326 BOTANICAL GAZETTE [MAY 
root renders these objections invalid, since it is the relation of 
the sensory zone only to the vertical which affects the move- 
ment. 
If the curvature is distributed according to the rapidity of 
growth the geotropic curvatures should, according to the theory 
of Sachs, resemble those obtained by a mechanical curvature of 
the root, since the normal extensibility of the walls may be 
assumed to be in direct proportion to the rapidity of elongation. 
The curves obtained by mechanical bending of roots are not in 
accordance with those attributed by Sachs to geotropism. The 
radius of curvature is shortest in the region of most rapid growth 
and gradually elongates in both directions. In geotropic curva- 
tures, however, the difference between the radii of curvature of 
the forward portion of the region of rapid growth and the apical 
and basal portions is abrupt and marked, showing that a special 
region has effected the greater part of the curvature. In this 
region the cortical and vascular cells have not attained more 
than 25 to 35 per cent. of their final length. The minor curva- 
ture, which includes the basal and apical portions of the root, 
may be explained entirely as mechanical results of the disturb- 
ance of tensions by the action of the cells of the specialized zone, 
and, as a matter of fact, are reproduced exactly in mechanical 
curvatures. At any rate these minor curvatures actually disap- 
pear with the fixation of the organ in its new position. In con- 
clusion of this detail, it is to be said that the formation of a 
sharper break or angle required by Sachs to establish the theory 
of a localized motor zone is not consequential in a body so plas- 
tic as the growing portion of the root. 
| In connection with the question of localization of curvatures 
the facts obtained as to the behavior of a root in recurvatures 
are of value. It has been quite generally asserted and received 
that if a geotropically excited root were allowed to effect only a 
small amount of curvature, and then placed in a position which © 
would induce a curvature exactly opposite, the first curve would 
be obliterated. _ 8 
TI have directed some experiments to this phase of the ques” 
