200 CLIMBERS, PARASITES, SAPROPHYTES. 



(e) What happens if a thick stick or other thick support is used 

 instead of a thin one ? Use a large pot or a box, and place in it a 

 thick cylinder of wood or a tube of cardboard ; try several different 

 thicknesses (diameters of 2, 4, 6, 8, 10 inches). The stem will not 

 twine round a thick support (over about 3 inches diameter), evidently 

 because the support is not curved strongly enough to allow the stem 

 to hold on while the free growing tip swings round the support. 



(/) Does the stem become twisted as it revolves? The best method 

 for finding out is to paint a line along the convex side of the stem and 

 watch what happens during a revolution. The line is seen to run 

 round the stem, so that at the middle of a revolution it is on the 

 concave side, while at the end of the revolution it is again on the convex 

 side. The growth of the stem is not equal all round ; a wave of rapid 

 growth passes round the revolving part of the stem. 



231. Native British twiners include Hop (Figs. 63, II.), whose 

 stem bears prickles which help in catching on to the support ; Honey- 

 suckle (Art. 398) ; Bittersweet (Art. 395) ; Great Convolvulus 

 (G. sepium, with large white firs. ) and Field Convolvulus (G. arvensis, 

 with small reddish firs.) ; Black Bindweed (Polygonum convolvulus), 

 like Field Convolvulus in habit, with small flowers and with the sheath 

 (ochrea, Art. 165) around stem at base of leaf, which is characteristic 

 of the Polygonaceae ; Black Bryony (Tamus communis), a Monocoty- 

 ledon (though its glossy dark-green stalked heart-shaped leaves are 

 net- veined) ; Dodder (Guscuta), which differs from other British 

 climbers in being a parasite (Art. 236) and in the fact that its stem 

 acts like a tendril, being irritable to contact and friction and not 

 transversely geotropic. 



232. Boot Climbers. A few plants climb by means of 

 attaching roots. The only native British example of a root- 

 climber is 



Ivy (Hedera helix), which climbs on walls, tree-trunks, etc., by 

 means of numerous short roots springing from the stem along the 

 shaded side ; sometimes it creeps along the soil, especially on banks. 

 It is not a parasitic plant ; when growing on a tree-trunk the roots 

 simply fix the plant to the bark and do not penetrate the trunk or tap 

 the tree's supplies of water and food. The Ivy gets its water and 

 dissolved salts from the soil by its main root, the climbing roots 

 serving for fixation to the support. 



Examine Ivy plants growing in different positions, e.g. on level 

 soil, on sloping banks, on trees, on walls. Note the arrangement and 

 form of the leaves. On flowering shoots the leaves are arranged 

 spirally, in five rows, and the blades are not lobed ; these shoots are 

 only formed when the plant is well exposed to the light (why ?), and 

 they project from the rest of the plant and have no climbing roots. 

 On the ordinary branches the leaves are arranged in two rows along 



