464 
PHYSIOLOGY 
The most thorough experiments, however, have been made upon roots, 
and these seem to show that perception takes place mainly in the very tip, 
within a zone little more than a millimeter long, including the root cap. 
Indeed, the inner portions of the root cap itself are believed to be the 
cells most concerned. But the results also show that the growing region 
has some perceptivity. 
This conclusion rests upon evidence derived mainly from three modes of experi- 
mentation: (a) Decapitation. Cutting off the terminal millimeter or two leaves 
the root still capable of weak response, after recovery from the shock. (V) Me- 
chanical deformation. If root tips are made to grow into glass slippers (figs. 695, 
696), or against a glass plate, so that the terminal millimeter is bent at right angles to 
FIGS. 695, 696. Roots of Vicia Faba with tips in glass slippers : 695, a, b, c, three 
stages in the curvature of the same root, o to 20 hours ; 696, a, b, two stages of the same 
root; b, 18 hours after being placed in position a. After CZAPEK. 
the body of the root and therefore can be placed in the position of stimulation while 
the rest is not (or vice versa), responses show the dominance of the excitation at the 
tip over that in the growing region ; but the conclusion that the tip alone is per- 
ceptive is not warranted, (c) Rotation. Experiments in which the roots are fixed 
on a centrifuge, deviating 135 from their normal position, permit responses to be 
varied at will, according to the extent of the root tip beyond the axis of rotation. 
In all cases, if the stimulus to the growing region is to determine the response, it 
must be several times greater than is needed at the tip. Anatomical facts, in con- 
nection with the statolith theory of geoperception, support the physiological evidence 
above cited (fig. 697). 
Statolith theory. In its original form this theory was purely specu- 
lative. It postulated in the protoplasts of perceptive cells minute vacu- 
oles, beyond the limits of microscopic vision, filled with a fluid in which 
there lay granules of slightly greater specific gravity, that would fall 
to the bottom of the vacuole, whatever position it occupied, and rest 
against the cytoplasmic membrane bounding it. In the normal position 
of parallelotropic organs this would lead to no excitation; but if the cells 
