A TEXT-BOOK OF BOTANY 



in other cases, as the grape-vine and star-cucumber, tendrils 

 are formed, which twine or hook about the supports (Fig. 

 48) ; in still other cases, as the woodbine, the tendrils pro- 

 duce suckers that act as 

 holdfasts and enable the 

 plant to cling to trees or 

 walls (Figs. 49 and 50). 

 It is in the dense forests 

 of the tropics that climb- 

 ing plants become espe- 

 cially conspicuous. There 

 great woody vines fairly 

 interlace the vegetation, 

 and are known as lianas 

 or lianes. 



If a young morning- 

 glory or twining bean be 

 watched, it will be dis- 



FIG. 48.-Branch of star-cucumber, with its covere d that the elonga- 

 tendrils in various conditions. 



ting stem is unable to 



stand upright, and that, as it bends over, the inclined part 

 begins to swing through a horizontal curve, which may bring 

 the stem in contact with a suitable support. If this hap- 

 pens, 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 peculiar 

 response of the plant to gravity. The influence of gravity 

 in directing plant organs will be considered later. 



Tendrils are illustrations of plant structures that are 

 unusually sensitive to contact. 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 presently the rest of it begins to curve a move- 



