Snow. — The Conduction of Geotropic Excitation in Roots. 49 
13. Twenty-four beans were treated as in Experiment 12, but pinned so 
that the two cuts and mica slips were lateral on the horizontally placed 
root. Twenty-one remained straight, one bent down, two bent slightly down. 
If the results of these two experiments are compared with those for 
decapitated roots in Experiment 1, it will be seen that there is no significant 
difference. 
It thus appears that the geotropic excitation cannot be conducted 
in a sinuous path so as to pass round two cuts made from opposite sides and 
filled with mica slips. There is indeed no reason to suppose that it travels 
otherwise than in straight lines. Further, since, as shown in Experiments 9 
and 10, geo-excitation can be effectively conducted back along either an upper 
or a lower half-root alone, it appears that these excitations, if they travel in 
straight lines, must be of two different kinds, one in the upper half leading to 
relative increase of growth, and one in the lower half leading to relative 
decrease. A similar state of things appears to hold for the responses to light 
and gravity of the Arena cotyledon. For there, according to Purdy ( 1921 ), 
photo-excitation can travel down from the tip either by way of the side 
farthest from the light alone, or, less easily, by the side towards the light 
alone, and in either case can still bring about positive curvature. Similarly, 
geo-excitation can travel back from the cotyledon tip by either the lower 
side or, less easily, the upper side alone. She reports, however, no experi¬ 
ments on the critical question whether in Arena conduction can or cannot 
take place by a sinuous path. 
Discussion. 
A decapitated root, as is well known, does not curve down in response 
to gravity, or only seldom. Yet this is not because it is unable to respond, 
for if the root is first exposed to gravity while intact, and then decapitated, 
it curves down normally. The simplest explanation of this, as suggested 
by Charles Darwin, is that the decapitated root is unable to perceive the 
stimulus of gravity, since perception takes place in the tip. In conformity 
with this, we have found that if, after decapitation, the tip is stuck on again 
with gelatine, the reconstituted root is able to perceive the stimulus of 
gravity and respond to it. It is, then, natural to suppose that the tip 
perceives the stimulus normally and transmits back excitation through the 
gelatine layer. 
And nothing now stands in the way of this explanation. For, firstly, 
it is beyond doubt that excitation can be conducted through a gelatine 
layer, as in the case of the Arena cotyledon ; and, secondly, it can hardly be 
doubted now that in the intact root excitation due to stimulus of gravity is 
transmitted back from the tip. For, as further evidence of this, besides 
E 
