ORIENTATION OF PRIMARY TERRESTRIAL ROOTS 315 
pearance of asymmetry in the case of the roots in the compact medium 
was generally i to 2 hours (at 20° C.) after the roots in air had shown 
the first trace of a reaction. 
From the time of the appearance of the first trace of curvature in 
the case of roots in air the curvature involved more and more the 
region behind the extreme tip until the zone of most rapid elongation 
began to curve. (See ^3 in figure 7.) Meanwhile the roots in com- 
pact sawdust had curved downward somewhat. The curvature was 
restricted, however, almost entirely to that part of the root which 
was now beyond the original position (indicated in the diagram by 
the broken lines) of the root tip. In other words the region of the 
root which lay behind the original position of the tip was curved 
little if any. (See 53.) The difference in the curvature of the roots 
in air and of those in the compressed sawdust up to this point seemed 
to have its cause in the relatively great resistance offered by the 
compact medium to any lateral displacement of the root. The termi- 
nal 4 to 5 millimeters of the roots in air were free to undergo a consider- 
able downward displacement. (See ^3 in figure 7.) This "swinging" 
displacement of the part of the root beyond the region of most active 
elongation was in some cases so extensive that a part of the root came 
to lie 2.5 to 3.5 mm. below the original position of the tip. 
After the region of most rapid elongation had become involved in 
the curvature the bending of the roots in air continued, principally 
by the activity of the region of most rapid elongation, until the 
maximum curvature was reached. As the maximum was approached 
the bending became slower. In most cases the maximum curvature 
of the roots in air did not amount to 90°. Up to this point the roots 
in very loose sawdust behaved in a manner similar to those in air, 
except that the "swinging" downward displacement was hindered 
considerably by the resistance of the medium. It is clear that the 
loose sawdust much more effectively opposes the lateral displacement 
of 3 to 4 mm. of the root than it does the advance of the root tip in 
the direction of the root's axis. As a result the maximum primary 
curvature of the roots in moist loose sawdust was considerably less 
than that of the roots in air. The roots in the compact medium bent 
promptly and acutely downward as soon as the terminal 2 to 3 milli- 
meters of the roots became distinctly curved. As the curved ex- 
tremity of such a root was pushed forward through the firm medium 
by the increase in length of the elongating region, there ensued a sharp 
