316 
RICHARD M. HOLMAN 
curvature downward into the perpendicular, just as in the case of the 
secondary curvature of roots in such media which has already been 
discussed. Thus in spite of the fact that in the compact sawdust the 
beginning of visible reaction was considerable later than in moist air 
the roots in the former medium frequently reached the perpendicular 
before the roots in air had attained their maximum curvature. (See 
54 and yl4 in figure 7.) The curvature of the root in loose sawdust 
at the time the roots in air and the compacted sawdust had reached 
their maximum curvatures was less than either; less than that of the 
root in air because even the loose sawdust hindered the "swinging" 
movement resulting from the curvature of the zone of elongation 
and less than that of the root in compact sawdust because the re- 
sistance offered by the loose medium to the curved tip as it was thrust 
forward by the growth of the root was insufficient to rapidly depress 
the root tip. 
These differences in the primary curvature of roots in the different 
media used are due apparently to the different degrees of resistance 
offered by the media to change in the form of the root and to the 
advance of the tip. On account of the relatively great resistance 
offered by the compressed sawdust to change in the form of the root, 
the beginning of the reaction is delayed in the case of roots in this 
medium. When curvature does begin it is largely restricted, as has 
been said, to the part of the root which has advanced beyond the 
original position of the tip. In air the root is quite unhampered in its 
reaction and the curvature begins sooner and the zone of active 
elongation takes a more active part in the curvature than in the 
compact medium. In loose sawdust the curvature is at first very 
slightly affected by the resistance of the medium. The root is able 
to displace easily the particles of the loose sawdust and at first the 
curvature is similar to that of roots in air. The medium lying just 
below the curving region is compressed by the bending of the root and 
the resistance offered to further bending increases until it is sufficient 
to prevent further active curvature of the elongating region. Sub- 
sequent curvature is rather favored than hindered by the resistance 
of the medium, for the extreme tip becomes acutely curved, that is, 
takes on the tip curvature which has been described earlier in this 
paper, and, being pushed forward by the elongation of the region 
behind it the root curves slowly downward in the manner of roots 
undergoing secondary curvature in the same medium. (See pages 
307 and 308 of this paper.) 
