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Biology essay: describe the adaptations shown by xerophytes to reduce water loss A Xerophyte is a type of plant that is well adapted to water. Water loss is something that is very bad for the plants if the ratio of water lost to water taken in is too drastic. The cells may lose their turgidity and may even submit to plasmolysis, which will result in the plant wilting and eventually dying. Water loss via transpiration (loss of water vapour from the aerial parts of a plant due to evaporation) is fundamentally inevitable due to the fact that plants exchange gases with the atmosphere, via their stomata-the pores in a leafs epidermis .
The bad aspect of this is the fact that the plants must photosynthesise in order to acquire the energy vital for their survival; for this exchange to occur the plant must be able to allow the gases in and out of the leaves, and to do this the stomata must open, meaning that water can be lost due to the opening of an exit, and also the change in the water vapour potential gradient.
Water potential is the measure of the tendency/ability of water to move freely in a solution. Water moves from an area of high water potential to an area of lower water potential, and this is what causes the water vapour in the plant to be lost to the outside atmosphere, due to the difference in the water potential gradient, and we call this “moving along the water potential gradient”.
If the water potential outside the plant was higher than the water potential inside the plant, then the plant would absorb water vapour rather than lose it, but because of the extreme weather conditions, and the difference in water potential the plant loses rather than gains water. The potential of water vapour is the same concept, and simply means the same thing but in terms of the gaseous form of water. Most plants can reduce water loss by structural and behavioural adaptations such as: * A waxy cuticle on the leaf will reduce water loss due to evaporation through the epidermis * The stomata are often found on the undersurface of leaves, not on the top surface- this reduces the evaporation due to direct heating from the sun
* Most stomata are closed at night, when there is no light for photosynthesis * Deciduous plants lose their leaves in winter, when the ground may be frozen (making water less available) and when temperatures may be too low for photosynthesis. However although xerophytes do execute these adaptations, they also have a number of adaptations specific to their own requirements that reduce the rate of water loss. Firstly, the surface area. Xerophytes have much smaller leaves, often shaped like needles. This reduces the surface area of the leaves significantly; hence the total leaf surface area is also reduced. This means that there is a much smaller area for the water vapour to escape from, this works well because the smaller the surface area, the smaller the quantity of water that can escape, therefore the less water lost. The thorn like structures reduce the area exposed for transpiration. Pine trees are prime examples of this, as they have small needle-shaped “leaves” that have a small surface area, therefore are able to retain more water as a result, because less of the area is exposed, and so transpiration cannot occur as abundantly.
Next, includes the way mesophyll, the spongy inner tissue of a leaf that is composed of loosely arranged cells of irregular shape, is densely packed together. This reduces the cell surface area that is exposed to the air inside the leaves, meaning that the space for water to have access to is reduced, because the cells are more compact, thus creating a sealed wall where water cannot escape into and less water will evaporate into the leaf air spaces as a result, hence reducing the rate of water loss. A third factor of xerophytes that they have adapted themselves to include the waxy cuticle, which appears on all plants, is a lot thicker than the typical cuticle. The waxiness reduces evaporation further, particularly cuticular transpiration, where water escapes from fissures through the cuticle. This is because the cuticle, found at the epidermal (outermost) layer of cells, is made up of a complex formula of waxy substances known as Cutin, which acts sort of like a waterproof layer to prevent the loss of water from the surface cells, therefore reducing the amount of water that could be lost to the atmosphere.
Fourthly, closing the stomata when water availability is low will reduce water loss and so reduce the need to take up water. This is because when the stomata is open for various reasons including gas exchange, water can escape from the openings made by the stomata, this is bad or a plant like a xerophyte which wants to retain as much after as possible, therefore keeping the stomata closed as much as possible increases the plants chances of retaining water, particularly when water is scarce. Next, hairs on the surface of the leaf trap a layer of air close to the surface. This air can become saturated with moisture and will reduce the diffusion of water vapour out through the stomata. This is because the gradient of the water vapour potential between the inside of the leaf and the outside has been reduced, for if there is a “barrier” of water between the inside of the cell and the out, then the gradient of water potential is significantly reduced, because the difference in water potential is less, hence water will not want to move from an area of high water potential to an area of low water potential.
Pits containing stomata at their base also trap air that can become saturated with water vapour, and so also reduce the rate of water loss. This will reduce the gradient in the water vapour potential between inside and outside the lea, so reducing loss by diffusion. Behavioural aspects of adaptations that xerophytes achieve include rolling their leaves up so that the lower epidermis is not exposed to the atmosphere which can trap air that becomes saturated. This is another way to reduce or even eliminate the water potential gradient. Another point to make is that some plants have a low water potential inside their leaf cells. This is achieved by maintaining a high salt concentration in the cells. The low water potential reduces the evaporation o water from the cell surfaces as the water potential gradient between the cells and the leaf air spaces is reduced.
An excellent example of a xerophyte is marram grass. A dense green plant with protruding spikes that appears in tufts, which you often see dotted along the coastal scenery. Its principal habitat is sand dunes and the conditions are very severe and can be particularly brutal at times, with winds and salty, dry terrain. The features described above mirror a lot of the characteristics that marram grass possess, such as rolling up its leaves to trap air inside as well as a thick waxy cuticle to reduce water evaporation rom surface cells, and hence is a very good example of a xerophyte. In conclusion, xerophytes are very durable plants that have adapted exceedingly well to living in such harsh conditions. Their features allow them to retain water incredibly well, and that provides them with an advantage to living in places such as the desert in comparison with a normal plant.
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