ORIENTATION OF PRIMARY TERRESTRIAL ROOTS 293 
curvature of parts of the root which have discontinued growth (cf. 
Simon, 1 91 2, S. 133 £f.) the residue of the primary curvature may 
become so flat as to be indistinguishable from the very gradual second- 
ary curvature. The failure of roots of Vicia faba and Lupinus alhus 
which are longer than 4 or 5 cm. when inverted to bend below the hori- 
zontal is probably due to a slackening of the geotropic sensibility with 
length. Roots of large diameter when executing a curvature must 
maintain a greater difference in the size of the cells of the upper and 
lower sides than would slenderer roots. It is not unlikely that on that 
account a relatively greater intensity of geotropic stimulus is neces- 
sary to bring about corresponding reactions. 
The behavior, described above, of roots variously placed in moist 
air is illustrated by the diagrams shown in figure 2. These diagrams 
were based upon a large series of camera drawings of roots of Vicia 
faba var. equina which were approximately 2 cm. long when displaced 
from the normal perpendicular position. Roots of Lupinus albus 
behaved in substantially the same manner. 
Observation of roots of these species and of Pisum sativum and of 
other leguminous species indicates that, as illustrated in figure 2, 
roots in air when displaced from the normal position by more than 
40° to 50° tend to take up a position in which the elongating region is 
directed obliquely downward, after which active curvature almost or 
entirely ceases. This fact does not however constitute conclusive 
evidence of the root having become plagiotropic. The failure of the 
root to undergo active curvature after this oblique position is reached 
can as well be explained by assuming that, after the primary curvature 
and its flattening have taken place, the relation between the autotropic 
and the geotropic impulse is altered in favor of the former and that a 
position of rest is then attained only after the inclination to the per- 
pendicular is reached at which autotropism and geotropism are in 
equilibrium. 
The crucial test for plagiotropism in the case of the roots in air 
is that which Nemec (cf. p. 290 of this paper) states that he apphed 
with positive results. I performed a number of experiments to deter- 
mine whether the supposedly plagiotropic roots would actually bend 
upward into an oblique position when directed perpendicularly down- 
ward. Roots which had grown for one or two days in an oblique 
direction in air (as shown at 4 and 5 in figure 2) were placed with the 
tip pointing directly downward, the roots remaining in moist air. 
