ORIENTATION OF PRIMARY TERRESTRIAL ROOTS 289 
Before discussing this question it is well to call attention to the 
curvature, often very acute, of the terminal i to 2 mm. of roots growing 
out of the normal position in air. Figure i illustrates the form of 
this curvature in the case of several roots used in my experiments. 
This curvature of the extreme tip of the root first makes its appear- 
ance after a root has begun to flatten its primary geotropic curvature. 
From that time on, it persists as long as active growth continues, 
provided that the root is well supplied with water and that the elon- 
gating zone of the root does not lie in the perpendicular. As the root 
elongates the curvature is maintained at the very tip. When the cells 
of the curved tip pass into the phase of active elongation, the difference 
in the size of the cells of the upper and lower sides of the root is com- 
pensated so that the elongating zone is only slightly or not appreciably 
curved. In my preliminary experiments, I was struck by the constant 
occurrence and conspicuousness of this curvature of the extreme tip 
of the root. Later I found that Nemec (1901, a, S. 93 ff.) had de- 
scribed it. It is remarkable that previous to the appearance of 
Nemec's paper, no one had reported this peculiar behavior of the 
root tip. This may, perhaps, be accounted for by the failure of 
earlier investigators to maintain a sufficiently high moisture content 
of the air in which they cultivated roots. One of Sachs's figures 
(1874, Fig. 10^) shows this curvature of the tip quite unmistakably, 
but his paper makes no reference to it. A similar geotropic ''counter 
curvature" of the tips of roots whose elongating zones were directed 
obliquely downward in consequence of a rheotropic reaction was 
described and figured by Berg (1899) and also mentioned by Juel 
(1900, S. 352) and Newcombe (1902, p. 269, ff.). The importance of 
this curvature of the tip in connection with the orientation of roots 
in different media will soon become apparent. 
Now an orthogeotropic organ may be defined as one whose only 
position of rest is the perpendicular. Such an organ, when placed 
in any other position tends to curve until its sensitive zone is again 
in the perpendicular (cf. Jost, 1908, S. 530). Similarly a plagio- 
geotropic organ is not merely one that undergoes no curvature when 
placed in a horizontal or oblique position but is an organ which tends 
to bend back into the original position when removed from an oblique 
or horizontal position.^ 
1 Except when the organ concerned is placed with its long axis parallel to an 
earth radius. Czapek has shown that for secondary roots the perpendicular is a 
labile position of rest, regardless of whether the root tips point upward or downward. 
