Xerophytes
The adaptation of xerophytic leaves are to conserve water to maintain a good water balance in the plant by reducing the rate of transpiration and storing water.
The main type of xerophytes are cacti with succulent tissues. Most cacti have leaves that are reduced to spines which are to protect the stored water inside their stems and leaves and the spines have a reduced SA:V ratio which also decreases the rate of transpiration. This also creates a coolness and a shade for the sunken stomata located on the upper epidermis of the stems. If xerophytes were to have leaves with a large SA:V ratio like hydrophytes and mesophytes, it will wilt and die very quickly as this would greatly increase the rate of transpiration but unlike hydrophytes and mesophytes, there’s no water coming in to replace the lost water. (bitesizeBBC, Water Balance, 2015) (ConservancyChichester, 2015) (PentecostCaroline, 2015)
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Other succulent plants with leaves have at least two layers of palisade cells as the intense light can penetrate further and therefore need more layers of light absorbing cells in their leaves and stems. These succulent plants have leaves that are thick and squishy but waxy. This is because they have a thick layer of reflective waterproof cuticle on top of the upper epidermal layer to reduce water loss through evaporation and transpiration and lower the temperature of the plant. There is a reduced number of leaves which are smaller or densely packed, mostly at the bottom of the stem to reduce the SA:V ratio of the overall plant. They also have succulent tissues in their leaves that store water. These plants have a reduced amount of stoma on the bottom side of leaves which are open only at night for gas exchanges while closing them in daytimes to conserve water and stop transpiration from occurring. The plant is still able to photosynthesise this way via CAM photosynthesis (Crassulecean Acid Metabolism) or C-4 photosynthesis by storing the carbon dioxide obtained in the night and using that stored carbon dioxide to photosynthesise during the day. However, transpiration still occurs during the night and sometimes the day as transpiration is equally as important as water conservation as transpiration needs to occur in order to transport fresh water and minerals around the plant. These stomata are also sunken like in mesophytes and have hairs around them creating a microclimate to reduce the concentration gradient between the inside of the plant and outside of the plant. The hairs also provide a windbreak and reduce air flow around the stoma. (DimmittMark, 2015) (PentecostCaroline, 2015) (aneyefortexas, 2015) (onlineBotanical, 2015) (onlinebotanical, 2015) (LearningEnchanted, 2015) (onlinebotanical, cactus adaptations, 2015) (HenryEllen, 2015) (Buscar, 2015)
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But there are other types of xerophytes that are not desert plants like marram grass which live in sand dunes. Just because it is by a mass of water, does not mean it has water in abundance as sandy conditions drain water quickly and the strong winds increase the rate of transpiration. Marram grass are non succulent plants and are not in the cacti family. They do not have succulent tissues or cacti spines but have long leaves like grass. This gives the leaves a big SA:V ratio which is not ideal for transpiration but this problem is not major as the leaves are rolled.Inside the rolled leaves are the sunken stomata with hairs on them which creates a microclimate. It is not exposed to harsh sunlight like other xerophytes and the rolled leaves essentially protects the stoma inside them from harsh conditions like wind and sunlight. (UruguayHereford, 2015) (snmichaels, 2015)
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