LIANES or LIANAS, a term applied originally to the climb ing plants—often of great size and with massive stems—of tropical forests. Schimper extended the term to include all climbing plants wherever occurring; the ivy, the honeysuckle and the traveller's joy (Clematis vitalba) of the English hedges would thus all fall into this class. The term is still, however, mainly used in its restricted sense and applied to tropical climbers. Climbing plants as a whole are those which take advantage of some other struc ture (other plant, wall, etc.) for the purpose of support, with the result of economy in the production of strengthening material in the stem; more material is thus available for the production of foliage and so for the production of food supplies. Plants differ very much in the degree of adaptation to this mode of life. Some may be described as ramblers since they merely scramble over the surrounding vegetation. Good examples of this class are the bramble or blackberry (Rebus fructicosus) and cleavers or goose grass (Galium Aparine) which raise themselves on plants associated with them in hedges. They are aided thereto by the thorns or prickles they possess which prevent the stem being pulled back once it has pushed itself upwards.
Others are root climbers. These are not very common in the European flora but ivy (Hedera Helix) is a familiar example. Large numbers of roots (adventitious in nature) are produced on the non-illuminated side of the shoot. These roots are not sensi tive to gravity and are negatively phototropic (see PLANTS : Physiology) ; as a result they penetrate any crevices in the wall and the bark of the tree on which they may be growing. These roots are "anchoring" roots ; they do not, as is commonly held, suck any nourishment from the tree on which they grow ; they may absorb a little rain water when they are young but they soon lose this power and become mere mechanical supports. Many members of the family Araceae and species of Ficus, Hoya, Norantea, Piper, Tecoma are root climbers.
Another form may be described as twining plants. These climb by winding their stems round suitable supports which must not be very thick nor too smooth, and must not be too far removed from the vertical position. Twiners generally possess long and thin stems and large internodes, and they show marked circum nutation, i.e., the apex of the stem, owing to regular cyclic variation in the rate of growth of its different sides, executes a circling movement in the air under the influence of gravity. Such a wide circling movement increases the chances of meeting a suitable support. When such a support is found the circumnuta tion causes the plant to wind round it in loose coils. Later the coils become tightened, i.e., their diameter is decreased, by the elongation of the stem. The spiral formed may be right- or left-handed according to the plant. It is best defined as "clock wise" or "counter-clockwise," i.e., whether, when seen from above, the tip of the stem twists round in the same or opposite direction as the hands of a clock. In the
British flora twiners are found in Humu/us (hop), Tams (black bryony), Calystegia (convolvu lus), Lonicera (honeysuckle), Solanum (bittersweet).
Another form is seen in the tendril climbers. These plants climb by means of slender whip like organs known as tendrils which are sensitive to contact and sometimes extremely so. If the tendril of vine (Vitis) or bryony (Bryonia) or Passiflora is lightly stroked on the lower side it will in a minute or two curve towards this lower side. As a re sult of this sensitiveness to a contact-stimulus (haptotropism as it is called) the growth rate on the side touched is decreased and that on the other side increased, and so the tendril winds round the support if of appropriate thickness, the act of winding bringing fresh surfaces into contact. Later strong mechanical tissue (sclerenchyma) develops in the tendrils, but only in those which have attached themselves to a support. The portion of the tendril between the support and its point of attachment to the plant frequently becomes coiled, the direction of the twist being re versed in the middle. This coil acts like a spring and renders the plant less liable to be torn from its support by a storm.