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A xerophyte or xerophytic organism (xero meaning dry, phyte meaning plant) is a plant which is able to survive in an ecosystem with little available water or moisture, usually in environments where potential evapotranspiration exceeds precipitation for all or part of the growing season. Plants like the cactus and other succulents are typically found in deserts where low rainfall amounts are the norm, but xerophytes such as the bromeliads can also be found in moist habitats such as tropical forests, exploiting niches where water supplies are limited or too intermittent for mesophytic plants. Plants that live under arctic conditions may also have a need for xerophytic adaptations, as water is unavailable for uptake when the ground is frozen. Their leaves are covered with silvery hairs (creates wind break & light reflective surface).
Adaptations of xerophytes include reduced permeability of the epidermal layer, stomata and cuticle to maintain optimal amounts of water in the tissues by reducing transpiration, adaptations of the root system to acquire water from deep underground sources or directly from humid atmospheres (as in epiphytic orchids), and succulence, or storage of water in swollen stems, leaves or root tissues. The typical morphological consequences of these adaptations are collectively called xeromorphisms.
Contents |
Mechanism table
| Mechanism | Adaptation | Example |
|---|---|---|
| Limit water loss | waxy stomata | prickly pear |
| few stomata | ||
| sunken stomata | pine | |
| stomata open at night | tea plant | |
| CAM photosynthesis | cactus | |
| large hairs on surface | Bromeliads | |
| curled leaves | esparto grass | |
| Storage of water | succulent leaves | Bryophyllum |
| succulent stems | Euphorbia | |
| fleshy tuber | Raphionacme | |
| Water uptake | deep root system | Acacia |
| below water table | Nerium oleander | |
| laterally extensive, shallow root systems | cactus | |
| absorbing surface moisture from leaf hairs or trichomes | Tillandsia |
Importance of water conservation
If the water potential inside the leaf is higher than outside the leaf, the water vapour will diffuse out of the leaf down this gradient. This loss of water vapour from the leaves is called transpiration, and the water vapour diffuses through open stomata in the leaf. Although this is a normal and important process in all plants, it is vital that plants living in dry conditions have adaptations that decrease this water potential gradient, and decrease the size of open stomata, in order to reduce water loss from the plant. It is important for a plant living in these conditions to conserve water because without enough water, plant cells lose turgor and the plant tissue wilts. If the plant loses too much water, it will pass its permanent wilting point, where the plant will die.
Types of xerophytic plants are:
- Succulent plants - typically store water in stems or leaves. They include the Cactaceae family which typically have stems that are round and store a lot of water. Often, as in cacti where the leaves are reduced to spines, their leaves are vestigial, or they do not have leaves.
- Bulbs - water is stored in their bulbs, at or below ground level. They may spend a period of dormancy during drought conditions underground, and are therefore known as drought evaders.
- Short-lived annuals can often germinate following rainfall. An example of this is the California poppy whose seeds lie dormant during drought and then, flower and form seeds within four weeks of rainfall.
See also
- Evaporation
- Hydrophyte
- International Center for Agricultural Research in the Dry Areas
- Kinetic Theory
- Mesophyte
- Transpiration
- Xerocole
References
- D. J. Taylor, N. P. O. Green, G. W. Stout (2001). Biological Science 1 & 2, third edition. Cambridge University Press. ISBN 0-521-56178-7.
Wikipedia content modification information:
- This page was last modified on 17 November 2008, at 13:46.
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