234 



ELEMENTARY STUDIES IN BOTANY 



tendrils are formed that either hook about a support, as the 

 grape-vine and star-cucumber (Fig. 201), or produce disk- 

 like suckers at their tips that act as holdfasts (Fig. 202), 

 as the woodbine or Virginia creeper (Fig. 203). The twin- 

 ing stem and the tendril exhibit responses to stimuli that 

 should be observed. 



If a young morning-glory or twining bean be watched 

 (Fig. 200), it will be discovered that the elongating stem is 



unable to stand upright, and 

 that, as it bends over, the in- 

 clined part begins to swing 

 through a horizontal curve, 

 which may bring the stem in 

 contact with a suitable sup- 

 port. If this happens, the 

 stem, continuing to swing in 

 a curve and growing in length 

 at the same time, winds itself 

 about the support. This 

 movement of the portion of 

 the stem which is in a hori- 

 zontal position is thought to 

 be brought about by a pe- 

 culiar response of the plant 

 to gravity. 



Tendrils are illustrations 

 FIG. 202. woodbine clinging to a wail of plant structures that are 



by means of tendril suckers. n , 



unusually sensitive to con- 

 tact. When the tip of a tendril in moving about touches 

 a suitable support, the side touched becomes concave and 

 the tendril hooks or coils about the support. This is 

 only the first response of the tendril to contact, for pres- 

 ently the rest of it begins to curve, a movement which 

 results in spiral coils, since the tendril is fastened at both 

 ends. This curving and twisting of the tendril between 



