All photosynthetic plants need to open stomata’s on leaves to allow carbon dioxide and oxygen in and out but this process allows for evaporation to occur. To retain optimum levels of moisture many plants have more stomata’s on their lower side away from direct sunlight.
The plant needs to be able to balance effective photosynthesis with reduced transpiration rates. One way most plants do this is by having a large surface area for absorbing light and a thin layer of photosynthetic cells so that the process can occur quickly with minimal energy use. This is a common adaptation and you can see this in a vast range of vegetation by looking at the thickness of the leaves compared to its surface area.
Plants in water deficient areas have developed a range of adaptations to help them cope with reduced water availability and increased temperatures. Vegetation found in desserts and tundra are a good examples of areas with reduced water availability but both have very different temperature and weather patterns, which will also influence adaptations. Plant species adapted to be able to withstand reduced water availability are known as xerophytes.
Adaptations can occur on three levels, firstly there are morphical features, which relate to the structure of the plant, physiological features, which relate to biological function and behavioural features which relates to plants choosing suitable places to inhabit.
Morphical features
These adaptations are obvious characteristics of plants and are often used as identification features, easily visible they can be seen in many dessert and water deficient areas, with most species present displaying at least one if not more of the following features.
- Fuzzy leaf surface – the hair on the leaf acts as a buffer for winds that contribute to increased transpiration levels
- Shiny reflective leaves reduce the amount of solar energy absorbed by the plant this means that less water is evaporated and the less energy required to perform actions that result in temperature reduction.
- Waxy leaves help to reduce water loss from transpiration
- Small leaves – reduced surface area means that less solar energy is absorbed preventing the plant drying out and over heating.
- Thicker leaves can hold more water allowing plants to store some water within their bodies rather than having to rely on moisture in the soil.
- Spines instead of leaves on many plants help to reduce impact of solar radiation reducing absorption and evaporation rates.
- Extensive shallow roots not only bind dry loose soil, which is vulnerable to erosion, but also allows plants to absorb as much rainfall as possible before it drains away or evaporates.
- Deep taproots allow some plants to absorb moisture from deeper soils where other plants cannot reach giving them a competitive advantage.
- Self-shading plants protect the majority of the plant from the effects of the sun by growing leaves, which shade out the rest of the plant.
Physiological features cannot always be seen without dissection and examination of the cellular structure and chemical composition of the plant. Water accumulation as seen in succulents is a visible feature and a common adaptation to extremely arid conditions, as is self pruning, in which a plant shuts off its circulation to particular leaves or branches to conserve water for the rest of the plant; the cut off parts wither and die eventually dropping off but the plant is able to survive.
Behavioural features can be observed in the areas in which the plant is found, certain plants will make use of microclimates that have more available water such as cracks in the landscape where water can accumulate
There are several ways of grouping plants with similar adaptation features.
Drought escapers make use of favourable conditions where they exist, more often than not they are short-lived annuals (Ephemerals) that complete their life cycle in a matter of weeks or even days when the right conditions arise. These plants put all their energy into reproduction and seed production and keep none for continued existence, their survival relies on the plants ability to ‘not exist’ for most of the year with seeds lying dormant in the soil through most of the year especially in places with predominantly unfavourable conditions.
Drought resistors are often perennial species that show adaptations that allow the plants to survive and grow throughout the year and onto the next. These species put a lot of energy into their adaptations and so generally do not initially flower for several years having finally established enough to put energy into reproduction.
The nature of the adaptation varies greatly over the spectrum of drought resisting plants and particular adaptations are favoured by plants depending on whether they are woody, herbaceous or succulent.
Typical adaptations on woody plants are small spiny leaves which reduce the impact of solar radiation, a waxy coating on the leaves help to keep moisture trapped and stomata’s are often sunken on the leaf to protect from evaporation from winds. The woody stems and trunks are often smooth and green contributing towards food production and preventing moisture loss. Hair can often be found over the trunk and stem this helps to protect from transpiration and hold onto moisture around stomata’s.
Trees and shrubs in areas with reduced water availability do not grow very tall, the lack of water means that growth is slow and production reduced. Many plants go into a state of dormancy for a large part of the year, conserving enough energy to survive until the next growth season begins with the arrival of precipitation.
Plants in dry areas often have thorns on stems and trunks; this adaptation helps to create self shade, which protects the plant from intense heat and thus reducing the rate of transpiration.
Some of the more dominant woody and shrub plants have developed deep tap roots that reach far down into the soil to draw water up that is unavailable to the other local species, plants that do this are known as Phreatophytes.
Succulents are the most well adapted Families to arid conditions showing adaptations that allow long-term plant survival. There are hundreds of species of succulent most notable of these is the Family Cactaceae, this is a relatively young group who have adapted rapidly into extremely specialised plants. Trunks and stems have evolved into green succulent structures containing chlorophyll and leaves have become the characteristic spines found on many species of cactus. While size and shape can vary greatly the function and internal make up is very similar.
Cacti stems have become swollen and enlarged with moisture storing tissue; this allows the plant to store its own supply of water in areas with uncertain future precipitation.
Many cacti have adapted to a spherical shape as this combines highest possible volume with lowest possible surface area. The spines shade the plant from high temperatures and protect from animals trying to gain water from plants.
Cactus roots are shallow but radiate out around the plant to maximise absorption of water when it finally rains, some species can begin to grow within hours of rainfall thanks to their shallow but widely radiated root structure. The stem of the cactus is also adapted to absorbing water, moisture in the air is just as important as in the soil and the presence of chlorophyll and spines
Cactus keep their stomata closed during the day when temperatures are highest, instead they store carbon dioxide within their bodies until night when temperatures are cooler, CO2 is then released and the stomata’s open, this means that transpiration rate are greatly reduced.
A long period of dormancy and short growing season help cactus to survive anything from 25 - 300 years. During this time initial growth can be very slow and it can be many years before a cactus reaches maturity and the ability to reproduce. Reproduction is also favoured at night and flowers will open to allow nocturnal animals, which frequent hot arid areas to transport and fertilise seeds.
Drought evaders manage to survive in areas of reduced available water by making use of micro-climates within an area grow in places where more moisture is constantly available such as desert caves and crevices, taking advantage of cool, moist conditions. These species show less morphical and Physiological adaptations in favour of behavioural adaptations because they are able to survive as they are in favourable conditions without the need for adaptation.
Plants in extreme high and low temperatures often show similar adaptations to vegetation in areas of reduced water availability, this is because in areas of extreme temperature water becomes less available to plants.
Plants of the tundra exhibit characteristics such low growing plants and hairs on leaves, this keeps the plant warm as well as preventing evaporation which is as important in the tundra as it is in the desert.
When temperatures drop as low as they do in the arctic circle a layer of permafrost covers the ground preventing plants from absorbing water, although surrounded by water they are in fact in drought conditions. One adaptation seen in plants of reduced temperatures is dark or red leaves; the dark pigmentation improves the plants light absorbing capabilities.
In areas with high levels of rainfall plants need to adapt to survive in waterlogged soils, in water oxygen is difficult for a plant to absorb, mainly because there are very few air spaces in the wet soil. Often what little air there is becomes utilised by decomposers and so plants need to develop sufficient adaptations to survive; these species are known as hydrophytes.
Aquatic plants have evolved in a very different habitat to terrestrial vegetation so much so that these plant types differ greatly in structure and behaviour.
Common Adaptations of Aquatic Plants:
- Less rigid stems as supported by the water
- Flexible stems/leaves that allow the plant to move with force of water currents
- Reduced external protective tissue, this would normally limit water loss on a plant but as water is abundant all around there is no need for cuticle formation
- Reduced or absent transportation system, water, nutrients and gases are taken directly from surrounding water and not from soils.
- Reduced or absent roots and root hairs, as materials are absorbed else where and water already supports the plant, roots are often used as anchorage but some species have adapted to free-float, increasing light absorption
- Air filled sacs/cavities in stem and leaves that work to hold the plant up and gain a competitive edge with increased light absorption
- Highly dissected/divided submerged leaves, this maximises surface area for absorption of light, water, nutrients and dissolved gases, minimises damage from resistance in water
- Chlorophyll restricted to surface leaves, this maximises absorption of light where it is most intense
- Waxy coating on surface leaves to repel water that would otherwise reflect light energy
- Floating seeds allow plants to spread across vast expanses of water broadening the gene pool and preventing local over-competition
References:
http://en.wikipedia.org/wiki/Cactaceae
http://www.desertusa.com/du_plantsurv.html
http://www.huntington.org/Education/lessons/BG-RP-leaf-adapt.pdf.
http://www.microscopy-uk.org.uk
http://www.4corners.net/ccyc/pl3.htm
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