82 Newcombe, Thigmotropism. of terrestrial roots. 



weak collodion sacs filled witli water and sunken in water, and 

 later against tongues of thin paper, rubber, and collodion, at- 

 tached at one end to a support and with tlieir cnrving surface at 

 the free end acting as constant but very weak Springs against 

 the sloping root-apex. In these experiments the responses were 

 few, but a majority were positive, so tliat the indication was 

 for a positive tliigmotroj)ic response of the root when an object 

 pressed against one side of the apical niillimeter. The responses 

 however were relatively so few and the angles attained so small 

 that one must conclude that the roots if sensitive in the part 

 pressed by the object are but feebly so. That the Immersion 

 of the roots in water destroyed their sensitiveness can hardly 

 be claimed, since such treatment does not destroy the sensitiveness 

 to Stimuli such as gravitation, light, a water-stream , etc. 



With the consideration of the sensitiveness of the apex of 

 the root to pressure, one thinks of the experiments of W i e s n er i), 

 and Detlefs e n^) which perhaps are of value in showing that the 

 root-tip is not negatively thigmotropic. 



The roots having shown but slight, if any, response when 

 the apical millimeter was pressed upon by some object, the elon- 

 gating Zone was next subjected to one-sided j^i'össure. When 

 the seedlings were kept in damp-chambers and pieces of paper, 

 glass, etc. were caused to adhere to one side of the elongating 

 Zone, no responses came. This result would indicate that one 

 may carry on such experiments with the elongating zone with- 

 out danger of interference from hydrotropism. Adhering ob- 

 jects however exert no pressure; but pressure was brought against 

 the elongating zone by setting against it a horizontally extend- 

 ing glass-rod. No curves were made by the roots in numerous 

 experiments. Drops of paste of plaster of Paris wdien allowed 

 to harden on one side of the elongating zone produced positive 

 curvature when in the growth of the root the plaster was 

 neither broken across nor loosened from its hold, and the drop- 

 let of plaster was always included at the apex of the angle of 

 curvature. It is evident that the curves were caused by the 

 resistance to growth offered by the hold of the plaster. 



A much more numerous series of experiments was niade by 

 causing pressure against the elongating zone while the roots were 

 immersed in water. The pressure of glass-rods brought no re- 

 sponse, not did the attachment of collars of paraffin left open 

 on one side, and grasping the elongating zone for about two- 

 thirds of its circumference. Tongues of collodion and rubber 

 tissue were caused to press as weak Springs against the elonga- 

 ting zone, but the results w^ere inconclusive. Collodion cylinders 

 one centimeter in diameter with very thin Avalls offering as pres- 

 sure only their own resilience, were creased when the roots were 



1) Wiesner, Das Bewegungsvennögen der PHauzon. Wien 1881. 

 p. 142. 



^) Detlef sen, Über die ■ von Darwin belianptete Geliirnfunktion der 

 Wurzeln. (Arbeit, d. bot. Inst. Würzburg. 1882. p. 627.) 



