TENDRIL-CLIMBERS. Ill 



a plant much higher when coiled about a 

 thin suj)port than about a thick one. The 

 upward movement of any part of the plant 

 does not cease when it has coiled itself about 

 a support, especially if the support is smooth. 

 The coil of a twiner may be aptly compared 

 to a compressed spiral spring ; the coil be- 

 comes looser and slides up the support. If 

 the support is not a high one the coil will 

 sometimes bound off its top, and often the 

 shoot begins again to revolve. 



The immediate cause of the revolving of 

 the shoots of twiners is the lengthening of 

 the cells on one side of the shoot more than 

 on the other. When the cells elongate on 

 one side the shoot is bent over, pushed over, 

 and it becomes convex on that side. If now 

 the cells elongate still more a little to one 

 side of the first elongation, the greater con- 

 vexity wiU occur at that point, and the tip 

 of the shoot will be moved from its original 

 position. If this elongation were to travel 

 gradually all around the stem from the point 

 of starting, all the sides would in turn as- 

 sume the greatest convexity, and the tip 

 would have made a complete revolution. 

 The convexity of the shoots of twiners is 

 readily verified by observation. Each inter- 



