282 
RICHARD M. HOLMAN 
air become geotropically neutral after they have performed a geotropic 
curvature and flattened that curvature. 
Quite independent of any change in the geotonus of the root, there 
is another factor which may be conceived of as entirely responsible 
for the difference in behavior of roots in air and earth or as co-operating 
with a weakened orthogeotropism to bring about the difference in 
behavior. This factor is the assistance of the root in soil or other 
consistent medium in the execution of its reaction by reason of some 
property of the medium which does not alter the sensibility of the 
root to the stimulus of gravity. The assisting factor might be, as 
Sachs suggested, a positive thigmo tropic reaction in the same direction 
as the geotropic reaction. On the other hand, it might be conceived 
of as acting more directly, without another tropism being concerned. 
Thus the physical properties of the medium might be such that the 
root would be assisted mechanically in the execution of a prompt and 
complete reaction. Numerous examples can be cited where the rapid- 
ity with which a body undergoes change in direction depends upon 
the resistance offered by the homogeneous material through which 
it is moving. Thus the rudder of a boat, although able to change the 
direction of the boat rapidly in water would, at the same rate of speed, 
be unable to cause any but a very slight change of direction if the boat 
were moving through air instead of water. In a medium offering still 
more resistance than water to the passage of a body through it the 
same steering device would cause a more rapid change of direction 
than in water. Such a case may be suggested as illustrative of the 
possibility that the physical properties of some media may assist the 
root in executing a prompt and acute curvature. 
I shall first consider whether a change in the geotonus of the 
root growing in air does actually take place, and then whether the 
root in earth or similar media experiences assistance or reinforcement 
of its curvature. 
Is there a permanent change in the geotonus of roots which have grown 
and curved geotropically in air? 
It is important to determine whether or not the change of geotonus 
of roots in air, if such a change does take place, is a permanent one. 
In order to answer this question it is only necessary to transfer roots, 
which have curved and flattened their curvature in air, to earth, with- 
out changing their position relative to the perpendicular. Of a great 
number of roots of Vicia faba var. major and V. f. var. equina and 
