42 
V. H. Blackman 
the “ creaming ” occurred and a difference of potential were produced. 
Since the potential, which is assumed to result from the movement 
of the particles, is, ex hypothesi , being dissipated by the current 
which it produces in the organ, it should soon disappear when the 
“creaming” is complete. In an organ kept horizontal it would seem 
that the difference of potential produced by the “creaming” should 
soon cease to exist, and, as the current produced by this difference is 
by the theory the cause of the curvature, the organ should soon cease 
to react to gravity. A root compelled to grow for a time through a 
short horizontal tube should, on becoming free at the other end, 
have lost the power of geo tropic response. The same difficulty arises 
as to the origin of the currents in the axial and lateral organs, which, 
Professor Small holds ( loc. cit. pp. 59 and 209), explain the relation of 
secondary branches to the primary root and stem. The normal polari¬ 
sation of the plasma membrane will not produce currents in uninjured 
cells. Also the apical meristems of, for example, the normal undis¬ 
turbed root system, would have been “creamed” from their first 
origin, so no difference of potential should arise in their case. 
The recent experiments of Bose (loc. cit.) do seem definitely to 
support the statolith hypothesis in the case of the stems he examined. 
When the stem is horizontal, a high e.m.f. is rapidly attained; it is 
at its maximum when a lead is taken from the endodermis, and no 
current is obtained when the electrode is in the centre of the stem ; 
also he found that when the starch grains had disappeared from the 
endodermis there was no electric response of the stem. Bose also 
described a striking case where there was no electric response when 
the flower stalk of Nymphaea was displaced by 33 0 , but “when this 
critical angle was exceeded by a single degree there was a sudden 
precipitation of geo-electric response” (p. 500). This would seem to 
be consistent with the sticking and displacement of large particles, 
but is quite inconsistent with the movement of ultra-microscopic 
ones in the protoplasm. 
That the hydrogen-ion concentration of the protoplasm plays an 
important role in cell processes, and that differences in this respect 
may explain the difference of geotropic response of the root and stem 
as assumed by the theory, would seem very plausible. If, however, 
a reaction which takes place as rapidly as the geotropic response is 
to be correlated with the movement of particles in a viscous medium 
it would seem certain that such particles must be comparatively 
large and heavy, not ultra-microscopic. 
Imperial College of Science and Technology, 
London, S.W. 7. 
