THE STEM 85 
toward the prostrate. Below the surface, ps, occur only 
underground stems. Is the prostrate habit an advantageous 
one for light 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- , $7 aL ee a Pay 
ventitious roots — poison ivy, com-_ B, convolvulus twining against 
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 
