A
xerophyte or
xerophytic organism (from
GreekGreek is an independent branch of the Indo-European family of languages. Native to the southern Balkans, it has the longest documented history of any Indo-European language, spanning 34 centuries of written records. Its writing system has been the Greek alphabet for the majority of its history;...
xero dry,
phuton plant) is a
plantPlants are living organisms belonging to the kingdom Plantae. Precise definitions of the kingdom vary, but as the term is used here, plants include familiar organisms such as trees, flowers, herbs, bushes, grasses, vines, ferns, mosses, and green algae. The group is also called green plants or...
which has adapted to survive in an environment that lacks water, such as a
desertA desert is a landscape or region that receives an extremely low amount of precipitation, less than enough to support growth of most plants. Most deserts have an average annual precipitation of less than...
. Xerophytic plants may have
adaptedAn adaptation in biology is a trait with a current functional role in the life history of an organism that is maintained and evolved by means of natural selection. An adaptation refers to both the current state of being adapted and to the dynamic evolutionary process that leads to the adaptation....
shapes and forms (
morphologyIn biology, morphology is a branch of bioscience dealing with the study of the form and structure of organisms and their specific structural features....
) or internal functions (
physiologyPhysiology is the science of the function of living systems. This includes how organisms, organ systems, organs, cells, and bio-molecules carry out the chemical or physical functions that exist in a living system. The highest honor awarded in physiology is the Nobel Prize in Physiology or...
) that reduce their water loss or store water during long periods of dryness. Plants with such morphological adaptations are called
xeromorphic.
Plants absorb water from the soil, which then
evaporatesEvaporation is a type of vaporization of a liquid that occurs only on the surface of a liquid. The other type of vaporization is boiling, which, instead, occurs on the entire mass of the liquid....
from their outer surfaces; this process is known as
transpirationTranspiration is a process similar to evaporation. It is a part of the water cycle, and it is the loss of water vapor from parts of plants , especially in leaves but also in stems, flowers and roots. Leaf surfaces are dotted with openings which are collectively called stomata, and in most plants...
. In dry environments, a typical (mesophytic) plant would evaporate water faster than the rate at which water was replaced in the soil, leading to wilting. To reduce this effect, xerophytic plants exhibit a variety of specialized adaptations to survive in such conditions. Xerophytes may absorb water from their own storage, allocate water specifically to sites of new tissue growth, or lose less water to the atmosphere and so convert a greater proportion of water in the soil to growth, or have other adaptations to manage water supply and enable them to survive.
CactiA cactus is a member of the plant family Cactaceae. Their distinctive appearance is a result of adaptations to conserve water in dry and/or hot environments. In most species, the stem has evolved to become photosynthetic and succulent, while the leaves have evolved into spines...
and other succulents are commonly found in deserts, where there is little rainfall. Other xerophytes, such as the bromeliads, can survive through both extremely wet and extremely dry periods and can be found in seasonally moist habitats such as tropical forests, exploiting
nichesIn ecology, a niche is a term describing the relational position of a species or population in its ecosystem to each other; e.g. a dolphin could potentially be in another ecological niche from one that travels in a different pod if the members of these pods utilize significantly different food...
where water supplies are limited or too intermittent for mesophytic plants to survive. Similarly,
chaparralChaparral is a shrubland or heathland plant community found primarily in the U.S. state of California and in the northern portion of the Baja California peninsula, Mexico...
plants are adapted to
Mediterranean climateA Mediterranean climate is the climate typical of most of the lands in the Mediterranean Basin, and is a particular variety of subtropical climate...
s, which have wet winters and dry summers. Plants that live under
arctic conditionsThe Arctic is a region located at the northern-most part of the Earth. The Arctic consists of the Arctic Ocean and parts of Canada, Russia, Greenland, the United States, Norway, Sweden, Finland, and Iceland. The Arctic region consists of a vast, ice-covered ocean, surrounded by treeless permafrost...
also have a need for xerophytic adaptations, since water is unavailable for uptake when the ground is frozen.
Types of xerophytic plants
Succulent plantSucculent plants, also known as succulents or fat plants, are water-retaining plants adapted to arid climates or soil conditions. Succulent plants store water in their leaves, stems, and also in roots...
s store water in their stems or leaves. They include the Cactaceae family, which has round stems and can store a lot of water. The leaves are often vestigial, as in the case of cacti where the leaves are reduced to spines, or they do not have leaves at all.
Water is stored in the
bulbsA bulb is a short stem with fleshy leaves or leaf bases. The leaves often function as food storage organs during dormancy.A bulb's leaf bases, known as scales, generally do not support leaves, but contain food reserves to enable the plant to survive adverse conditions. At the center of the bulb is...
of some plants, at or below ground level. They may be dormant during drought conditions and are therefore known as drought evaders.
Importance of water conservation
If the
water potentialWater potential is the potential energy of water per unit volume relative to pure water in reference conditions. Water potential quantifies the tendency of water to move from one area to another due to osmosis, gravity, mechanical pressure, or matrix effects such as surface tension...
(or strictly, water vapour potential) inside a leaf is higher than outside, the water vapour will
diffuseMolecular diffusion, often called simply diffusion, is the thermal motion of all particles at temperatures above absolute zero. The rate of this movement is a function of temperature, viscosity of the fluid and the size of the particles...
out of the leaf down this gradient. This loss of water vapour from the leaves is called
transpirationTranspiration is a process similar to evaporation. It is a part of the water cycle, and it is the loss of water vapor from parts of plants , especially in leaves but also in stems, flowers and roots. Leaf surfaces are dotted with openings which are collectively called stomata, and in most plants...
, and the water vapour diffuses through the open stomata. Transpiration is natural and albeit inevitable for plants, and much water is lost through this. However, it is vital that plants living in dry conditions are adapted so as to reduce this water loss and decrease the size of the open stomata, in order to reduce unnecessary 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 pointPermanent wilting point or wilting point is defined as the minimal point of soil moisture the plant requires not to wilt. If moisture decreases to this or any lower point a plant wilts and can no longer recover its turgidity when placed in a saturated atmosphere for 12 hours...
, and die.
In brief, the rate of transpiration is governed by the number of stomata, Leaf area (allowing for more stomata), temperature differential, the relative humidity, the presence of wind or air movement, the light intensity, and the presence of a waxy cuticle. It is important to note, that whilst it is vital to keep stomata closed, they have to be opened for gaseous exchange in photosynthesis.
Morphological adaptations
Xerophytic plants may have similar shapes, forms and structures and look very similar, even if the plants are not very closely related, through a process called
convergent evolutionConvergent evolution describes the acquisition of the same biological trait in unrelated lineages.The wing is a classic example of convergent evolution in action. Although their last common ancestor did not have wings, both birds and bats do, and are capable of powered flight. The wings are...
. For example, some species of cacti (members of the family
Cactaceae), which evolved only in the Americas, may appear similar to
Euphorbias, which are distributed worldwide. An unrelated species of
caudiciformCaudiciform plants, or fat plants, are water-retaining plants adapted to arid climate or soil conditions. Caudiciform plants store water in their stem bases, stems and/or roots. Many species of plant from different plant families have developed this form of storing water rather than in foliage or...
s, plants with swollen bases which are used to store water, may also display such similarities.
Reduction of surface area
Xerophytic plants can have less overall
surface areaSurface area is the measure of how much exposed area a solid object has, expressed in square units. Mathematical description of the surface area is considerably more involved than the definition of arc length of a curve. For polyhedra the surface area is the sum of the areas of its faces...
than other plants, so reducing the area that is exposed to the air and reducing water loss by evaporation. Xerophytes can have smaller leaves or fewer branches than other plants. An example of leaf surface reduction are the
spinesIn botanical morphology, thorns, spines, and prickles are hard structures with sharp, or at least pointed, ends. In spite of this common feature, they differ in their growth and development on the plant; they are modified versions of different plant organs, stems, stipules, leaf veins, or hairs...
of a cactus. An example of compaction and reduction of branching are the
barrel cactiBarrel cacti are classified into the two genera Echinocactus and Ferocactus, both of which are found in the Southwest Desert of North America. Their pineapple-shaped fruits can be easily removed but are not recommended for eating. The barrel cactus may reach over a metre in height. Its ribs are...
. Other xerophytes may have their leaves compacted at the base, as in a basal rosette, which may be smaller than the plant's flower. This adaptation is exhibited by some
AgaveAgave is a genus of monocots. The plants are perennial, but each rosette flowers once and then dies ; they are commonly known as the century plant....
s and
EriogonumEriogonum is the scientific name for a genus of flowering plants, in the family Polygonaceae. The genus is found in North America and is known as wild buckwheat. This is a highly species-rich genus, and indications are that active speciation is continuing...
s, which can be found growing near
Death ValleyDeath Valley is a desert valley located in Eastern California. Situated within the Mojave Desert, it features the lowest, driest, and hottest locations in North America. Badwater, a basin located in Death Valley, is the specific location of the lowest elevation in North America at 282 feet below...
.
Reduction in air flow
Some xerophytes have tiny hairs on their surface to provide a wind break and reduce air flow, thereby reducing the rate of evaporation. When a plant surface is covered with tiny hairs, it is called
tomentoseTomentose is a term used to describe plant hairs that are flattened and matted, forming a woolly coating known as tomentum. Often the hairs are silver or gray-colored...
.
In a still environment, the areas under the leaves/spines where transpiration is taking place form a small localised environment that is more saturated than normal with water vapour. If this is not blown away by wind, the water vapour potential gradient is reduced and so is transpiration. Thus in a windier situation, this localization is not held and so the gradient remains high, which aids the loss of water vapour. Spines trap a layer of moisture and also slow air movement over tissues.
Reflectivity
The color of a plant, or of the waxes or hairs on its surface, may serve to reflect sunlight and reduce evaporation. An example is the white chalky wax (
epicuticular waxIn botany, the plant cuticle is covered by epicuticular wax or bloom mainly consistingof straight-chain aliphatic hydrocarbons with a variety of substituted groups, serving to decrease moisture loss and decay...
) coating of
Dudleya brittoniiDudleya brittonii Dudleya brittonii Dudleya brittonii ((DUD-lee-yuh brit-TON-ee-eye), common name Chalk Dudleya, is a succulent plant in the Crassulaceae family. It is found in Baja California. Leaves grow in a basal rosette and are covered with a dusty, chalky, mealy white epicuticular “wax”...
, which has the highest ultraviolet light (UV)
reflectivityIn optics and photometry, reflectivity is the fraction of incident radiation reflected by a surface. In general it must be treated as a directional property that is a function of the reflected direction, the incident direction, and the incident wavelength...
of any known naturally occurring biological substance.
Physiological adaptations
Some plants can store water in root structures, trunk structures, stems and leaves. Water storage in swollen parts of the plant is known as succulence. A swollen trunk or root at the ground level of a plant is called a
caudexA caudex is a form of stem morphology appearing as a thickened, short, perennial stem that is either underground or near ground level . It may be swollen for the purpose of water storage, especially in xerophytes...
and plants with swollen bases are called
caudiciformCaudiciform plants, or fat plants, are water-retaining plants adapted to arid climate or soil conditions. Caudiciform plants store water in their stem bases, stems and/or roots. Many species of plant from different plant families have developed this form of storing water rather than in foliage or...
s.
Tiny pores on the surface of a xerophytic plant called stomata may open only at night, so as to reduce evaporation.
Plants may secrete resins and
waxesthumb|right|[[Cetyl palmitate]], a typical wax ester.Wax refers to a class of chemical compounds that are plastic near ambient temperatures. Characteristically, they melt above 45 °C to give a low viscosity liquid. Waxes are insoluble in water but soluble in organic, nonpolar solvents...
(
epicuticular waxIn botany, the plant cuticle is covered by epicuticular wax or bloom mainly consistingof straight-chain aliphatic hydrocarbons with a variety of substituted groups, serving to decrease moisture loss and decay...
) on their surfaces, which reduce evaporation. Examples are the heavily scented and flammable resins (volatile organic compounds) of some
chaparralChaparral is a shrubland or heathland plant community found primarily in the U.S. state of California and in the northern portion of the Baja California peninsula, Mexico...
plants, such as Malosma laurina, or the chalky wax of
Dudleya pulverulentaDudleya pulverulenta is a succulent plant known by the common names chalk lettuce and chalk liveforever.-Distribution:This dudleya is native to the southwestern United States and northern Mexico, where it is found in steep open rocky areas in coastal and inland mountains and desert foothills, such...
.
Plants may drop their leaves in times of dryness (
drought deciduousDrought deciduous refers to plants that drop their leaves during dry season or periods of dryness, such as plants of the California Coastal sage scrub community. This may be contrasted to plants that drop their leaves during cold periods....
), or modify the leaves produced so that they are smaller.
During dry times, xerophytic plants may stop growing and go dormant, change the kind of
photosynthesisPhotosynthesis is a chemical process that converts carbon dioxide into organic compounds, especially sugars, using the energy from sunlight. Photosynthesis occurs in plants, algae, and many species of bacteria, but not in archaea. Photosynthetic organisms are called photoautotrophs, since they can...
, or change the allocation of the products of photosynthesis from growing new leaves to the roots.
Seeds may be modified to require an excessive amount of water before germinating, so as to ensure a sufficient water supply for the seedling's survival. An example of this is the
California poppyThe California poppy is a perennial and annual plant, native to the United States, and the official state flower of California.- Description :...
, whose seeds lie dormant during drought and then germinate, grow, flower and form seeds within four weeks of rainfall.
Modification of environment
The leaf litter on the ground around a plant can provide an evaporative barrier to prevent water loss. Waxes shed from some plants coat the ground, so reducing evaporation in the immediate vicinity of the plant, as in the case of
Dudleya pulverulentaDudleya pulverulenta is a succulent plant known by the common names chalk lettuce and chalk liveforever.-Distribution:This dudleya is native to the southwestern United States and northern Mexico, where it is found in steep open rocky areas in coastal and inland mountains and desert foothills, such...
. A plant’s root mass itself may also hold organic material which retains water, as in the case of the arrow weed (
Pluchea sericeaPluchea sericea, commonly called Arrowweed, is a rhizomatous evergreen shrub of riparian areas in the lower Sonoran Desert and surrounding areas...
).
Mechanism table
| Mechanism |
Adaptation |
Example |
| Limit water loss |
waxy stomata |
prickly pear Opuntia, also known as nopales or paddle cactus , is a genus in the cactus family, Cactaceae.Currently, only prickly pears are included in this genus of about 200 species distributed throughout most of the Americas. Chollas are now separated into the genus Cylindropuntia, which some still consider...
|
|
few stomata |
|
|
sunken stomata |
pinePines are trees in the genus Pinus ,in the family Pinaceae. They make up the monotypic subfamily Pinoideae. There are about 115 species of pine, although different authorities accept between 105 and 125 species.-Etymology:...
|
|
stomata open at night |
tea plant |
|
CAM photosynthesis |
cactus |
|
large hairs on surface |
Bromeliads |
|
curled leaves |
esparto grass |
| Storage of water |
succulent leaves |
KalanchoeKalanchoe , also written Kalanchöe or Kalanchoë, is a genus of about 125 species of tropical, succulent flowering plants in the Family Crassulaceae, mainly native to the Old World but with a few species now growing wild in the New World following introduction of the species.Most are shrubs or...
|
|
succulent stems |
Euphorbia |
|
fleshy tuber |
Raphionacme |
| Water uptake |
deep root system |
AcaciaAcacia is a genus of shrubs and trees belonging to the subfamily Mimosoideae of the family Fabaceae, first described in Africa by the Swedish botanist Carl Linnaeus in 1773. Many non-Australian species tend to be thorny, whereas the majority of Australian acacias are not...
,"prosopisProsopis is a genus of flowering plants in the pea family, Fabaceae. It contains around 45 species of spiny trees and shrubs found in subtropical and tropical regions of the Americas, Africa, Western Asia, and South Asia. They often thrive in arid soil and are resistant to drought, on occasion...
" |
|
below water table |
Nerium oleander |
|
absorbing surface moisture from leaf hairs or trichomes |
TillandsiaTillandsia is a genus of around 540 species in the Bromeliad family , found in the forests, mountains, and deserts, of Central and South America, and Mexico and the southern United States in North America....
|
External links
See also
- Hydrophyte
- International Center for Agricultural Research in the Dry Areas
International Center for Agricultural Research in the Dry Areas is an Arab center for International Agricultural Research located at Tel Hadya near Aleppo in the Syrian Arab Republic....
- International Crops Research Institute for the Semi-Arid Tropics
The International Crops Research Institute for the Semi-Arid-Tropics is a non-profit organization in India.It was founded in 1972 by a consortium of organizations convened by the Ford and the Rockefeller Foundations. Its charter was signed by the FAO and the UNDP...
- Kinetic Theory
The kinetic theory of gases describes a gas as a large number of small particles , all of which are in constant, random motion. The rapidly moving particles constantly collide with each other and with the walls of the container...
- Raunkiær plant life-form
- Xerocole
Xerocoles are animals adapted to live in a desert.Under the scorching rays of sun, the arid and baked sands of deserts may appear to be devoid of animal life. But surprisingly a large number of animal species may thrive there. The animals are either drought evaders or drought resistors. Drought...