116 BRITISH PLANTS 



from those of prostrate habit. The latter may easily, if 

 circumstances favour, become scramblers. True scram- 

 blers, however, are provided with special organs to aid 

 them hooks, prickles, barbs, or recurved bristles. These 

 grappling structures are not to be associated with the 

 sensitive climbing organs of true climbers, for they are 

 not sensitive to contact, and have no movements of their 

 own. The bramble or blackberry (Rubus fruticosus) and 

 the dog-rose (Rosa canina) are provided with large re- 

 curved hooks, which arise as emergences upon the stems 

 and leaves. In the moss-rose and Rosa spinosissima the 

 emergences are straight, and not fitted for climbing ; they 

 serve only for defence. The goosegrass, or cleavers 

 (Galium Aparine), scrambles up the hedges by its rough 

 stems and leaves, which are armed with a multitude of 

 small reflexed bristles. 



2. Twiners. The weak stems of these plants twine 

 round supports, provided that these are not too thick 

 nor too smooth. The growing tips are sensitive, and 

 describe slowly and regularly circles or ellipses. This 

 spontaneous movement is known as " circumnutation " 

 (Lat. circum, around ; nuto, I nod). In the black bryony 

 ( Tamus communis) the apex describes one complete revolu- 

 tion in about two and a half hours. By means of these 

 movements the stem is enabled to come into contact with 

 supports which it otherwise would fail to reach. On 

 reaching the support, the stem twines round it. Some 

 plants twine clockwise, others counter-clockwise, but for 

 every plant the direction is fixed, the habit having 

 become hereditary. 



(a) Twiners climbing counter-clockwise, the usual 

 habit : PTiaseolus vulgar is (scarlet runner), Wistaria 

 chinensis, Convolvulus sepium (Fig. 34), Polygonum Convol- 

 vulus, Cuscuta (dodder, Fig. 42). 



(b) Twiners climbing clockwise : Humulus Lupulus 

 (hop, Fig. 35), Tamus communis (black bryony), Lonicera 

 (honeysuckle). 



Wistaria and hop connect this group with the scram- 

 blers, the former having recurved spiny stipules, and the 

 latter T-shaped stiff hairs. 



Solanum Dulcamara {woody nightshade) is a feeble 

 climber, and may climb either way. In the open, where 

 the light is strong, it is a low bush, not climbing at all. 



