364 BOTANICAL GAZETTE [MAY 
latent period is consumed by the preliminary changes in the 
motor zone. 
7. The motor zone. The movement of a root is caused by 
changes in the region in which the energy of the periblem is 
turned from cell division to cell enlargement. The motor zone 
includes a length of 2-3" of the root. The curvatures of roots 
apical and basal to the motor zone are mechanical accompani- 
ments of the action of the motor zone. 
8. The mechanism of curvature. The curvature of roots is 
due to the excessive active elongation of the internal layers of 
the cortex, of the side becoming convex, made feasible by the 
increased stretching capacity of the longitudinal membranes. 
The extension of the membranes is accompanied or preceded by 
changes in the quality of the membranes as indicated by their 
reaction to staining fluids. In consequence of the stretching the 
membranes of the convex side become thinner. As a later effect 
of the compression upon growth of the concave side, the mem- 
branes of that side become thicker. Seventy to one hundred 
hours later the difference is obliterated by growth, 
The peripheral layers of the convex side are stretched pas- 
sively in the longitudinal axis, and decrease in radial diameter 
during curvature. The peripheral tissues of the concave side are 
compressed longitudinally and show an increase in radial diame- 
ter during curvature. Roots witha peripheral layer of mechani- 
cal tissue exhibit only a slight increase of the radial diameter of 
the concave side and a marked increase of the radial diameter 
of the inner layers of cortex of the convex side. Roots with- 
out a peripheral layer of mechanical tissue exhibit a marked 
increase of the radial diameter of the inner cortex of the con- 
cave side, and a decrease of the radial diameter of the cortex of 
the convex side. 
g. Recurvatures of stems in response to an excitation to 
movement in a direction opposite to the first curvature are not 
_ accompanied by a relaxation of the extended cells of the con- 
vex side of the first curvature, but by the greatly accelerated — 
extension of the forward cells of the sensory and motor zones, 
