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 Hox) (Humulus Lupulus). 



1 Free end of a slioot recently emerged above tlie ground. 2 siioot of Hop twining round an elder-stem; natural size. » \ 

 portion of tlie Hop stem magnitied. *, s single, anvil-shaped clirabing-hooks detached from the stem; more highly 

 magnitied. 



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 



