THE STEM 



85 



toward the prostrate. Below the surface, ps, occur only 

 underground stems. Is the prostrate habit an advantageous 

 one for Hght exposure ? Can you think of any compensat- 

 ing advantages a plant might derive from it ; for example, 

 in regard to warmth and moisture ? 



96. Climbing stems. — ■ These are such as lift themselves 

 from the ground and attain the advantages of the upright 

 position by clinging to supports of 

 various kinds — usually, in a state 

 of nature, the stems and boughs of 

 other plants. The means of climb- 

 ing may be : (1) by merely leaning 

 upon or propping themselves up by 

 the aid of the supporting object — ex- 

 amples, the rose, wistaria, star jessa- 

 mine {Jasminum officinalis) ; (2) by 

 coiling their main axes spirally 

 around the support — hop, bean, 

 morning-glory ; (3) by means of ad- 

 ventitious roots — poison ivy, com- 

 mon English ivy, trumpet vine 

 ( Tecoma radicans) ; (4) by organs specially developed for 

 the purpose, called tendrils — gourd, cucumber, grape, pas- 

 sion flower. 



97. Tendrils. — The part assigned to do the work of climb- 

 ing may be a secondary branch, a flower stem, a leafstalk, a 

 leaf, a leaflet, or a group of leaflets (Fig. 98). Tendrils behave 

 in general very much like twining stems, except that they 

 are more sensitive and respond more quickly to any cause 

 that may influence their movement. While young, their 

 tips revolve just as do the tips of twining stems, until they 

 meet with an object round which they can coil. When this 

 happens, not only the part in contact with the object coils, 

 but the free part between it and the main axis will usually 

 respond by twisting itself into a helix (Fig. 99). As the 

 distance between the base and tip of the tendril is shortened 



A 

 Fig. 97. — Twining stems : 



A, hop twining with the sun ; 



B, convolvulus twining against 

 the sun. 



