324 BOTANICAL GAZETTE [MAY 
The assumption is justified that the great difference in the 
latent period is due to the greater mechanical inertia to be over- 
come in Phaseolus than in Zea, and that only a comparatively 
small proportion of it is concerned in the production and trans- 
mission of the impulse. That many changes preliminary to 
curvature do ensue is suggested by the results of Kirchner (11), 
who found that a marked difference was to be noted between the 
specific gravity of the tissues of the convex and concave sides 
of aroot in one to two hours after stimulation, or long before 
the slightest curvature was to be seen. 
1X... THE MOTOR ZONE. 
The region of the rout which exhibits curvature is to be 
termed the motor zone. Hofmeister asserted that the region 
capable of curvature occupies a position immediately back of 
the root cap, and found by twenty measurements that it lies at 
a distance of 1.75 to 3™ from the tip of the root cap in roots of 
Pisum (10). Frank (8), N. J. C. Miiller (15), and Cieselski 
(1), on the other hand, held that the curvature occurs in the 
region of greatest growth; and Sachs (27), in consideration of 
these conflicting views, asserts that the entire growing region of 
the root participates in the action, and that naturally the region 
of most rapid elongation exhibits the curvature of the shortest 
radius. The proper determination of this matter is of the great- 
est importance in the consideration of the mechanism of curva- 
ture. If the entire growing region participates in the movement 
it would be a very weighty bit of evidence in favor of the theory 
that curvature results purely from growth, to the exclusion of 
any idea of ductile extension. If, however, only a special 
region is concerned the case is left open for the interposition of 
specialized action on the part of the root. This specialized 
action might consist of accelerated growth or might consist in 
changes in extensibility of the walls. 
An examination of my preparations reveals the fact that the 
region of greatest curvature lies in that portion of the root 
where the energy of the periblem or cortex has become diverted 
