xerophytes:
xerophytes:Xerophytes are plants that are adapted to living in two types of temperature extremes. They live in places really hot and dry places and also places that are really cold. Both places have very little water available but plenty of sunlight, this causes xerophytes to have many adaptations to maintain water balance and to prevent water loss but little adaptations for photosynthesis and getting enough light. It is essential for Xerophytes to have water balance adaptations, these fall into four categories; Stomata adaptations, stem and leaf adaptations, vascular system adaptations and root adaptations because they need the water to carry out life processes such as photosynthesis (as water is a raw material for the process) , mitosis and respiration. Some types of xerophytes include: Succulents and Cacti which live in areas such as deserts, certain Bromeliads which are plants that can survive both really wet and really dry periods, chaparral plants which have really wet winters but extremely dry summers and some plants which live in arctic conditions. (Mulroy, 1979)(Powell, 2013)(Roberts, 2011)
What affects the rate of transpiration in Xerophytes?With xerophytes the factors that affect the rate of transpiration are light intensity, soil moisture and temperature; for this plant group, wind speed and humidity are not as big factors. Light intensity affects the rate of transpiration for xerophytes because they are found in a habitat that generally has plenty of sun and not as much clouds, resulting in light intensity being a major factor that contributes to water loss in xerophytes. Soil moisture affects the rate of transpiration in xerophytes because xerophytes live in places where it is usually dry or the water is frozen in the ground, this decreases the rate of transpiration because less water is available for the plant to lose. Temperature is a main factor that affects the rate of transpiration in xerophytes because some xerophytes live in very hot temperatures, this causes them to lose water via transpiration to keep cool and so that their enzymes don’t denature and they can’t carry out life processes, thus causing the rate of transpiration to increase. Also to do with transpiration the water is evaporated faster at higher temperatures meaning the plants is losing more water trying to decrease the concentration gradient. Although these four factors affect the rate of transpiration in xerophytes, xerophytes have adaptations to maintain water balance and to stop them from losing water to transpiration. (Roberts, 2011)(Jones, 2015)(Class notes,2015)
Adaptations Xerophytes have to maintain water balance:Stomata adaptationsAdaptations that the stomata in xerophytes do to prevent water loss. This includes things such as how the stomata are in sunken pits. This is a structural adaptation which allows for the stomata in xerophytes to be more sheltered and protected from factors such as wind speed and light intensity, which therefore decreases the rate of transpiration this is seen in marram grass. Also by having their stomata in sunken pits it creates a microclimate or a mini-environment with water trapped in it, causing the air humidity or moisture to increase and therefore lowers the concentration gradient between the inside of the leaf and the outside of the leaf so less water is lost via osmosis (as osmosis is the process in which water molecules travel through a semi-permeable membrane from an area of high concentration to an area of low concentration) which therefore decreases the rate of transpiration and thus helps the plant to maintain water balance. The stomata only open at night in a process called crassulacean acid metabolism (CAM) photosynthesis, thus reducing the rate of transpiration by the plant as no water can get out of the stomata during the day when the temperature is up, it is also cooler at night which reduces the amount of water evaporated. This process is when the stomata close during the day and absorb the sunlight and open at night to collect gas such as oxygen and carbon dioxide, meaning half of the plants photosynthesis happens at night and the other half of the plants photosynthesis happens during the day, this is a physiological adaptation. Another structural adaptation is that the stomata in xerophytes are also located on the underside of the leaf or if the plant doesn't have any leaves, they are located on the stem. This helps to reduce the water lost as it is cooler on the underside of the leaf and the sunlight isn't directly hitting the whole stem at all times which causes the rate of transpiration to decrease and therefore the plant conserves more water which it can use to keep the cells turgid or use for other processes like photosynthesis as water is a raw material for photosynthesis. (Feildtrip to Hamilton Gardens)(Roberts, 2011)(Jones, 2015)(Class Notes,2015)
Stem/leaf adaptationsAdaptations that xerophyte plants have in their leaves and stems that are different to the other plant groups. This includes a structural adaptation which is how the leaves and sometimes the stem have tiny hairs called trichomes, by having hairs it breaks up the surface area and also helps the water that has transpired out of the stomata to not be swept away by the wind, which therefore decreases the rate of transpiration, this also decreases the concentration gradient. In some xerophytes another structural adaptation is that the leaves are rolled in to protect the stomata from the wind and the harsh sun which reduces the water lost due to transpiration and evaporation because the light intensity, temperature and wind speed has decreased. They also tend to have less stomata to reduce the amount of lost via transpiration. Xerophytes tend to have very thick waxy cuticles, this is also a structural adaptation which helps the plant to hold water and not lose water by transpiration. If xerophytes were to have thin waxy cuticle like hydrophytes, it would not help them to survive or reproduce as they would lose too much water out of the leaves or stem which they already don’t have. A structural adaptation that the stems of xerophytes have is that it has ridges which in some way shades the stomata which decreases the rate of transpiration by lowering the temperature but it also provides more surface area, so that the plant can absorb as much sunlight that it can for photosynthesis. Also a physiological adaptation is that on or should I say in some xerophytes is that the stem is where the chloroplasts and stomata are which helps reduce the rate of transpiration from the leaves. Xerophytes also have little or no leaves and instead have spikes. By having this structural adaptation it allows for them to have less or no surface area for transpiration and evaporation to occur which reduces the amount of water lost out of the leaves. Another physiological adaptation is that xerophytes have little chloroplasts and can be a silver colour, this is because they live where light is in abundance so they don’t need lots of chloroplasts to absorb light. It is also because having more chloroplasts and chlorophyll causes the plant to be a darker green. Dark colours attract light and heat whereas light colours, like silver, reflect it. It also helps the plant to control its temperature. If the xerophytes are too dark it causes them to absorb more heat which could cause the enzymes which help the plant to carry out life processes such as photosynthesis to denature, they then use the water they have stored to cool the plant down and causes the plant to lose water.(Feildtrip to Hamilton Gardens)(Roberts, 2011)(Jones, 2015)(Class Notes,2015)
Vascular system adaptationsA structural adaptation that xerophytes have is that they have a thick fleshy vascular system which is in the stem. This thick fleshy stem allows for the plant to conserve lots of water which helps the plant to maintain water balance as it can store water when there is water around and use it if water is limited. The water is stored in swollen parts of the plant which are known as succulence. It is also able to move up the xylem freely as the xylem has been adapted to hold lots of water. The stems are also lignified to hold them upright so that they can absorb light. (Roberts, 2011)(Jones, 2015)(Class Notes,2015)
Root adaptationsAdaptations that xerophyte plant roots do or have in order to maintain water balance. Xerophyte roots travel close to the surface of the ground or travel deep down to a water table. These are structural adaptations. Xerophyte roots travel close to the surface of the ground because when it does rain, it usually is very little so it doesn’t go down deep into the ground or it is so hot it gets evaporated quickly, which means that if xerophytes had their roots too deep, it means that they would not be able to gain water through the roots and would therefore not be able to carryout out life processes such as respiration and it would certainly limit their chances of survival, it would also not help them to maintain water balance. By having the roots they can absorb the water quickly and efficiently before it is evaporated but it presents a limitation that the plant isn’t very anchored in the ground. Xerophyte roots can also travel deep under the surface to get water from underground springs so that they can get water to maintain water balance.(Singh, 2015)(Roberts, 2011)(Jones, 2015)(Class Notes,2015)
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Examples of xerophytes
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