ORIENTATION OF PRIMARY TERRESTRIAL ROOTS 3OI 
body through the medium, which would increase as the compactness 
of the medium was increased. I shall consider in turn the possibility 
of each of these properties contributing to the difference in the second- 
ary curvature in loose and in compacted sawdust or sphagnum and 
A 
2CtT2 
u 
Fig. 4, Curvature of three roots of Vicia faha placed horizontally in air (A), 
loose moist sawdust {B), and firmly compressed moist sawdust (C) after the roots 
had first curved downward in air and flattened the curvature. 
to the difference in the curvatures taking place in air and in earth. 
Except when the roots in air are almost constantly sprayed, they are 
less abundantly supplied with water than when growing in moist soil. 
Moreover gas interchange is free in the case of roots in air and no 
appreciable resistance is offered to the advance of the root tip. 
Clearly the amount of water at the disposal of the roots is greater 
