Climbing plants are unable to stand upright by themselves, and make use of other plants as supports. In this way a climbing plant obtains a plentiful supply of light and air, and at the same time gains the advantage of being able to economize in the production of mechanical tissue, since the weight of the foliage is borne by the supporting plant. Theof are weak, bend easily and have little or no fibrous tissue. The xylem consists chiefly of large vessels, which in woody climbers are very long. The phloem is well developed and in some climbers, e.g. Cucurbita, occurs on both sides of the xylem. Climbing plants are common in hedges and woods, and reach their maximum development in tropical forests, where they grow to an enormous length, and are known as lianas. Climbing plants may be divided into the following classes, according to the way in which they climb :—
Such plants as the blackberry and wild rose have weak stems which scramble loosely over vegetation, and are provided with curved hooks. The hooks prevent the stems from slipping back and so enable the scrambler to keep its. Goosegrass is a small which clings to other plants by means of very numerous small prickles found on the edges of the and .
The bindweed is a plant which climbs by means of a twining. The tip of the stem moves round in a circle, and when it comes into contact with a support coils round it. The circular movement of the stem is caused by the unequal rate of growth of the sides, and takes place in a counter-clockwise direction in Convolvulus and runner bean, and in a clockwise direction in black bryony and honeysuckle.
A more specialized form of climbing is by tendrils. In white bryony these are long whip-like structures with a slightly curved tip. When a support which is not too thick is touched, the tip of the tendril coils round it, and thus the bryony plant is anchored to the support. Then the remaining part of the tendril twists in the form of a spiral, and since the tendril is fixed at both ends, the direction of the twist is reversed after several coils. The spiral coiling draws the bryony closer to the support, and also acts as a spring, so preventing the tendril from being torn off in a strong wind.
In thethe tendrils are developed from a part of the , and represent the modified upper leaflets. The whole of the of the yellow vetchling is modified as a tendril, and the function of assimilation is carried on by the enlarged stipules. The two tendrils at the base of the leaf stalk of Smilax probably represent stipules, and in clematis the leaf stalks themselves are able to coil round the stems of other plants, and so act as tendrils. The tendrils of the vine occur opposite the , and it has been suggested that they represent modified stems and that the continual growth of the vine axis has been carried on by axillary buds developing between the leaf and the tendril. The branched tendrils of the Virginia creeper also occur opposite the leaves, and are attached to the support by adhesive discs. When the tips of the tendrils touch a wall or the stem of another plant they flatten out into small discs which become firmly applied to the support, and also become stuck to it by the secretion of a cement-like substance.
The ivy climbs by means of numerous adventitiouswhich develop on the side of the stem next to the support. These small grow horizontally and become closely attached to the surface of a wall, or fit tightly into the crevices of the bark of a tree.