ey EE ee eee pe ee ee ee 
Se, eS 8) Pe ES ee Ae ep ee en See ae 
1897 | THE CURVATURE OF ROOTS 327 
tion in the following manner: Seedlings of Zea were placed in 
such position that the roots were pointing nearly vertically 
upward for a period of five hours and a curvature of 50° had 
ensued. At this time the root was reversed and placed in such 
position that a curvature would be induced in exactly the oppo- 
site direction. This curvature was allowed to proceed for fif- 
teen hours, until it was much more marked than in the first 
instance. The roots were then killed and hardened in chromic 
acid in the usual manner. The sections thus obtained show that 
such curvatures cannot be obliterated (see table XII). 
While in most roots the motor zone lies forward of the root 
hairs, sometimes the hairs may attain considerable length 
before the curvature is entirely accomplished. In Zea the papilla 
like extensions were to be seen often in the apical part of the 
motor zone three hours after excitation, and the tubes had 
attained a length equal to many times their diameter in curva- 
tures eight hours after excitation. That is to say, the zone of 
root hairs had moved forward until it embraced the region of 
curvature before motion had entirely ceased. No difference 
of structure or form could be made out between those of the 
convex and concave sides. It is to be seen that the movement 
would often result in the rupture of the hairs on the region of 
curvature, especially on the convex side. 
In this zone the annular vessels are represented by great cells 
with a length of 0.75 to 1.™, and a diameter of 0.2 to 0.3™. 
The nuclei are still present and a distinct lining layer. The 
remaining vascular elements are still in the form of elongated 
cells in which the protoplasmic content and no differentiation of 
the wall have appeared. The cortical parenchyma is in the form 
of short cylindrical cells with the ends in some instances slightly 
rounded and in others distinctly plane. | 
X. THE MECHANISM OF CURVATURE. 
In the examination of the curvatures of roots in order to 
€rmine the forces active in producing curvature I have used 
cimens of Zea mais, Phaseolus vulgaris, Pisum sativum, Ari- 
Ui 
