Epiphytes are plants that grow on other plants but they do not absorb any food from their hosts, even though they have no connection with soil. They depend on either rain or dew for water. Sometimes, they absorb moisture from the atmosphere. They derive their nutrients from wind-borne dust and also from the decaying bark of the supporting part.
So let’s find out what are epiphytes, their characteristics, adaptations, and more.
What are epiphytes?
Epiphytes are common on trunks of forest trees but are uncommon on shrubs as well as on submerged plants. The association between epiphytes and host plants is not symbiotic in nature, since they do not derive any substantial benefit from these associations. However, the epiphytic plants derive nutrients from the bark of the host without doing damage to their hosts.
Facultative epiphytes are an important type of epiphytes that grow on trees and on ground.
The terrestrial flora is very rich in epiphytes. In cold and dry climates, algae, bryophytes, lichens, etc form the chief epiphytic flora. During warm wet climates such as in tropical rainforests, the epiphytic flora is represented by ferns, angiosperms, etc.
Many angiosperms belonging to the family Bromeliaceae (pineapple) and Orchidaceae form the common epiphytes in tropical regions. Vando, and Dendrobium, are important examples. Epiphytic plants are not always seen on other plants. They can grow on walls, rocks, etc as well.
Some epiphytes start their development as rooted plants and after some time, they lose this connection and grow only on other plants. Thus, they form true epiphytes. But others start as epiphytes and later develop their roots with which they are fixed to soil. These epiphytes have no more relation to the host plants.
Classification of Epiphytes
According to the mode of life, epiphytes are classified into four by Schimber.
Proto epiphytes
- Derive water and minerals from the surface of the host plant and the atmosphere.
- Often produce velamen roots.
- Eg. Ferns and Vanda.
Hemiepiphytes
- Similar to epiphytic plants as they grow on host plants.
- Gradually become independent later on by connecting to the ground with their own roots.
- Similar to ordinary terrestrial plants with their mode of nutrition.
- Eg. Root climbing plants such as Sciandaspus officinalis.
Nest epiphytes
- Collect large amounts of water and humus in their entangled roots that form nest-like structures.
- Eg. Orchids such as Cymbidium form typical nest epiphytes.
Tank or Cistern epiphytes
- Roots form just the organs of anchorage and do not take part in nutrition.
- Such roots are very thick due to the presence of thick-walled fibers.
- The absorption of water takes place through peltate or funnel-shaped scales present on the surface of leaves.
- Eg. Tillandria, Nidularium
Epiphytic Adaptations
Epiphytes show a good number of adaptations. Since they have no connection to soil they are confronted with scarcity of water, nutrition, etc. Hence they develop adaptations similar to that of xerophytic plants. Epiphytes are more common in tropical rainforests, either rain or moisture supply water to the roots.
- Generally, epiphytes are xeromorphic.
- Their plant parts, especially the leaves develop thick cuticles to prevent cuticular transpiration.
- Moreover, stomata present on the leaves are sunken to reduce the transpiration rate.
- The plant parts are fleshy to contain water and the water-binding carbohydrates help retain water.
- Plants like Vanda, Drymoglossum, etc develop thick and fleshy leaves which retain water considerably. In Bulbophyllum, the leaflets form bulb-like structures that store water.
Leaf Adaptations
In many cases, the plants reduce their transpiring surfaces or leaves to a minimal number. Plants from Bromeliaceae develop funnel-like scales that absorb water and they are present on the upper surface of the leaves. These funnel-like structures open to the atmosphere to collect rainwater which they send down to the leaf cells.
In plants like Drymoglossum asplinius (bird nest epiphyte), certain leaves form some leaf pockets in a peculiar shape or rosette arrangement. These pockets collect humus and water, from which roots absorb nutrients. Since the plant has no connection to the soil, these kinds of adaptations serve as an effective mechanism for the absorption of water and minerals.
Epiphytic Root Adaptations
Epiphytes depend on atmospheric moisture, rain, dew, snow, etc for their water supply. To capture his moisture, they have special epiphytic roots having certain adaptative features for their survival. These adaptations help them collect and store water as well as reduce water loss.
Features of Epiphytic Roots
Epiphytes have extensively developed root systems having the following features.
- The normal absorbing roots of epiphytes help absorb water, nutrients, and minerals from the decaying matter present in the crevices of their supporting plants.
- Clinging roots help them attach to the surface of their host and absorb nutrients from their bark.
- Several epiphytes absorb water through their leaf and stems, hence the roots form anchors to support the plants. These roots clasp the support to form clasping roots. They are very thick and have no physiological function.
- Aerial roots are green-colored velamen roots having a spongy texture. They hang from the plants and can absorb moisture from the atmosphere. These roots are photosynthetic due to the presence of chlorophyll pigments in them.
- In nest epiphytes, a large number of roots entangle to form a nest. Such roots collect and accumulate large masses of dust and debris, which hold water. This water will later be absorbed by the roots. Moreover, the plants get some nutrients from them.
Dischidia nummularia is perhaps the most specialized epiphyte where one of the leaves forms a pitcher to collect rainwater with the help of penetrating absorbing roots. Sometimes the pitcher contains ants and other insects. Their excreta and dead remains serve as a nitrogenous source for the plant.
Velamen Roots
Vanda develops special roots to absorb moisture from the atmosphere. Such epiphytic roots are called velamen roots and contain pale greenish tissue called velamen on the surface. This is a modification of epidermis and their cells are empty, spiral, or reticulate, and thick. Velamen roots can absorb water from rain or moisture just like a blotting paper.
After this tissue, there is a single-layered suberin-containing cell forming exodermis. This helps prevent the loss of moisture absorbed by the roots. There also are some thin-walled passage cells on the exodermis through which water from the velamen passes to the cortex.
Stem Adaptations
Epiphytic adaptations are seen in stems as well. Since these plants face water scarcity and nutrients, the plant body is poorly developed and the stem is reduced and condensed.
In Drymoglossum, the stem is twine-like with a very small conductive system. The Drynaria stem is rhizomatous and fleshy to store and retain water. Moreover, rhizomes contain hair or lament to retain water. Vanda, Dendrobium, Bulbophyllum, etc., have highly reduced stems.
Anatomical Adaptations Of Epiphytes
The transverse section of the roots of epiphytes shows the following features.
- Thick cuticles and sunken stomata help reduce water loss. These are usually absent in plants that have absorbing organs.
- Development of thin-walled parenchymatous tissue for water storage in succulents.
- Aerial hanging roots in tropical epiphytes of Orchidaceae and Araceae have thin-walled greenish-white tissue called velamen.
- Velamen: Velamen is a hygroscopic tissue that quickly absorbs moisture from a saturated atmosphere, like a sponge. It is a modification of the epidermis and is multilayered. In dead or empty cells, these cells will have spiral or reticulate thickening.
- Exodermis appears in the inner layer of velamen which can be thick-walled with lignin or thin passage cells permeable to water.
- Velamen retains this water until it is absorbed by the exodermis or passage cells.
Seed Dispersal of Epiphytes
Many epiphytes contain seeds that are small and powdery for easy dispersal by wind. In some cases, seeds are surrounded by a pulpy mass to enable dispersal by birds and insects. Usually, these seeds and fruits are carried by the birds to the top of trees.
Conclusion
Thus epiphytes form a very interesting group of plants that grow over other plants not for any kind of nutrition but for getting more light in congested areas. This is the reason why epiphytes are abundant in regions of thick forests like tropical rainforests.
References
- Zhang, T., Liu, W., Hu, T., Tang, D., Mo, Y., & Wu, Y. (2020). Divergent Adaptation Strategies of Vascular Facultative Epiphytes to Bark and Soil Habitats: Insights from Stoichiometry. Forests, 12(1), 16. https://doi.org/10.3390/f12010016
- Shukla, R.S. and Chandel, P.S. (2001) Plant Ecology. S. Chand and Company Ltd., New Delhi.
- Verma, P.S., Agarwal, V.K. (1999). Cell Biology Genetics Molecular Biology Evolution and Ecology. New Delhi: S.Chand Co.(Pvt) Ltd.
