ORIENTATION OF PRIMARY TERRESTRIAL ROOTS 299 
gating region reached the perpendicular by a uniform curvature of 
the root. Roots treated as above but grown at temperatures between 
6° and 9° C. often execute so flat a curvature that the horizontal is 
not reached by the elongating zone for from 8 to 10 days. During 
this time, in the case of Vicia faba and Lupinus alhus the root may 
elongate as much as 6 cm. Yet there is no evidence of a plagiotropic 
condition, for the roots always attain the perpendicular eventually. 
Do roots which have undergone geotropic curvature in air lose their 
geotropic sensibility? 
It has already been shown that roots which have grown in an extra- 
perpendicular position in air for a considerable time curve clear to the 
perpendicular when brought into earth or even less compact media 
such as loose moist sawdust. It might be supposed that roots which 
have performed a geotropic curvature in air lose their sensibility to 
geotropic stimulus. Sachs, in fact, made this suggestion (1874, PP- 
455-456). It is easy to show that such roots are still capable of a 
geotropic reaction even while growing in air. If a root which has 
elongated for some time in an oblique direction is displaced until the 
elongating region is horizontal or inclined upward an active downward 
curvature takes place, provided only that the root is still actively 
growing. Of a large number of roots in air which I have tested I have 
found none in which it was not possible to call forth a distinct geotropic 
reaction as long as active growth continued. 
Do roots diverted from their normal position in earth undergo a rein- 
forcement of their geotropic curvature? 
There is no necessity for assuming an agency reinforcing the geo- 
tropic curvature of roots in soil in order to explain the oblique position 
in air and the perpendicular position in soil of roots which have 
grown twenty to thirty hours in these media after having been placed 
in a horizontal position. The fact that the root in soil is not free to 
flatten its primary geotropic curvature is sufficient to explain why 
the root in soil does not take up an oblique position as does the root 
in air. It is true that roots growing in air do not generally reach the 
vertical as the result of the primary geotropic curvature but the 
incomplete primary curvature of such roots is probably due to inter- 
ruption of the curvature by the autotropic counter reaction (cf. Simon, 
1912, Table X). 
But on the other hand the prompt curvature into the perpendicular 
when placed in soil of roots which have grown in the oblique position 
