VELOCITY OF TRANSMISSION OF EXCITATION 453 



Since the conduction of excitation takes place by the 

 transmission of protoplasmic changes, it is evident that it must 

 occur most easily along those paths in which there is greatest 

 protoplasmic continuity. It is clear, then, that certain 

 elements in the fibro-vascular bundles will furnish the best 

 conducting medium. Cells of indifferent tissue, on the other 

 hand, like the parenchyma of the leaf, are divided from each 

 other by more or less complete septa, the fine filaments, by 

 which neighbouring cells may be protoplasmically connected, 

 being so minute that the conduction of stimulus through such 

 imperfect channels must be comparatively feeble. Such 

 tissues are, therefore, indifferent conductors of excitation, 

 the stimulus remaining more or less localised in them. 

 Plant-organs, then, which contain fibro-vascular elements, 

 such as the stem, peduncle, and petiole, are for that reason 

 relatively good conductors. Conductivity in such an organ, 

 again, is, as we should expect, much greater along the length 

 than across. 



I shall now describe an important method by which the 

 relative conductivity of a tissue in different directions may 

 be experimentally determined, verifying by its means the 

 difference in the power of a tissue to transmit stimulus 

 longitudinally and transversely. For this purpose I took 

 a thick peduncle of Musa, and made two electrical con- 

 nections, of which one was at a fixed point B, transversely 

 situated as regards C, the point of application of stimulus. 

 The second point, A, was longitudinally above C, and its 

 distance from it could be varied in successive experiments 



(fig. 2 73). 



If we now take a point, A, in such a position that CA is 

 equal to CB, then, on account of the better conductivity along 

 CA, the excitation will reach the A contact earlier than that 

 at B, making that point galvanometrically negative. The 

 direction of the first responsive current, therefore, will be 

 from A->B in the tissue. If, next, the longitudinal contact 

 be moved to A", that is to say, so far that the excitation 

 reaches the B contact first, then the responsive current will 



