1897] THE CURVATURE OF ROOTS $3 
after excitation (concave 29, convex 43) show that in Zea an 
elongation of both sides of the root takes place during curvature. 
It is apparent, however, that the epidermal and sub-epidermal cells, 
which have been in a state of passive tension previous to curva- 
ture, will show purely mechanical changes. These mechanical 
changes will depend upon the angle and rapidity of curvature 
as well as upon the thickness of the root. It is possible that 
the passively stretched tension of the epidermal cells in young 
roots may be converted into a compressed tension in older 
organs. 
A comparison of the radial diameters of the cells of the two 
sides exhibits changes of a similar nature. The radial diameters 
of the cells of the convex sides of roots steadily decrease in 
Zea as the angle of curvature increases, while the reverse is true 
of the concave side. The decrease is most marked in the 
peripheral layers of the convex side, and the cortical layers of 
the concave side in Zea. The radial diameter of the convex side 
in table III is 7.56, in table VI 6.98. The average diameter of the 
cells of the concave side in table I is 8.25, in table VI is 11.4. 
It seems well demonstrated that the extension in the length 
of the cells of the convex side of the root of Zea is accompanied 
by a decrease in radial diameter, and that the slight elongation 
of the cells of the concave side is attended by an increase in 
radial diameter. Such conditions lead to the conclusion that the 
elongation of the convex side is a ductile extension of the longi- 
tudinal walls. The ductile extension is accompanied by the 
usual amount of growth. The longitudinal compression of the 
cells of the concave side permits only a minimum of growth in 
this direction and facilitates extension in a radial direction. 
TABLE XII. 
Median longitudinal section of root of Zea mais, allowed to curve geo- 
tropically six hours and then reversed five hours. The measurements are 
taken from the portion of the old curvature, which had decreased from 40” 
to 15°. The new curvature was formed at a distance of 2™ apical from the 
frst curvature. 
