R. Snow 
248 
through a galvanometer two points in a tissue at different potentials, 
he finds that a current flows round the circuit, but this does not show 
that the same current was flowing before the circuit was made. 
Moreover, when the current does flow, since it flows from positive 
to negative in the wire, it must flow from negative to positive in the 
tissue, just as in the Voltaic cell, when on closed circuit, the current 
in the cell flows from negative to positive pole, being forced up 
against the potential gradient by the internal forces of the cell. Thus 
in the tissue, when a current is artificially produced, it will flow in the 
opposite direction to that required by Professor Small. This point 
of view is in agreement with that of animal physiology, and also of 
Bose in his Comparative electro-physiology , though it is true that in 
earlier writings (cf. Plant response , p. 33) he found some objection 
to it. But it is of no use to go further into the question of direction 
of current, until it is made clear what bearing the question has upon 
Professor Small’s theory. On this point we have only his brief state¬ 
ment that a downward flowing current in the root will produce greater 
effect at the lower side “on account of the resistance in the circuit.” 
(This Journal, 19 , No. 3, p. 53.) As I suspect that others besides myself 
have been puzzled by this statement, may I ask for further explana¬ 
tion? 
But the chief source of difficulties is surely the practice of speaking 
of ‘'currents” in tissues when the evidence only shows that there are 
potential differences. There appears indeed to be no direct test for 
currents in tissues in a state of nature, though it is evident that 
such must occur, by local short-circuiting through cell walls or 
other paths. 
Such currents must, in fact, occur if the two points at different 
potential are connected also by tracts of conducting tissue other 
than those along which the forces are acting which produce that 
potential difference: but then the resulting currents will be circular, 
flowing in one direction along the one path and in the other direc¬ 
tion along the other. 
PRINTED IN ENGLAND BY J. B. PEACE, M.A. 
AT THE CAMBRIDGE UNIVERSITY PRESS 
