688 



CLIMBING PLANTS. 



a favourable influence on the growth of the shoot as a whole. This pressure must 

 be regarded as a stimulus, just like the pressure which incites the tendrils, to be 

 described below, to luxuriant growth. We may therefore conclude that twining 



Fig. 160. Twining Hop (Humulus Lupulus). 



i Free end of a shoot recently emerged above the ground. 2 Shoot of Hop twining round an elder-stem ; natural size. 8 A 

 portion of the Hop stem magnified. *, * Single, anvil-shaped climbing-hooks detached from the stem ; more highly 

 magnified. 



stems are irritable, although the irritability in this case is not so conspicuous as in 

 tendril-forming structures. 



In the temperate zones the majority of twining stems have only a short life. 

 The twining Polygonum is an annual; hops and bindweeds are indeed perennial, 

 but their stems sent up fresh each year from the underground stock always perish 



