110 Newcombe, Gravitation seiisitiveness not confined to apex of root. 
vai of 2 mm from the apex, gave curves in two thirds of their 
number, while under the same circumstances, the roots of Ricinus 
gave no curves. 
Summary. 
1) AU the attempts thus far made to demonstrate the limi- 
tation of gravitation-sensitiveness to the apical two mülimeters of 
the root have failed, for the phenomena accord equaUy weU with 
the hypothesis of the extension of sensitiveness thru the elongating 
Zone, but diminishing from the apex backward; or the phenomena 
accord with the hypothesis of a more equable sensitiveness thru 
the elongating zone, and a stronger autotropism in the posterior 
than in the anterior part. 
2) Experiments on the centrifnge with beheaded roots show 
that gravitation sensitiveness is present, in most of the species em- 
ployed, more than 2.5 mm posterior to the tip, and in some species 
more than 4 mm distant from the tip. 
3) The kind of geotropic curve which an orthotropic root 
will make when displaced from its position of equilibrium de- 
pends upon the relation of its geotropism to its autotropism. 
In frequent cases, the autotropism of the root prevents the 
horizontaUy placed seedling from sending its roots vertically 
downward. 
4) The length of the elongating zone cannot be seen 
to have any relation to the extent of the geotropically sen¬ 
sitive Zone. 
University of Michigan, Ann Arbor, (U. S. A.). 
Nov. 30, 1907. 
Explanation of plate. 
The lower figure is made from a photograph of a preparation of Cucur¬ 
bita pepo^ 3.5 mm of the root-tips having been removed, and the preparation 
subsequently revolved on the centrifnge at 8 g for 6 hours. All the roots that 
have grown show outward curves. 
The upper figure is a similar preparation of Vicia faha. The roots had 
3.5 mm removed, and the preparation was subsequently revolved on the centri- 
fuge at 8 g for 6 hours. Here there are 6 pronounced outward curves, one 
inward, one obliquely outward, and 4 roots nearly straight. 
