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.