A plant's stem can provide structure, storage and resources. The xylem and phloem vessels make up a plants vascular bundle, and run through the stem of a plant. The xylem is the tube that carries water and minerals up the plant, it is made of continuous dead cells that are strengthened with lignin. The phloem vessel carries nutrients all over the plant, it is made of live cells that are connected by sieves. Hydrophytes, mesophytes and xerophytes all have stems and vascular bundles that are adapted to their habitats and needs.
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The majority of hydrophytes are supported by the water they live in and so have thin, flexible stems which make floating and moving in the water easy. Their stems have large air spaces in them to allow for easy diffusion of molecules into the plant. Unsurprisingly, most hydrophytes have little or no xylem as the majority of the plant has access to water. This means water can diffuse into different parts of the plants and therefore doesn't need to be transported around it. The absence of a well-developed xylem and consequently lignin, means that most stems of hydrophytes have little rigidity. Hydrophytes don't have adaptations for protecting their water supplies as they are surrounded by it. Therefore they have unlimited access to the resource and can easy get more if need be.
Water is essential for a plant to survive and unlike hydrophytes, mesophytes don't have water surrounding nearly all of their structure. Not only does this mean they have to provide their own support, but that water has to be transported to all parts of the plant, as it can't diffuse directly in. Mesophytes' stems provide support and a water transport system for the plants. When a plant's cells are turgid with water, they provide structure for the plant. In contrast to hydrophytes, mesophytes have a developed xylem structure that provides the cells with the water they need. It is essential for mesophytes to have xylem structures, as without them, water would not be transported around the plant and the plant's cells wouldn't be turgid, causing the plant to wilt. Also, cells wouldn't be able to carry out life processes such as photosynthesis. In contrast from xerophytes, mesophytes don't usually have spines or hair covering their stems. This is because they do not need such a high level of protection against predators. Mesophytes usually have access to enough water to create extra glucose and consequently energy, to quickly repair themselves if damaged.
Xerophytes often have woody, sturdy stems. Most xerophyt's stem have ridges which allow it to expand quickly in order to store a maximum amount of water in sudden rainfalls. More adaptations can be seen on the stem of a xereophyte as apposed to other groups of angiosperms. Many xerophyte's stems are adapted to store water, photosynthesize and protect the plants water stores. The Elephant Cactus (Pachycereus Pringlei) has a large quantity of succulent tissue in its stem. This allows the stem to swell and store huge amounts of water wet conditions that can be distributed throughout the plant in dry conditions. As many xerophytes' have these succulent tissues, nearly all of them have defensive mechanisms that protect them against predators. Thorns, spines or hair cover many species of xerophytes stems, in an attempt to scare off predators such as mammals and birds. If the stem didn't have these defenses, it is likely that thirsty animals would bite at the plant, attempting to reach the water stores inside. One bite would lead to the plants precious water supplies being exposed to the heat, wind or cold, as it would remove the outer, protective layers of the plant, which reduce water loss. The amount of water lost through this hole would be detrimental to the plant as many xerophytes are slow growing due to the minimal amounts of water available. This slow rate of growth and the high concentration gradient of water inside of the plant compared to its environment, would mean the plant would lose huge quantities of its water supply, possibly leading to its death. If xerophytes had thin, flexible stems like hydrophytes, they would immediately die. The thin stem would collapse under the weight of the plant, allow water to easily diffuse out of it and would be the perfect resource for predators to access. As xerophytes have large amounts of storage tissue, don't have easy access to water and posses resources predators need, a hydrophyte's stem would be unpractical. The defenses that cover the stem also have other uses. The hair or spines provide shade and lower the concentration gradient of water between the inside and outside of the plant. This means transpiration is reduced, as less water will evaporate out of the stomata if the outside environment is cooler. Again, if a xerophyte had a mesophyte's unprotected stem, a greater amount of water would be lost via transpiration as the concentration gradient would be higher. Xerophytes need their specific adaptations to survive.
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Xerophytes often have woody, sturdy stems. Most xerophyt's stem have ridges which allow it to expand quickly in order to store a maximum amount of water in sudden rainfalls. More adaptations can be seen on the stem of a xereophyte as apposed to other groups of angiosperms. Many xerophyte's stems are adapted to store water, photosynthesize and protect the plants water stores. The Elephant Cactus (Pachycereus Pringlei) has a large quantity of succulent tissue in its stem. This allows the stem to swell and store huge amounts of water wet conditions that can be distributed throughout the plant in dry conditions. As many xerophytes' have these succulent tissues, nearly all of them have defensive mechanisms that protect them against predators. Thorns, spines or hair cover many species of xerophytes stems, in an attempt to scare off predators such as mammals and birds. If the stem didn't have these defenses, it is likely that thirsty animals would bite at the plant, attempting to reach the water stores inside. One bite would lead to the plants precious water supplies being exposed to the heat, wind or cold, as it would remove the outer, protective layers of the plant, which reduce water loss. The amount of water lost through this hole would be detrimental to the plant as many xerophytes are slow growing due to the minimal amounts of water available. This slow rate of growth and the high concentration gradient of water inside of the plant compared to its environment, would mean the plant would lose huge quantities of its water supply, possibly leading to its death. If xerophytes had thin, flexible stems like hydrophytes, they would immediately die. The thin stem would collapse under the weight of the plant, allow water to easily diffuse out of it and would be the perfect resource for predators to access. As xerophytes have large amounts of storage tissue, don't have easy access to water and posses resources predators need, a hydrophyte's stem would be unpractical. The defenses that cover the stem also have other uses. The hair or spines provide shade and lower the concentration gradient of water between the inside and outside of the plant. This means transpiration is reduced, as less water will evaporate out of the stomata if the outside environment is cooler. Again, if a xerophyte had a mesophyte's unprotected stem, a greater amount of water would be lost via transpiration as the concentration gradient would be higher. Xerophytes need their specific adaptations to survive.
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