Leaves are essential to most plants survival as their cells generally contain chloroplasts which are the sites for photosynthesis. Leaves are usually the site for transpiration because they contain stomata, making them the site for evaporation. All three plant groups have both internal and external adaptations linked to their leaves, to combat transpiration and maintain the correct water balance within their cells.
The leaves of hydrophytes can differ depending on whether they are submerged or floating on the surface. Submerged leaves tend to be finely divided or ribbon (thread) like to increase surface area for diffusion of minerals, nutrients, gases like CO2 and water. There tends to be little or no waxy cuticle because there is no need to conserve water in a submerged leaf. Floating leaves tend to be large, wide and float on top of the water to increase surface area and maximise the absorption of sunlight for optimum rates of photosynthesis. Due to this the leaves tend to be dark green indicating the presence of large amounts of chlorophyll. Floating leaves have a large amount of stomata but only on the top of the leaf - if stomata were on the underside of the leaf they would be blocked by water resulting in transpiration not being able to occur. Which is an extremely important process that ensures water and nutrients are pulled up into the plant to replace water that has evaporated. This is an extremely important adaptation for plants as their survival depends on being able to draw up nutrients and water via transpiration (TaliaPowell and Profile ) & (Hydrophytes ). The top of leaves tend to have a thin waxy cuticle to protect the stomata and help to keep water off the leaf (waxy surface makes it easier for water to roll of the leaf) and to prevent the stomata from being submerged. Floating leaves have long petioles to enable them to move up or down with changes in the level of water, therefore keeping the stomata from being submerged. The stomata will always be open because the abundance of water means that the cells will always be turgid, resulting in there being enough turgor pressure for the stomata to constantly remain open. This is so that water can transpire - subsequently allowing more nutrients and water to be drawn up the plant - and so that gases can efficiently diffuse in and out, resulting in optimum rates of photosynthesis, respiration and the plants' other life processes.
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The leaves of hydrophytes can differ depending on whether they are submerged or floating on the surface. Submerged leaves tend to be finely divided or ribbon (thread) like to increase surface area for diffusion of minerals, nutrients, gases like CO2 and water. There tends to be little or no waxy cuticle because there is no need to conserve water in a submerged leaf. Floating leaves tend to be large, wide and float on top of the water to increase surface area and maximise the absorption of sunlight for optimum rates of photosynthesis. Due to this the leaves tend to be dark green indicating the presence of large amounts of chlorophyll. Floating leaves have a large amount of stomata but only on the top of the leaf - if stomata were on the underside of the leaf they would be blocked by water resulting in transpiration not being able to occur. Which is an extremely important process that ensures water and nutrients are pulled up into the plant to replace water that has evaporated. This is an extremely important adaptation for plants as their survival depends on being able to draw up nutrients and water via transpiration (TaliaPowell and Profile ) & (Hydrophytes ). The top of leaves tend to have a thin waxy cuticle to protect the stomata and help to keep water off the leaf (waxy surface makes it easier for water to roll of the leaf) and to prevent the stomata from being submerged. Floating leaves have long petioles to enable them to move up or down with changes in the level of water, therefore keeping the stomata from being submerged. The stomata will always be open because the abundance of water means that the cells will always be turgid, resulting in there being enough turgor pressure for the stomata to constantly remain open. This is so that water can transpire - subsequently allowing more nutrients and water to be drawn up the plant - and so that gases can efficiently diffuse in and out, resulting in optimum rates of photosynthesis, respiration and the plants' other life processes.
http://www.rna.ca/photography/water_lily/index.shtml
| The leaves of mesophytes are greatly varied in both size and shape. Leaves are generally thin and large to increase chance of sunlight being rebounded off the spongy mesophyll for chlorophyll to absorb and convert into ATP for photosynthesis. The second picture shows how the leaf is very thin so that there are greater chances of sunlight absorption. A waxy cuticle is present on the upper epidermis (occasionally on lower epidermis) to reduce water loss from transpiration. Stomata are only on the lower epidermis where it is cooler, this is so that the water molecules have lower kinetic energy (due to temperature) and therefore have a less of a driving force for transpiration (resulting in less water loss (Roberts 2011). During periods of high temperatures and/or hot, dry winds the stomata will close to conserve water within the plant. Deciduous trees shed their leaves during winter because when water is frozen in the soil it cannot be pulled up into the plant therefore no transportation stream would occur (Mesophytes ). Some leaves have white patches which indicates that there is no chloroplasts and therefore no photosynthesis occurring, this is to save energy as the white parts of the leaf may not be exposed to the sun as much as the other parts of the leaf. |
| The leaves of Xerophytes tend to have been replaced with spikes or fur, however in some cases there are leaves but they have become small to reduce surface area and have a very thick waxy cuticle to prevent water loss through evaporation; e.g. the needle like leaves of Conifers and the spines of cacti (Roberts 2011). The leaves are rolled and hairy with the stomata inside, generally sunken in and covered in furry spikes to create a sub-climate where moist air is trapped, increasing humidity and reducing water loss (Revision:Xerophytes and hydrophytes -...). The stomata being sunken in results in the light intensity it is subjected to being lower, meaning that the water molecules with have less of a driving force for transpiration because they will have lower kinetic energy. Some species of xerophytes only open their stomata at night when the temperature is lower meaning that while there will be immediate water loss it will not be as significant due to lower kinetic energy. The leaves of xerophytes have a very thick waxy cuticle to prevent water loss and they have two rows of palisade cells compared to the one row that mesophytes and hydrophytes have. This is because xerophytes are subjected to a much higher light intensity, so having another row of palisade cells means that there is a greater chance of sunlight being absorbed for photosynthesis (TaliaPowell and Profile ). The leaves (and stems) of xerophytes tend to be a very pale green in colour, this because of reduced number of chloroplasts and the pale colour reflects light, meaning that the temperature the plant is subjected to is less resulting in less water loss, e.g. Aloe Vera (picture 5) |