52 Snow .— The Conduction of Geotropic Excitation in Roots. 
side leading to decrease of growth, and one on the convex side leading to 
increase, then clearly we need first to determine whether both of these can 
pass through the gelatine, or one of them only. It is hoped shortly to 
investigate this question, for, until it is settled, suggestions as to the relation 
between conduction in the gelatine and conduction in the living tissue are 
perhaps premature. Meanwhile, in Avena the feeble response when con¬ 
duction is only possible by the near side, and the stronger response after 
decapitation and replacement of the tip with gelatine, seem to indicate that 
at least the excitation conducted by the far side can pass through the 
gelatine. 
It may, however, be noted that Brauner ( 19 . 22 ) has observed in the 
Avena cotyledon a vigorous protoplasmic streaming, which might enable 
soluble substances to pass down it much faster than they could by simple 
diffusion. 
Summary. 
1. If the tips of decapitated roots of Vicia Faba are stuck on again 
with gelatine, the roots in most cases become again capable of bending down 
in response to gravity. It appears that excitation set up in the tip must 
be conducted back through the gelatine. 
2. If a cut is made half-way through the root from one side, at a point 
2 mm. from the vegetative apex, and a mica slip is inserted into the 
cut, to prevent diffusion across it, the root can still respond to gravity 
by bending down in the elongating zone when placed horizontal, whether 
the cut be on the upper or the lower side. Hence either the upper or lower 
side alone can conduct back excitation from the tip. 
3. If two mica slips are inserted half-way through from any two 
opposite sides at 2 mm. and 275 mm. from the apex, and the root placed 
horizontal, it does not curve down, or no more frequently than a decapitated 
root. Only curvatures above the second cut are here referred to. The 
excitation cannot therefore be effectively conducted back from the tip by 
a sinuous path. 
These results are briefly discussed. 
I wish to thank Professor Sir Frederick Keeble for encouragement 
during this investigation. 
The Department of Botany, 
Oxford, 
October 1922. 
