28o 
RICHARD M. HOLMAN 
III. Cause of the Difference in Curvature of Main Roots 
Growing in Earth and in Air 
As Sachs pointed out, there are two particulars in which the 
behavior of primary roots in air which are considerably diverted from 
their normal position differs from the behavior of such roots similarly 
placed in soil. 
First, the roots in the more consistent medium do not flatten their 
geotropic curvature. They undergo no change in the form of their 
curvature after the perpendicular is reached. The roots in air on 
the contrary, as soon as the geotropic curvature has reached a maxi- 
mum, flatten this curvature. Thus the terminal portion of the root 
comes to form a considerable angle with the perpendicular. 
Second, the roots in air after they have undergone geotropic curva- 
ture and flattening of that curvature may elongate in an oblique 
direction for several days and during this time only very slight further 
curvature, if any, generally takes place. On the other hand, roots in 
soil which have executed a geotropic curvature, if again diverted from 
the perpendicular and directed obliquely downward, again curve into 
the normal position. 
Now, with reference to the first of these points, there is no reason 
to doubt that roots which have curved downward in earth and other 
firm media possess the same tendency to flatten their curvature as do 
roots in air. That the autotropic tendency is not absent from roots 
growing in media which do not permit of a change of the form of the 
root's curvature is indicated by the results of experiments performed 
by Simon (1912, S. 137 ff.) with roots which had curved under the 
influence of gravity while growing in moist sawdust. When removed 
from this medium and kept in air these roots gradually flattened 
their curvature. In some cases the flattening was so extensive as to 
decrease by 80 degrees the angle formed by the portions of the root 
above and below the original curvature. This, however, demonstrates 
with certainty only that growth in a relatively firm medium, even for 
several days after the geotropic curvature has been completed, does 
not prevent the flattening tendency being realized after such roots are 
brought into the air. As I have frequently observed, however, the 
roots of Vicia faba and Lupinus alhus may immediately flatten their 
curvatures to a considerable extent when, after having executed a 
geotropic curvature in loose sawdust, they are carefully freed from the 
