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4.2: Internal leaf characteristics

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    Leaves have three main internal regions; the epidermis, the mesophyll, and the veins. The epidermis is the outermost layer, being present on the top and bottom of the leaf, the upper and lower epidermis, respectively (Figure \(\PageIndex{1}\)). The epidermis is usually one cell layer thick, but it can be several layers thick in plants growing in arid environments, to prevent excessive water loss. A waxy layer known as the cuticle is found outside of the epidermis. The cuticle is made of a waxy substance that helps to reduce water loss. Some leaves may have small hairs (trichomes) on the leaf surface, which help to decrease herbivory either mechanically (reducing insect movement) or chemically (producing chemicals). They can also reduce transpiration by reducing airflow on the leaf surface or by reflecting sunlight, protecting the plant against UV damage.

    DutraElliott modified.png
    Figure \(\PageIndex{1}\): Internal leaf anatomy. Modified to replace Russian with English labels and added chloroplast. By Chan, CC BY 3.0, via Wikicommons.

    The epidermis protects the inner leaf tissues and helps in the regulation of gas exchange. It possesses small openings called stomata, that facilitate gas exchange (Figure \(\PageIndex{2}\)). Carbon dioxide (CO2), needed for photosynthesis, enters the leaf through the stomata, while oxygen, a product of photosynthesis, and water vapor from transpiration exit. In most plants, stomata are usually located in the lower epidermis. The opening and closing of the stomata are controlled by guard cells, which surround each stoma. Climatic conditions, water availability in the soil, and the time of the day affect the opening and closing of stomata. For example, most plants close their stomata during the nighttime as the CO2 required is only needed when sunlight is available for photosynthesis.

    Figure \(\PageIndex{2}\): Stomata on the leave of purple heart (Zebrina sp.) By DutraElliott is licensed under CC BY-NC-SA 4.0 via Flickr. A derivative from the original work “Stomata top view” by AioftheStorm, Public Domain, via Wikimedia Commons. Modified to include labels.

    In between the upper and lower epidermis is the mesophyll, which has two differentiated regions. The palisade mesophyll is found just below the upper epidermis and it is composed of parenchyma cells closely arranged in a brick-like fashion. These cells are packed with chloroplasts and it is here where most of the photosynthesis takes place. The other region is called the spongy mesophyll and it is composed of loosely arranged parenchyma cells that have numerous air spaces in between (Figure \(\PageIndex{1}\)). The veins are the third tissue present inside the leaves. Veins contain bundles of vascular tissue, xylem for transporting the water for photosynthesis and transpiration, and phloem for transporting the sugars produced in photosynthesis.

    This page titled 4.2: Internal leaf characteristics is shared under a CC BY-NC-SA 4.0 license and was authored, remixed, and/or curated by Daniela Dutra Elliott & Paula Mejia Velasquez.

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