RICHARD M. HOLMAN 
between the media in which the secondary curvature of primary 
roots is acute and those in which the curvature is relatively flat or 
lacking, as is often the case in air after the roots have reached an 
oblique position. It seems to be the last of these points of difference 
which we have considered, that is, the varying resistance offered by 
the different media to the advance of the root tip, which is the only 
one concerned to any extent in bringing about the differences in 
curvature. This varying resistance does not influence the form of 
the curvature by making possible a thigmotropic reaction which 
assists the geotropic curvature. It is rather the direct mechanical 
effect of the medium, and probably that alone, which causes the 
differences in secondary geotropic curvature. In the next section 
experiments will be reported which had for their object the deter- 
mination of the effect of the medium upon the primary geotropic 
curvature, that is, upon the curvature which roots which have previ- 
ously grown in the normal position perform when placed in a position 
of one-sided geatropic stimulation. 
What influence does the medium exert upon the primary geotropic 
curvature of primary roots? 
So striking is the difference in the secondary curvature in media 
offering different degrees of resistance to the advance of the root tip 
that the question naturally suggests itself, whether or not the primary 
curvature also is influenced by the resistance of the medium. The 
only reference which I have found relating to the course of the primary 
curvature in different media is the comparison made by Sachs (1874, 
S. 444-445) of the curvatures of roots of Viciafaba, Pisum, Phaseolus 
and Aesculus placed horizontally in air and earth. He stated that 
after 4 to 6 hours at 18° to 20° C. the roots growing in both media 
took on the form of an arc of a circle, the curvature involving the 
whole growing region. He stated further that after that time the 
radius of the curvature decreased, the decrease in curvature radius 
being most active in the region of most active elongation. Thus the 
curvature took on a parabolic form. After this the first difference in 
the form of the curvature in earth and air was observed by Sachs. 
He stated that in the latter medium the regions above and below 
the most strongly curved zone straightened somewhat so that the 
curvature became more localized and acute. Roots in earth, on the 
other hand, although somewhat more acutely bent in the elongating 
region than the roots in air, also showed considerable curvature above 
