CLIMBING PLANTS 
171 
vegetation, the etiolation-characters thus produced gradually 
leading to a twining habit and becoming hereditarily fixed. 
The actual mechanism of twining is not clearly understood. 
Nutation alone is not sufficient to account for it. The aid 
of negative geotropism, which is strongly marked in many 
twiners, is therefore invoked, but we cannot clearly explain 
the exact mechanism. 
The spiral may be right- or left-handed ; the same plant 
or species usually twines throughout in the same way. The 
direction may be defined as clockwise, i.e. in the direction 
of the hands of a clock, or counter-clockwise ; the terms 
right and left are used in opposite senses by different writers. 
The stem usually becomes twisted upon its own axis, but 
there is no relation between the number of turns of the spiral 
and the number of twists. The length of the internodes is 
usually sufficient to ensure the leaves not overlapping each 
other, and no special forms of phyllotaxis are found. 
In the British flora twiners occur in the genera Tamus, 
Humulus, Aristolochia, Polygonum, Calystegia, Convol- 
vulus, Cuscuta (this genus has sensitive stems, like tendrils), 
Solanum, Lonicera. Other genera of interest are Lygodium 
(fern with twining midrib of leaf), Ruscus, Cassytha, many 
Lardizabalaceae, Menispermaceae, and Malpighiaceae, Wis- 
taria, Phaseolus, many Loasaceae and Combretaceae, Plum- 
bago, Dipladenia, Cynanchum, Ceropegia, Hoya, Ipomoea, 
Thunbergia, Mikania, &c. Many of these have hooks on 
the stem, aiding them in clinging to their supports, e.g. 
Humulus, Dipladenia. 
II. Climbers with sensitive organs. These possess 
organs which are sensitive to continued contact and which 
move in response to this stimulus. As in other cases of 
adaptation, various parts have become adapted to this end. 
In the majority of these plants the organs are tendrils , long 
thread-like structures with rapid growth and marked nuta- 
tion. If one side of a tendril come into contact with a 
support, it grows less rapidly than the other side, and thus 
the tendril curves towards the support. This brings a new 
surface into contact and the movement becomes more 
marked, and so on until the free end of the tendril is all 
wound round the support. Afterwards the free portion of 
the tendril twists into a spiral and becomes woody, thus 
