In tree-ferns they form a dense coating around, and completely concealing, the stem ; such is also the case in some Dra caenas and palms. In epiphytes, or plants growing in the air, attached to the trunks of trees, such as orchids of warm climates, the aerial roots produced do not reach the soil ; they continue always aerial and green ish, and they possess stomata. Delicate hairs are often seen on these epiphytal roots, as well as a peculiar spongy investment formed by the cells of the epidermis which have lost their succulent contents and are now filled with air. This layer is called the velarnen, and serves to condense the moisture contained in the air, on which the plant is dependent for its water-supply. Some leafless epiphytic orchids, such as species of Angraecum, depend entirely upon their aerial roots for nourishment; these perform the functions both of leaves and roots. A respiratory or aerating function is performed by roots of certain mangroves (q.v.), growing in soil or water and sending vertical roots up into the air which are provided with aerating passages by which the root system below can com municate with true outside air.
Parasitic plants, as the mistletoe (Viscum), broom-rape (Oro banche), dodder (Cuscuta) and Raftlesia, send root-like processes into the substance of the plants whence they derive nourishment. Leaf-buds are sometimes formed on roots, as in plum, cherry and other fruit trees ; the common elm affords an excellent ex ample, the young shoots which grow up in the neighbourhood of a tree arising from the roots beneath the soil. In some plants no roots are formed at all; thus in the orchid Corallorhiza, known as coral-root, a stem-structure, the shortly branched underground rhizome, performs all the functions of a true root, which is absent. In aquatic plants the root acts merely as a holdfast or is altogether absent as in Salvinia, Utricularia and others. The well known epiphyte Tillandsia usneoides (Spanish moss) is rootless.
For the interval structure of the root see PLANTS: Anatomy.