Snow .— The Conduction of Geotropic Excitation in Roots. 51 
the lower zones of the grass cotyledons when connected with their tips, and 
loss of sensitivity in their absence. It seems rather that when the root tip 
is replaced with gelatine it must bring about response of perceiving the 
stimulus of gravity and transmit back the true geo-excitation through the 
gelatine, though a definite proof of this cannot yet be given. 
Incidentally, this result of Brauner’s shows, as he points out, that what 
is lacking in the decapitated cotyledon is neither the power of perceiving 
nor that of responding, but some intermediate phase of the tropistic pro¬ 
cesses. To call it insensitive is therefore perhaps misleading. 
As to the light which the gelatine experiments throw on the nature 
of conduction, the processes most naturally suggested as capable of passing 
through the gelatine are, as pointed out by Paal, the diffusion of soluble 
substances and the electric current. The latter he considers excluded in the 
case of Avena , as conduction was not found to take place through a platinum 
plate. But this conclusion seems hardly convincing, firstly because metals 
in contact with living tissues polarize very rapidly, and secondly, because 
short-circuiting would occur in the metal plate so that no current produc¬ 
tion in the tissues on one side of it could have any effect on those on the 
other side. 
Still, it certainly seems more likely that such conduction through non¬ 
living media is due to diffusion, especially since Ricca ( 1916 ), working with 
Mimosa , and Stark ( 19 . 21 , p. no), with Avena , appear to have extracted 
such soluble stimulating substances. 
Naturally, it does not at all follow that throughout all its course 
the conduction is effected by any such purely physical process. Indeed, it 
is most unlikely that it should be, in view of Fitting’s experiments ( 1907 , 
p. 219 seq.) on the effects of the local application of warmth and anaes¬ 
thetics to the conducting region in Avena. 
These agents were often found to prevent conduction, although not 
killing the tissue. In this respect Avena differs strikingly from Mimosa , in 
which conduction, now known to take place in the wood, can pass through 
stretches of anaesthetized or even dead stem. 
It seems, then, that in conduction of the Avena type the living tissues 
must in some way be actively concerned ; and accordingly it has been 
s uggested (Paal, 1918 , p. 432) that the conduction may be a composite pro¬ 
cess, in which excitation at one point leads to the production of soluble 
stimulating substances, which diffuse away to neighbouring points of the tissue 
and excite them in turn, with production of more of the substances, and so on. 
The purely physical part of this process might be able to pass through 
a layer of gelatine, but yet not be able to travel far through the living tissue 
unless aided by renewal of the excitation. 
But if, as the experiments on the paths of conduction have led us 
to suppose, there are two kinds of conducted excitation, one on the concave 
