Read this article to learn about Epiphytes: Distribution and Features !
Epiphytes (Epi = above, phyton = plant; i.e., plant growing upon plant) are those autotrophic plants which grow on the surface of some other supporting plants and are not permanently rooted in the soil Orchids, Bo tree (Pipal tree), some algae, lichens and mosses are some of the familiar examples.
These plants absorb sufficient moisture from the atmosphere and mineral nutrients from the decaying bark of the supporting plants upon which they are situated.
As they are autotrophic in nutrition they manufacture their own food (carbohydrates) from water and CO2 in presence of sunlight. These plants differ from parasites because they do not derive nutrients and water from the living parts of supporting plants, and also they differ from lianas (woody stem climbers) because epiphytes, in the real sense of term, are not permanently rooted in the soil. Epiphytes are also called Aerophytes or air plants.
Some epiphytes grow on the surface of submerged aquatic plants, while others may be aerial Some are found growing on the surface of tree trunks, some on the horizontal forks of the trees and some may grow even on the surface of leaves (i.e., epiphyllous epiphytes). Some epiphytes show specificity in selection of their supporting plants. Tortula pagorum an epiphytic moss is peculiar the sense that it grows on the tree trunks within the urban limits.
This moss grows well in the city atmosphere presumably because it requires high temperature and smoky air for its normal growth. Both these factors are available to the plants in the city area S epiphytic species may often grow on rocks, and some may grow rarely even on the poles and horizontal telephone wires.
Epiphytic vegetation is very rich in moist and cold regions but poor in dry and cold areas. In north-western Himalayas the epiphytic species are much less in number as compared to those present in the eastern Himalayas. In warm and wet regions, members of the families Bromeliaceae and Orchidaceae are found in abundance. In tropical rain forests, epiphytic species found at the tops of trees are xerophytic in nature but those occurring at lower levels are hygrophilous (moisture and shade loving).
Since the epiphytes depend directly for their water supply on rains, atmospheric moisture snow and dew, they develop certain structural adaptations for water storage and for reducing excessive water loss.
The important features are listed below:
1. Morphological Features:
(i) Root system:
In the epiphytic vascular plants, the root system is extensively developed. In these cases, the roots may be of the following three types (Fig. 9.1).
(a) Normal absorbing roots:
Which absorb water, minerals, and organic nutrients from tie moist crevices of decaying barks of supporting plants.
(b) Clinging roots:
These roots fix the epiphytes on the surface of the supporting object firmly and also absorb nutrients from the humus and dust that are accumulated on the surface of bark.
(c) Aerial roots:
These are spongy and green roots which hang downwardly in the atmosphere and absorb moisture from the air. These roots can photosynthesize in light because of the presence of green colour in them. In some epiphytes, the roots collect on their surface good amount of dust that holds water which will finally be absorbed by the roots.
Stem in epiphytic vascular plants may or may not be extensively developed. Some epiphytes develop succulence in their stems and become pseudo-bulbous or tuberous (Fig. 9.1).
The majority of epiphytes show considerable reduction in leaf number. Some orchis develop only a single leaf in a growing season. Leaves in some may be fleshy and leathery In Dischidia nummulana, Platycehum and Aspleniumnidus leaves are modified into the pitchers.
Dischidia nummulana, an epiphytic species of family Asclepiadaceae, growing very commonly in Sunder-ban shows peculiar types of leaf pitchers. The pitchers have openings through which the adventitious roots enter inside. The roots branch copiously into a number of very delicate rootlets which spread on the entire inner surface of pitcher and form a network (Fig. 9.2). The inner surface of pitcher is coated with wax. Pitcher collects and accumulates rain water, humus and minerals that are absorbed by the root network.
Sometimes ants and insects enter the cavity of the pitcher through hole where they may be killed and digested. The dead remains of animals serve as nitrogen source for the plants. Myrmecophily which is a sort of symbiotic association between ants and plants is of common occurrence in the epiphytic vegetation. In the family Bromehaceae, some species develop spoon-like leaves in rosettes. These leaves collect and store rain water which is finally absorbed by the epidermal hairs present on the concave surface of the leaves.
(iv) Fruits, Seeds and their dispersal:
The fruits and seeds are usually dispersed by wind insects and birds. When the seeds reach the suitable surface and get favourable environment’ they germinate over there and give rise to new independent epiphytes.
2. Anatomical Features:
Important anatomical peculiarities in epiphytes are as follows:
(i) Presence of thick cuticle and sunken stomata:
These two structures greatly reduce the loss of water from the plants. Generally the surface cells of water absorbing organs (roots and some leaves) are not cuticularized.
(ii) In succulent epiphytes, thin-walled parenchymatous tissue that stores water develops extensively.
(iii) The aerial hanging roots of many tropical epiphytes belonging to the families Araceae and Orchidaceae develop on their surface a characteristic greenish white thin-walled massive tissue. That is called velamen (Fig. 9.3). The velamen is hygroscopic tissue that rapidly absorbs moisture from the saturated atmosphere like a sponge. It is modification of multilayered epidermis. Its cells are empty (i.e., dead) and cell walls show spiral or reticulate thickenings. Inner to the velamen there is present a peculiar layer called exodermis.
Exodermal cells are of two types:
(a) Lignified and thick walled cells.
(b) Thin-walled cells or passage cells, walls of which are permeable to water. The velamen absorbs and retains moisture till that is absorbed by passage cells of exodermis.
(iv) Other structures are similar to those found in mesophytes.
Types of Epiphytes:
Schimper has classified epiphytes into four subgroups which are as follows:
These plants derive their nourishment partly from the surface of the supporting plants and partly from the atmosphere. They do not develop any adaptive feature in them except perhaps, aerial roots with velamen. Examples: Peperomia, Dischidia and some ferns belong to this group.
These plants grow on the supporting plants in the beginning like true epiphytes but later on they establish connection with the soil by their roots. Epiphytic Fig trees some root climbing Aroids, Scindapsus officinal is, etc. are important plants of this group.
Some stem climbing plants grow in the soil but their stems die from below upward and terminal portions live independently like hemiepiphytes. Such plants are termed as Pseudoepiphytes.
(3) Nest epiphytes:
These plants have appropriate devices to collect large quantity of water and humus for their own use. Orchids are familiar examples of this group.
(4) Tank epiphytes:
These plants develop fibrous anchoring roots which do not take part in the water absorption. Leaves, that are variously modified, absorb water and manufacture food Nidularium, Tillandsia, and other epiphytic species of Bromeliaceae are common plants of this group.