ORIENTATION OF PRIMARY TERRESTRIAL ROOTS 28 I 
surrounding medium. As a result of this immediate autotropic 
flattening the angle of the terminal portion of the root with the per- 
pendicular was in my experiments sometimes increased by as much as 
40 degrees. Table I shows the extent of this immediate flattening in 
the case of two roots taken at random from the large number observed. 
In this case moist sphagnum was the growth medium instead of saw- 
dust but in the latter material the results were similar. 
Table I 
Roots of Seedlings of Vicia faha var. equina Placed Horizontally in Loose Moist 
Sphagnum 
After 40 hours the curved portion was freed from the sphagnum. 
Root 
Number 
Original Length 
Angle with Perpen- 
dicular After 40 Hours 
Angle After Release 
Region Involved in 
the Flattening 
I 
2 
6.2 cm. 
7.0 cm. 
14° 
9° 
40° 
5.8 cm. 
3.0 cm. 
This immediate flattening or "springing" of the root after release 
from the relatively firm medium surrounding it is evidence that in 
such media the same changes in the curved region of the root take place 
which result in the flattening of the curvature in the case of roots in 
air. The resistance of soil prevents any change in the form of the 
root's curvature. There is nevertheless an autotropic reaction which, 
however, results only in a pressure being exerted by the root upon the 
material above it. No further explanation than the mechanical 
hindrance to change in the root's form is necessary, then, for the first 
of the two mentioned particulars in which the behavior of roots in air 
and in earth differ. 
There is no such simple and obvious explanation for the second 
point of difference — i. e., the fact that roots in air may grow almost 
straight ahead in an oblique position for several days while in earth 
they grow straight only when in the normal perpendicular position. 
According to Sachs's (1874, 456) explanation, roots in air undergo a 
lessening of their geotropic sensibility and later Elfving also concluded 
that there is a lessening of the geotropic sensibility of the roots in air. 
Nemec (1901, b) advanced the idea that the roots in air undergo such 
a change in the perceptive apparatus that they become actually plagio- 
tropic. In addition to these two hypotheses, there is a third possi- 
bility relative to the geotonus of the root, which is that the roots in 
