LOCALIZATION OF THE PERCEPTIVE ZONE 23 



32. Determination of the Character of the Bodies which may 

 Act as Contact Stimuli. Dip a glass rod in liquid gelatine, and 

 after it has solidified so that it will not drop from the rod, touch 

 the sensitive surface of the tendril with it. Does a reaction fol- 

 low ? Set the rod aside until the gelatine hardens and presents 

 a rough outer surface. Repeat the experiment, and note result. 

 The occurrence of a response to the contact of a tendril with a 

 body would appear to depend upon the smoothness of the surface, 

 or rather the size of the minute projections which press against the 

 convex outer surfaces of the receiving surface. Soften the dried 

 gelatine in water and dip in fine sand. Touch the tendril again, 

 and observe results. 



33. Transmission of Impulses in Tendrils. Fasten a small 

 thread taut between the arms of a pair of calipers, moisten in In- 

 dia ink and touch the sensitive surface of a tendril at one point 

 which will be marked with a coating of ink. Note the limits of 

 the region in which curvature ensues. To what extent is this 

 stimulus-effect transmitted ? 



• 34. Tetanized Condition of a Tendril. Fasten a cord or wooden 

 rod in such position that the tip of a tendril will come in contact 

 with it, and coil around it. Remove the cord or rod two hours 

 after the beginning of the experiment. Repeat, allowing the 

 tendril to clasp the support for three, four or five hours. How 

 long may the tendril remain in a contracted condition and return 

 to the normal ? All of the curvatures are fixed after a time by 

 growth-alterations in the cells of the tendril so that it is unable to 

 uncoil or relax as did the leaflets of Mimosa or the stamens of 

 Opuntia. The fixation of the curves of the tendril in this manner is 

 a secondary phenomenon which enables the slender climbing plant 

 to fasten its body permanently to a support. The pressure on the 

 inner surface of the tendril acts as a second stimulus upon the 

 growth processes, causing an exaggerated elongation of the con- 

 vex side of the organ and decreased extension of the concave flank. 

 35. Localization of the Perceptive Zone. Tendrils which are 

 flattened or show a bilateral structure are not sensitive on both 



