Certain plants always grow as vines, while few grow only for part of their life as low shrub or herbs when support is not available, and become vines in the presence of proper support. Climbing plants differ from self-supporting plants, such as shrubs and trees, in a range of characteristics; most notable is the mechanical properties of the stem. A vine is a growth-form based on long but flexible stems. This has two purposes. It may use rock exposure, other plants or supports for growth. This enables the plant to reach sunlight with minimum investment of energy. There are also some tropical vines that develop skototropism, and grow away from the light - a negative phototropism. Growth away from light allows the vine to reach a tree trunk, on which it can climb to brighter regions. Climbers constitute a large and important sector of ornamental horticulture. The annual flowers, of course, are an indispensable component of gardens, contributing, brilliance and an astonishing range of colour. Certain plants always grow as vines, while few grow only for part of their life as low shrub or herbs when support is not available and become vines in the presence of proper support.