THE ORIENTATION OF PRIMARY TERRESTRIAL ROOTS 
WITH PARTICULAR REFERENCE TO THE MEDIUM 
IN WHICH THEY ARE GROWN 
Richard M. Holman 
1. Introductory and Historical 
The orientation of plant organs relative to external agencies has 
long been a subject of interest, not alone to botanists but to those 
without botanical knowledge as well. The fact that the trunks of 
the trees on a steep mountain slope orient themselves without refer- 
ence to substratum and grow parallel to the direction in which the 
attraction of gravity operates illustrates no less forcibly than does 
the familiar bending of the stems of house plants toward the window 
from which they receive light the importance of external factors in 
directing the plant's growth. Similar phenomena are not uncommon 
among animals, although, aside from our own dependence upon gravity 
for the orientation of our bodies, there are about us fewer examples 
which are obvious to the untrained observer, of the directive effect 
of gravity, light and other external factors upon animals. There are, 
however, among those animals which, like most plants, remain at- 
tached during all or a part of their existence, many cases in which the 
orientation of the organism is dependent upon gravity, one-sided 
illumination or other external agencies acting in a definite direction. 
Unattached and motile animals can, in many cases, be shown to have 
the direction of their movements definitely determined by these and 
other external factors. Plants offer, however, more favorable material 
for the study of the directive influence of these agencies which are 
not diffuse in their application to the organism but which operate or 
can be caused to operate in a constant direction. The subterranean 
parts of the plant as well as the structures above ground are under the 
influence of various agencies which affect the direction of their growth. 
Chief among these is gravity, and the terrestrial root furnishes a par- 
ticularly favorable object for the study of the directive influence of 
gravity. More investigation has probably been devoted to the study 
of the geotropism of roots than to any other subject related to the 
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