108 LI IK : OUTLINES OF GENERAL BIOLOGY 



barbed branches which help them to insinuate themselves. Very 

 familiar, too, is Jack- Run- the-Hedge {Galium aparine), entirely 

 unable to siipix)rt its<If, yet helped up by the backward-turned 

 hooks on its stem and leaves. In the end it is often highest of all 

 the plants in the hedge, sometimes crowning the successful bramble. 

 .\nother common hedgerow plant is the Hittersweet {Solanum 

 dulcamara), a near relative of the nightshade and the potato, which 

 is on the whole a "leaner" on its neighbours, but illustrates the grada- 

 tions that everywhere occur by being also to some extent a twiner. 



RooT-CLiMBi'RS. — Conveniently by themselves are the root- 

 t limbers, like the linglish Ivy, in which the stem gives off adventi- 

 tious roots binding tlie plant to its support. In the Ivy these 

 attaching roots, usually turning brown in colour, have entirely lost 

 the normal absorbing function. They are negatively heliotropic — that 

 is to say, they grow out on the shaded side of the stem. In experi- 

 mental conditions of very dim light they emerge all round. Among 

 the other root -climbers may be mentioned the common Tecofna 

 radicans of tlie Southern States, the Vanilla orchid, the often- 

 cultivated (limbing Fig, various Arums and Bignonias. It should 

 be noted that some have absorptive as well as attaching rootlets, 

 and that attaching rootlets may occur on climbers that also use 

 other methods, as in some Virginia Creepers. 



TwiNKRS. — A higher grade is that of the twiners, well illustrated 

 by hop and honeysuckle, and by many of the lianas of the tropical 

 forest. The growing tip of the main stem is constitutionally compelled 

 to grow against the pull of gravity, and it exhibits "a bending and 

 bowing movement" or "revolving nutation", usually to the left, 

 or opposite to the hands of a watch, as in dodder and bindweed, but 

 sometimes following the sun, as in hop and honeysuckle. As Darwin 

 said, the youngest part of the growing hop shoot "may be seen to 

 l)end to one side and to travel slowly round towards all points of 

 the compass, moving, like the hands of a watch, with the sun". 

 Some twiners, such as the Chili Nettle (Loasa) and the Bittersweet, 

 move indifferently in either direction. Darwin found that the 

 average pcTiod for a complete revolution in the hop on a summer day 

 is 2 hours <S minutes; but the rate varies greatly for different plants. 

 It is slower at night. If a growing twiner is inverted, it reverses the 

 direction of its movement; and it will not twine unless gravity is 

 acting on it in the erect position, and unless an almost vertical 

 support is at hand. 



If the growing part strike on a suitable support, it is arrested by 

 the contact ; but as the free portion continues growing and nutating, 

 the same occurs higher up; and .so the twining goes on. As some of 

 them attain to a great height, as is sometimes illustrated by the 

 honeysuckle, it is not surprising to fmd a special development of 

 sap-conducting tissue. 



