) 
Geotropic Sensitiveness of the Root-tip 1 . 
BY 
W. PFEFFER. 
RADICLE placed horizontally bends, as is well known, 
-<tV. until its tip points vertically downwards, that is until it 
has attained its normal position of equilibrium. This geotropic 
movement is occasioned by gravitation, which however merely 
operates as a stimulus ; that is, it affords the impulse in 
obedience to which the plant executes the necessary move- 
ment of curvature by means of the factors of growth at its 
command. 
In principle then, the case is something like that of a man 
in darkness to whom a single lingering ray of light gives in 
the same way the impulse to so change his movements as 
to make his way towards the light. It is plain that the 
processes involved in the movement by which such an act 
is accomplished are to be distinguished from the perception 
of the stimulus. Furthermore, as is well known in the case of 
the higher organisms, and as also frequently occurs in plants 
the sensitive organ may be separated by some distance from 
the organs that perform the external action. 
Such a relation exists in the root, in which the tip alone 
is geotropically sensitive. The root-tip therefore is the part 
1 Preliminary Communication. Read before Section D of the British Association, 
Oxford, August it, 1894. 
[Annals of Botany, Vol. VIII, No. XXXI. September, 1894.] 
