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INFLUENCE OF THE MEDIUM UPON THE ORIENTATION OF SECONDARY TERRESTRIAL ROOTS
Author(s) -
Holman Richard M.
Publication year - 1916
Publication title -
american journal of botany
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.218
H-Index - 151
eISSN - 1537-2197
pISSN - 0002-9122
DOI - 10.1002/j.1537-2197.1916.tb05422.x
Subject(s) - citation , orientation (vector space) , library science , computer science , mathematics , geometry
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 curvature 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 reaction to the stimulus of gravity is generally restricted, after the completion 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 sphagnam, which 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.