Page:Researches on Irritability of Plants.djvu/156

 any tissue analogous to the nerve, then it is in the fibrovascular 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 electrical 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 galvanometric 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 fibrovascular 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 excitation. 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 transmission 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