April 27, 1882] 
NATURE 
599 
away from the side on which the card was fastened, so 
that in some cases the root made complete loops by its 
continued curved growth. We believe that the tips of 
roots are sensitive to contact, and that when a root meets 
with an obstacle in its way the pressure on one side of the 
tip causes the growing part of the root to 
rapidly cn the side of the obstacle, and thus curve away 
from it. It is conceivable that a root should curve away 
from an obstacle, owing to the forcible bending of the 
root, just as any other ductile object would give way when 
forced against a fixed object It was from observing roots 
bending away from fixed obstacles placed in the way of 
their growth that we were led to suspect that the curvature 
was not due to forcible bending, but to a special kind of 
curvature, due to stimulus transmitted from the sensi- 
tive tip. 
Wiesner confirms our facts, but differs entirely in his 
interpretation of them. His explanation rests on certain 
facts observed by us, namely, that if the extreme apex of 
a root be wounded on one side by an oblique cut with a 
razor, or by a touch with lunar caustic, the same curvature 
occurs as when pieces of card are attached. Wiesner be- 
lieves that the spirits of wine contained in the shellac 
varnish which was used in many of the experiments to 
fasten on the cards, injures the root on one side, and that 
this accounts for the curvature which we ascribe to con- 
tact. But Wiesner’s criticism is incomplete on this point. 
He takes no notice of the experiments in which we used 
thick gum, instead of shellac varnish, for attaching the 
pieces of card to the tips of the roots. It is difficult to 
believe that gum injures the roots, since gum by itself 
produces no effect ; and it seemed pretty clear that it was 
the card, and not the gum, which acted, for when, as often 
occurred, the gum absorbed water and swelled up so that 
the card was separated from the root, no effect was pro- 
duced. Nor does Wiesner mention our experiment in 
which the shellac varnish was not attached directly, but to 
a small piece of gold-beater’s skin applied to the root. 
If Wiesner’s view is correct, then since the thick shellac 
varnish which we used hardens on the surface in a few 
minutes, it is clear that the injury to the root must take 
place in this short period. This however has not been 
- shown to be the case: 
Wiesner makes a great point of an experiment in which 
he causes small pieces of wood (p. 144) and sand to ad- 
here, by pressing them against one side of the root-tip 
without using shellac: under these circumstances he says 
that no curvature took place. This experiment does not 
seem to me so important as Wiesner would have us be- 
lieve, but for reasons given in the next paragraph I defer 
, further discussion of the whole question. 
A few words must be given to some of Wiesner’s other 
experiments on this subject. He caused roots to grow 
ugainst various more or less yielding objects in such a 
way that he could estimate the pressure against the root- 
tip, and found that the root did not bend from the touch- 
ing obstacle till the pressure is sufficient to cause forcible 
| bending. Here Wiesner’s experiments seem not to be quite 
{ conclusive, and I have begun to make experiments which 
1 hope will prove to be crucial. But I have been obliged 
to stop, because of the curious want of sensitiveness in 
the tip of the root which has occurred in a large propor- 
{ tion of the beans tried. I have repeated the experiment of 
gtow more 
fastening on pieces of sand-paper with shellac to one side 
of the root-tip, and in the large majority of cases no cur- 
vature en-ued. I have no means of knowing what the 
meaning of this failure may be (I am inclined to believe 
it is owing to the experiments being made in the winter), 
but it is clearly useless to pursue the experiments with 
such abnormal material. 
Wiesner seems to be in doubt whether or not the cur- 
vature of the root away from the injured side implies that 
a stimulus travels from the sensitive tip to the growing 
part of the root, whether, in fact, the injury to the root 
acts asa stimulus. The alternative proposed by Wiesner 
(p. 145), and which he thinks more probable, is that in 
consequence of injury, ‘‘the uninjured part of the root 
gets into a state which directly increases its capability of 
growth without any stimulus coming into action,” Wies- 
ner’s conception of stimulus (“ Reiz”) is not quite easy to 
understand, thus, for instance his definition of “ Reiz” 
(“Elemente der Anat. und Physiol. der Pflanzen,” 1881, 
pp. 249, 250) is an “‘influence whose mechanical effi- 
ciency (Leisting) is out of proportion to the resulting 
mechanical effect. A stimulus in this sense does not 
directly cause the movement, it merely releases the 
efficient forces, just as the pressure of a finger on th= 
trigger of a gun produces the explosion indeed, but 
stands in no relation to the force with which the ball is 
driven through the barrel.” In accordance with this 
definition he describes (“‘Anatomie,” p. 253) the nocturnal 
(nyctitropic) movements of the leaves of Robinia as 
‘€4 phenomenon of stimulation (Reiz-erscheinung) depend- 
ing on differences of Turgor.’’ Again, in his present 
work (“ Bewegungsvermégen ”) he gives (p. 25) a defini- 
tion of “ Reiz,” which is essentially the same as that 
already given. He adds, however, the characteristic of 
transmission to his definition :—“ Every irritable part has 
the power of transmitting the stimulus to neighbouring 
parts which are also usually irritable.” He goes on to 
say that the sleep-movements of leaves do not realise the 
characteristics of a Reiz-~Erscheinung, thus contradicting 
what he says in his other work. Whether we take the 
first or the second definition, it seems surprising that 
Wiesner should doubt that the curvature of a root when 
its tip is injured is a phenomenon of stimulation. If it 
were, as Wiesner is inclined to believe, the direct mechani- 
cal effect of the injury, the result would be curious ; 
for if injuring one side of the root causes increased 
growth on the same side as the injury, it is clear that 
injuring both sides symmetrically ought to increase the 
whole growth of the organ. And accordingly when the 
tip is cut off by a section perpendicular to the axis of the 
root, its growth ought to be accelerated ; which would be 
directly contrary to the results of Wiesner’s experiments 
on this point. 
Symmetrical injury to the tip looked at from our point 
of view could not of course be expected to lead to conse- 
quences of this sort. 
The next chapter in Wiesner’s book begins with the 
question of ‘‘spontaneous” nutations, and under this 
heading occur the hook-like curvatures of the ends cf 
shoots of Amrelopsis. This curvature Wiesner considers 
I It seems to me inconceivable that a stimulus should act indirectly ly 
releasing potential energy, as in the case cf discharging a gun, unless some 
kind of transmission of the stimulus takes place ; and if this is so, Wicsner’s 
two definitions are identical. 
