24 RELATIONS OF PLANTS TO MECHANICAL FORCES 



sides, or equally sensitive over all of the perceptive zone. Make 

 a number of tests to determine the sensitive surface of tendrils of 

 the species named in the above experiment. If possible compare 

 with some tendril of radial structure. Make a second series of 

 tests to determine the relative delicacy of sensitiveness of the dif- 

 ferent parts of the perceptive zone. Variations among different 

 species will be found. 



36. Summation of Stimulus-effects. Touch the irritable surface 

 of a tendril with a wooden rod for two or three seconds. Repeat 



with a second tendril which should 

 be irritated a second time after an 

 interval of five seconds. Note the 

 difference in the amplitude of the 

 resulting curvatures. The uncer- 

 tainty of giving the stimuli of equal 

 strength is such that the experi- 

 ment should be repeated two or 

 three times with different sets of 

 tendrils. 



37. Measurement of Force of 

 Contraction. Allow a tip of a vine 

 of the Passion-flower {Passiflord) 

 to rise above the top of a shelf or 

 table and fasten it firmly to an up- 

 right post by a cord tied around the 

 stem at the base of a tendril in a 

 Shoots of Smilax showing proper condition of irritability. Ir- 

 ritate the tip of the tendril until it 

 has clasped the arm of a spring dynamometer. Then fix the 

 dynamometer firmly in an upright position at such distance from 

 the stem that the tendril will be extended its full length. After the 

 contraction due to handling has been lost, again adjust the dyna- 

 mometer so that the tendril is extended its full length. Now rub 

 the sensitive surface with a pencil and note the contractile force 

 exerted upon the dynamometer. A stress of . 5 gram is quickly set 



Fig. 9 

 tendrils. 



