TWINERS AND CLIMBERS. 215 



winds up into a close spiral (fig. 33, a), but if it 

 attaches its extremity to any object it winds itself 

 into a more open spiral for some distance, then 

 reverses, and winds in the opposite direction (fig. 33, 

 •b, c). The reason for this will be obvious if we 

 attempt to twist a piece of twine with its extremity 

 fixed ; the torsion will soon become so great that we 



Fig. 34. — Cleavers {Galium aparine). 



must cease or reverse the spiral. The latter move- 

 ment relieves the torsion, and the twist in the second 

 direction soon compensates the first. If any ten- 

 dril with its extremity attached be examined, this 

 reversal of the twist will be found of universal occur- 

 rence. Indeed, it must be so, as a physical necessity, 

 to which the tendril is compelled to submit. The 



