised. The 

 parenchyma in the leaf is thus an indifferent conductor, 

 whereas the midrib and veins are good conductors of exci- 

 tation. In stems also great difference is found, as regards 

 power of conduction, between the fibro- vascular strands and 

 the ground tissue. The results of electrical investigation 

 thus give strong support to the conclusion that plants 

 possess conducting-tissues by means of which the excitatory 

 state may be transmitted to a distance. 



The prevailing opinion, however, up to the present 

 has been that in plants like Mimosa there is merely a trans- 

 mission of hydro-mechanical disturbance and no transmission 

 of true excitation comparable with that of animal nerve. 

 That this conclusion is erroneous will be shown from the 

 results of varous inquiries fully described in the next chapter. 

 In all these investigations it is necessary to determine the 

 velocity of transmission with the highest accuracy ; and in 



