Most cultivars of ornamental and fruit species and forest trees are highly heterozygous. Consequently, their seed progeny is not true-to-type. To preserve the unique characters of selected cultivars of horticultural plants nurserymen practice vegetative propagation, using stem, leaf or root cuttings or propagules such as tubers, corms, bulbs or bulbils. For plants which do not set seeds, such as edible bananas, grapes, citrus, petunia, rose and chrysanthemum, vegetative propagation is the only means of multiplication. A population of plants derived from a single individual by vegetative propagation is genetically uniform and is called a clone. The conventional methods of clonal propagation are slow and often not applicable. For example, the only in-vivo method for clonal multiplication of cultivated orchids, which are complex hybrids is "back-bulb" propagation. It involves separating the oldest pseudobulbil to force the development of dormant buds.