The below mentioned article provides an essay on epiphyte.
Epiphyte forms an ecologic group of plants that grow in substrate where there is no soil. They grow and remain attached to another plant. Epiphytes use the other plant merely for support. They do not grow parasitically.
Trunks and branches of trees serve as supports of epiphyte but cannot retain water for their provision. The habitats of epiphyte are always dry except during rainfall. The climate, whether humid or arid, has no special effect on habitat. So epiphytes are regarded as special kind of xerophytes.
There exists no relation between an epiphytic seed plant and substrate. As for example Tillandsia usneoides (so named because the plant resembles the lichen Usnea) belonging to the family Bromeliaceae grows on telephone wires, depending entirely upon air-borne nutrients. The whole plant is thickly covered with scaly hairs with thin cuticle, which collect and absorb rainwater. The plant is not adapted to store water.
Epiphytes are common among monocotyledons especially among the families Orchidaceae, Bromeliaceae and Araceae. These seed plants are usually succulent. The plants have three kinds of root—clinging root, absorbing root and freely suspended aerial roots.
An epiphyte is attached to the supporting plant with the help of clinging root. Moreover the clinging roots form a network where humus accumulates in meshes. The absorbing roots originate from clinging root and draw nutrition from humus. The hanging aerial roots are covered by a velamen.
Velamen is a multilayered tissue formed by periclinal divisions of protoderm. So velamen is regarded as multiple or multiseriate epidermis. Each cell of velamen is thick walled, devoid of any contents, nonliving, horizontally elongated or polygonal in shape. The cells are compactly set and air-filled.
Some cells have band like or reticulate secondary wall thickenings. The cells have fissures and pores on their walls. Velamen becomes filled with water during precipitation. Exodermis occurs just below velamen. The continuity of exodermis is interrupted by the presence of thin walled passage cells.
It is assumed that water from velamen is transported through passage cells to the inner tissues that are living, but this has not been verified experimentally. Though velamen is interpreted as absorptive tissue, but some physiological studies on orchid velamen indicate that velamen is concerned with mechanical protection of inner tissues and prevention of excess loss of water from cortex.
Special structure termed pneumathode occurs in the velamen. Pneumathode has some loosely packed cells with dense spiral wall thickenings. These cells form an outlet of the ventilating system of epiphyte. Exchange of gases is facilitated through pneumathodes when velamen is saturated with moisture.
Aerial roots regularly have chloroplasts in the cortex. In Taeniophyllum, the main photosynthetic activity is restricted to cortex of aerial roots. Apart from normal functions, leaves are also concerned in water absorption and it is of special importance among epiphytes. Epiphytic Bromeliaceae develops scale-like hairs with thin cuticle that facilitate water uptake.
Some special features of the anatomy of suspended aerial epiphytic roots are mentioned below taking Vanda (Fig. 29.14) as an example:
(1) Presence of protective tissue – velamen,
(2) Chlorophyllous cortex,
(3) Aerenchyma in cortex,
(4) Radial and polyarch xylem with exarch protoxylem,
(5) Centralized mechanical cells like sclerenchymatous conjunctive tissue, and
(6) Sclerotic pith in mature root. 