In fact, some plants live only in water. Plants living in water have special porous passageways that supply the roots with air. Other plants that grow outside of the soil are ones containing parasitic roots. These grow high in the air and live off the sap streams of other plants. Yet other roots live and multiply without having any true roots.
Plant propagation can fall into two categories, sexual and asexual propagation.
Sexual propagation is usually performed through seeds, and it involves two individuals, usually a male and a female. In addition to this, most forest trees are also formed sexually. On the other hand, asexual propagation is quite useful when identical genotypes of a plant must be reproduced. There are several forms of asexual reproduction, most of which I won't go into detail. These include grafting, vegetative reproduction, budding, and marcottage.