4 6 4 



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. (b) 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 



