INFLUENCE OF THE MEDIUM UPON THE ORIENTATION 
OF SECONDARY TERRESTRIAL ROOTS ^ 
Richard M. Holman 
In a recent paper^ I called attention to the inadequacy of the 
explanations put forward by earlier investigators for the striking 
difference in the behavior of primary roots which have been diverted 
from their normal position while growing in air or water, on the 
one hand, and earth, sand or other non-fluid medium, on the other 
hand. My experiments led to the conclusion that, after the primary 
root in air or water has flattened the primary geotropic curvature, a 
considerable resistance on the part of the medium to the advance of 
the root tip is a necessary condition for a subsequent complete curva- 
ture of the root. After the flattening of the primary curvature, the 
root in air grows straight ahead in an oblique position. Although the 
increasing length and weight of the root may result in its reaching 
the perpendicular through bending under its own weight, active re- 
action to the stimulus of gravity is generally restricted, after the com- 
pletion of the autotropic flattening, to the extreme tip. This tip 
curvature, although varying in intensity, is maintained as long as the 
root is well supplied with water, is actively growing and has not 
attained an approximately perpendicular position. In such media as 
earth, sand, sawdust, and sphagnum, w^hich offer more or less resistance 
to the root's advance, a root with such a curvature of the tip curves 
downward into the normal position in a curve whose radius is smaller 
the greater the resistance offered to the advance of the root. This 
downward- curvature is apparently due to a passive depression of the 
root resulting from the non-symmetrical application (relative to the 
axis of the root) of the force opposing the advance of the root tip. 
These conclusions, arrived at as the result of a study of the behavior 
of the primary roots of Vicia faba, Lupinus albus, and Pisum sativum, 
naturally suggested the question, whether or not the secondary roots 
of these species showed any relation between geotropic behavior and 
medium such as exists in the case of primary roots. 
1 The orientation of primary terrestrial roots with special reference to the medium 
in which they are grown. Amer. Journ. Bot. 3: 274-318. 19 16. 
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