Ecological Niches

Most angiosperms can be split into three different categories depending on their ecological niches and the availability of water in their habitats. Each group lives in drastically different conditions to the others due to climate and availability of water. All three plant groups have numerous adaptations, that allow them to live in their different environments. With each environment come limitations, but because of the plant's adaptations, they are able to live successfully with these limitations. 

Hydrophytes live in conditions where water is plentiful such as swamps, ponds, streams and lakes.  Some hydrophytes have leaves on the surface of the water while others are completely submerged. The limitation hydrophytes face is the huge quantity of water they are exposed to. Therefore, hydrophytes have minimal adaptations specific to conserving water as they are often surrounded by it. Instead, their adaptations specialise in allowing them to survive in their aquatic environments and enabling the movement of water through the plant. Hydrophytes have adaptations that help eliminate competition with other plants so that lack of other resources don’t become limiting factors to processes such as photosynthesis.

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Mesophyte plants live in environments like those experienced in New Zealand; where there aren’t extreme climate conditions. As mesophytes aren’t exposed to extremely wet or dry conditions, water loss is only a major problem during the hotter time periods such as midday in summer. There isn’t much competition with other plants as far as water is concerned, as there are ample water supplies.

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Xerophytes live in dry, arid conditions such as deserts. The obvious limitation of this environment is the lack of water. Xerophytes are often exposed to extreme heat, cold and wind, making water preservation one of their key focuses. They have adaptations that allow them to gather and store water. It is because of these adaptations that xerophytes are able to survive in their environments. There is a large amount of competition for water as rainfall is minimal. Consequently, most xerophytes’ adaptations are specific to retaining moisture and protecting their water supplies.

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Water Balance

The adaptations of many plants are designed to minimise water loss and balance the amount of water in the plant's cells. This is of significance as many of a plant’s life processes require water. Plants are autotrophs’ and  make their own food by photosynthesizing. The equation for photosynthesis is;

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As shown, plants need water in order to photosynthesize and produce glucose, the sugar they use to create energy. Without access to water or a way to conserve it plants wouldn't be able to complete this process or create food for itself. Without food the plant would die which is why plants have adaptations. These allow the plant to control water loss and therefore ensure processes such as photosynthesis can be carried out.

Photosynthesis isn't the only reason plants need water, for mesophytes and hydrophytes, water is needed to support their structures. As plants don’t have skeletons, the cell walls and vacuoles in their cells provide support. When the cells’ vacuoles are turgid and full of water, the plant is able to stand upright, without water the plant would wither and wilt. For hydrophytes, the water often supports the plant itself. Water is also necessary for keeping the cells’ membranes moist which allows for efficient gas exchange. Water is also used to control the temperature of the plant. Without water, plants’ cells wouldn't be able to function or produce substances such as glucose, which the cells need to survive.

The issue plants face when it comes to water conservation is transpiration. As water evaporates from the plant’s leaves and stomata, more water moves up the plant to replace the molecules lost. This is due to cohesion; water molecules are attracted to each other. They stick together to make a chain and as water evaporates from the plant’s leaves and stem, more molecules are pulled up to fill the space the evaporated water has left. Unfortunately for some plants, this means they are nearly always losing water.  Transpiration is the main cause of water movement in plants yet there are other smaller factors contributing to this movement. Root pressure pushes water up the plant and capillary action means that water sticks to the sides of the xylem and is pulled upwards. Although transpiration results in a loss of water from the plant, it also helps keep the plant cool and allows for movement of water and other minerals around the plant. 

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The rate of transpiration increases when a plant's stomata are exposed to high winds and hot or dry environments, as there is a high water concentration gradient between the inside of the plant, and its environment. This means that as there is less water outside of the plant, the H20 molecules will automatically diffuse into this low concentration area in an attempt to reach an isotonic state. Subsequently, the higher the concentration gradient, the more water the plant will lose.

Root Adaptations

Each group of angiosperm have different root systems that are adapted and specialized to living in their habitats.

Hydrophytes root systems are generally not hugely developed unless the plant is exposed to strong currents or tides. In these cases, larger root systems are grown to ensure the plant is securely anchored and is not at risk of being pulled from the soil due to the currents. However hydrophytes in still water such as ponds and lakes, often have very small root systems. This is because the majority of the nutrients the plant needs can be taken from its watery environment. Therefore the primary need for the roots is anchorage. The root systems are grown specifically, depending on the species of plant, as there is no point in wasting valuable energy growing unnecessary roots. For example,  water lilies have small root systems and use their energy to grow large, flat leaves, so that transpiration can occur and water and minerals are able to move through the plant.

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Mesophytes rely on their roots to provide them with anchorage, water and nutrients. As many mesophytes are exposed to a range of weather conditions, anchorage is highly important to ensure plants are supported and can withstand heavy winds and rains. As hydrophytes get both water and nutrients from their roots, it is essential the roots stay embedded in soil, as if they are ripped up because of weather conditions, they will not be able to provide the plant with the resources it needs to live. Unlike hydrophytes, mesophytes don't have continuous access to water and therefore, their roots must be able to supply them with the resources necessary to sustain life. Different species of mesophytes have different root systems, at different times, depending on their environment and water availability. Most mesophytes are planted or begin germination in Spring. During this time, there is typically ample rainfalls which allow new plants to establish themselves and grow their roots, without having to be especially concerned about water conservation. By the time summer approaches, new mesophytes have grown bigger root systems that penetrate the earth and collect moisture from deeper soils. This is essential as the reduction in rain means there is less water available in the top layers of soil. Without these quick growing root systems, new mesophytes wouldn't be able to access water and would most likely die. Older and more established mesophytes such as oak trees have huge root systems which develop over many years. They often spread several meters under the ground. These extensive roots allow the tree to take advantage of any water available in the soil. As water is continually lost via transpiration, it is essential that the tree has enough water to continue with its life processes. The developed root systems allow this to happen. 

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The roots of xerophytes come in a range of forms, depending on the water or moisture available for the plant. Some xerophytes roots grow deep into the soil, in an attempt to maximise the amount of water they have access to. This is crucial as in the habitats of many xerophytes, there is no rain for long periods of time. This means the top layers of soil stay bone-dry. By having deep and penetrating roots, xerophytes have access to reserves of water deep beneath the ground and are able to sustain life in long, dry periods. The Acacia tree has be known to have roots stretching over 8 metres below the surface.

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Other xerophytes have very shallow root systems; the Saguaro cactus has a root system that is extensive yet shallow. This type of root system allows for these xerophytes to take advantage of any rainfall. They quickly suck up the water as soon as it sinks into the soil, ensuring they collect as much water as possible. The specialised roots systems are adaptations that xerophytes could not survive without. If Acacia trees had a shallow and undeveloped root systems, like water-lilies, they would immediately die. They would not be able to survive in the arid conditions of their habitat with roots designed solely for anchorage.

Stem and Xylem Adaptations

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. 

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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. 

Leaf Adaptations

Leaves are hugely important to plants as many of their cells contain chloroplasts, the sites of photosynthesis. Also, leaves are generally where transpiration occurs, making them the place water evaporates from. All three groups of angiosperm have external and internal adaptations related to their leaves, which allow them to efficiently manage the water in their cells.

Some adaptations of leaves can include the structure, the presence or absence of a cuticle and the location of stomata. All three of the adaptations mentioned are highly important to a plants survival.

Submerged hydrophytes, such as Elodea, often have leaves that are small and dissected. They rarely have a cuticle or stomata. This is because of their environment. They have large holes in their leaves which allow water and nutrients to move through the plant. Floating hydrophytes with surface leaves usually have leaves which are large and thin. Their structure makes it easy for them to float and provides a large surface area through which sunlight can be easily absorbed. Its crucial that the leaves float as their stomata need to have access to air, in order to exchange gases or breathe. Floating hydrophytes have stomata on the tops of their leaves. This is an uncommon location as most plant's stomata are on the cooler, underside of the leaf, in order to reduce water loss through transpiration. Stomata are controlled by guard cells which become turgid and open the stomata when the plant has plenty of water, and flaccid, closing the stomata in its absence.

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If a floating hydrophytes stomata were on the underside of the leaf, they would be blocked by water. Therefore, their stomata are on the top of the leaf, a necessary adaptation to ensure that transpiration can still occur. This is essential as transpiration leads to new water and nutrients being pulled through the plant. If water couldn't evaporate through the stomata, the plant wouldn't be able to have fresh water and nutrients moving through it. Consequently, it's cells wouldn't be able to produce the energy it needs to survive. To protect the stomata from being blocked by water, most floating leaves have a thin cuticle. The cuticle acts as a waterproof barrier, ensuring that water rolls off of the leaf and prevents the stomata from being submerged.

Mesophyte leaves come in all different shapes and sizes but in general, mesophytes have thin leaves with a large surface area compared to volume ratio. A common Dock leaf (Rumex Obtusifolius) is a great example of a specimen that has typical mesophyte-leaf qualities. Generally, the inside of a mesophyte's leaf consists of different structures which help to complete life processes as efficiently as possible. The palisade layer ensures the maximum amount of light is absorbed and the air spaces in the spongy mesophyll layer allow for efficient movement of gases in the leaf.

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As mesophytes can be exposed to dry and wet conditions, they need to balance the water in their cells effectively. The adaptation which helps them do this, are the stomata. The guard cells can open and close the stomata depending on water availability, and so help mesophytes manage their water. When there is plenty of water in the plant,  the stomata are opened. C02 is able to enter, and water can evaporate. When there is limited water in the plant, the stomata close, meaning that there is a reduction in water loss as it is harder for it to evaporate.

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Many mesophytes have a large quantity of stomata, most of which are situated on the lower epidermis. The location of the stomata means they are shaded and kept cool. This lowers the water concentration gradient between the inside and outside of the leaf, and reduces the amount of water lost during drier times. If the stomata were on the top of the leaves, like floating hydrophytes, the concentration gradient would be very high, as the stomata would be exposed to the wind and sun, therefore a large amount of water would be lost. It is still important for transpiration to occur, and for water to move around the plant. The quantity of stomata on mesophyte's leaves allow for a continual stream of water and nutrients to be moved around the plant by the transpirational pull. This is essential as without the numerous stomata and the intense transpirational pull, the plant's cells wouldn't get the nutrients they need. This would result in a slower production of energy and rate of growth. Also designed to help reduce the water, is a waxy cuticle which covers the top of the leaves. This waterproof barrier also helps to reduce water loss during dry periods pf time.

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Some species of xerophytes such as cacti, don't have leaves. Leaves are  often replaced by spikes or hair. This offers the plant protection against predators but also creates coolness and shade for the few sunken stomata, which are situated on the upper epidermis of the stem. This is important as the shaded areas create a lower concentration gradient and consequently reduce the amount of transpiration. Many xerophytes stomata are often sunken into the epidermis. The pits the stomata are in are called micro-climates and often have hair growing from them. The hair traps moisture and cools the environment around the stomata. Again, this helps to lower the concentration gradient and reduces the amount of water lost through transpiration, as the stomata is not exposed to the harsh sun and winds. This adaptation specifically helps xerophytes survive in their habitat. Xerophytes often close their stomata during the day when is are high temperatures. At night, when there are cooler temperatures, they open. This ensures that only a minimal amount of water is lost due to transpiration, as a higher amount would be lost during the hot day than at night. If they had large quantities of stomata like a mesophyte, they wouldn't be able to replace the water at the same rate they would be losing it, and would consequently die. 

The inside layers of nearly all xerophytes have two or more rows of palisade cells under their upper-epidermis, (this can be on leaves or stems). This is because of the intense light, which is able to penetrate the layers of the leaf or stem. By having two or more layers of palisade cells, there is a greater chance of the deep penetrating light being absorbed.

Some xerophytes have thick, succulent leaves with substantial cuticles. The succulent tissue is used to store water and the thick, waterproof cuticle reduces the amount of water lost through transpiration and evaporation. This is important because of the limited amount of water available to the plants. Every drop of water is precious, and every measure is taken to preserve any water stores the plant has. Xerophytes with leaves, just like those without leaves, have very few stomata. The decreased amount is essential as it reduces the amount of water that can be lost through transpiration. The Echeveria Hybrid is a xerophyte which has succulent leaves arranged around the stem on upward tilits. This structural adaptation allows it to catch rain and direct the water to the plant's roots. 

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One specie of xerophyte called a window plant (Haworthis Truncata) has adaptations that allow it to survive in the harshest of environments. Its leaves are succulent tissues, used to store water, and look as though they have been cut in half. The translucent ends of the leaves allow the plant to survive in environments with fierce winds, as when sand and debris covers the rest of the plant, sunlight can still be absorbed through the 'windows'. This is highly useful as it means the plant can use its stored water and access to sunlight to grow, even when the majority of the plant is covered up.

All species of angiosperm have leaves sporting different adaptations which allow them to survive in their various habitats.

In Conclusion...

The three groups of angiosperm, hydrophytes, mesophytes and xerophytes, all have adaptations that give them the ability to survive in their habitats. Each of the habitats have limiting factors, but because of the plant's adaptations, they are able to survive in their varying conditions. 

Water is essential for plants to sustain life as their cells need it to complete various life processes. Without water, plants wouldn't be able to produce glucose, the sugar they convert into energy. Without energy, plant cells couldn't function and the plant would die. 

All types of angiosperm have to balance the water in their cells to ensure they survive. As their habitats have varying amounts of water available for the plant, many plants adaptations are specific to dealing with water conservation. These adaptations allow the different plants to survive in their habitats; if they didn't have the adaptations, the plants wouldn't be able to manage their water supplies and would most likely die. Their adaptations allow the plants to survive, despite the limiting factors of their ecological niches.