272 MR. F.-DARWIN ON A METHOD OF INVESTIGATING THE 
. Experiments with Peas. 
My assistant Mr. Elborn made a series of observations on peas 
with the root-lever, of which the results were in one respect 
more satisfactory than my own experiments on beans. 
The tips were fixed in small quills filled with gypsum at the 
beginning of each experiment. A very thin and flexible silver 
wire was wound spirally round root- and tube; it was not fixed 
at either end, but merely tended by friction to keep the root in 
place. The water-supply was led by a thread so that the dis- 
turbance of water actually falling on to the root was avoided. 
In something like half the experiments a distinct result was 
obtained *—that is to say, the cotyledonary end of the seedlings 
curved downwards and then beyond the vertical. In a con- 
siderable number of experiments the root curved through 180°, 
z.e., 90° beyond the downward position. But it never reached 
such a degree of curvature as is shown in figs. 7 and 8. 
Conclusions. 
The evidence (in spite of the numerous failures in the case of 
the bean) seems to point clearly to the conclusion that there is a 
strong tendency, in the root of the bean and pea, to continue 
curving when the tip is fixed horizontally and the other end of 
the seedling is free to move. 
The conclusion to be drawn from this result is not so simple 
as at first appears. It is certain that the results are explicable 
on the assumption that the tip is the only part of the root 
which is sensitive to gravitation. But is this the only possible 
explanation ? 
It seems possible that just as apogeotrovic organs curve until 
their free ends are far beyond the verticar, so roots supported by 
their apices might, by the geotropism of the region close to the 
tube (A, fig. 9), assume the appearance shown in fig. 9, B, in which 
the base of the root between C and the cotyledons is beyond the 
vertical. In apogeotropic stems this over-shooting of the vertical 
is corrected by a new gravitational stimulus arising in the oblique 
part. But in roots in which the region of curvature is short it 
is conceivable that the region C may have ceased to grow, and 
therefore that the curvature may not be reversible: in other words, 
* T . . : 
in 7 out of 18 experiments the curvature was through 180°, in 3 it was 
between 135° and 180°, 
