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5: Leaves

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    123927

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    Leaves are specialized organs of the shoot for performing photosynthesis. A leaf is often a relatively large, flat surface used to optimize sunlight capture. Most leaves have determinate growth, meaning that growth stops once the leaf reaches a certain size. This is in contrast to the overall plant body, which grows indeterminately. Leaves arise from the shoot apical meristem through leaf primordia.

    Usually, true leaves are easily distinguishable, but in some species, modified stems (cladophylls and phyloclades) or leaflets (parts of leaves) superficially appear like whole leaves. True leaves subtend an axillary bud in the axil and are lateral (rather than at the tip, or apex of the shoot; Figure \(\PageIndex{1}\)). New shoots and leaves do not arise from true leaves.

    A potted plant with the root system, shoot system, and organs labeled.
    Figure \(\PageIndex{1}\): Leaves are lateral structures of the shoot system. The plant here is divided into a generative shoot system, vegetative shoot system, and root system. The generative shoot system consists of the floral unit, which is supported by the pedicel. The vegetative shoot system consists of stems and leaves. Leaves attach at nodes on the main stem. Stem segments between nodes are internodes. Leaves consist of a flattened blade and stalklike petiole, and they subtend axillary buds (labeled axial bud here). Some axillary buds have developed into secondary shoots. Terminal (apical) buds are found at the end of stems. In the root system, lateral roots arise from the main root (taproot). Adventitious roots arise from the stem. At the base of the main stem, just before it transitions to the main root is the hypocotyl.

    Most leaves are usually green, due to the presence of chlorophyll in the chloroplasts of their cells. However, some leaves may have different colors, caused by other plant pigments that mask the green chlorophyll. The thickness, shape, and size of leaves are adapted to the environment. Each variation helps a plant species maximize its chances of survival in a particular habitat.

    Attributions

    Curated and authored by Melissa Ha using the following sources:

    • 5.1: External Structure of Leaves
      Leaves commonly consist of a petiole and lamina (blade). The edge of the leaf is called the margin, and the vascular bundles form veins. Leaves differ in their arrangement  and whether they are simple or compound. Additionally, leaves are distinguished by their venation, margin, and shape.
    • 5.2: Internal Leaf Structure
      There are three main tissue types in the leaf: the epidermis, mesophyll (modified parenchyma cells that conduct photosynthesis), and vascular bundles (veins). Specialized leaf adaptations allow different plant species to grow at differing water availabilities. Such leaves are described as mesophytic, hydrophytic, or xerophytic. Pine and corn leaves have unique adaptations as well. Developing in the sun versus the shade also influences leaf structure.
    • 5.3: Leaf Modifications
      While photosynthesis is the main function of most leaves, some leaves are modified for other purposes. These include storage leaves, succulent leaves, spines, tendrils, bracts, and insect traps. Particularly when the leaf blade is modified for another function, the petiole may be wide and flat to conduct photosynthesis (phyllode). Plantlets are small propagules that form along leaf margins functioning in asexual reproduction.
    • 5.4: Chapter Summary

    Thumbnail image: The red petal-like structures of the poinsettia plant are actually modified leaves called bracts, which are adapted to attract pollinators. The true flowers tiny structures where the bracts meet. Image by Scott Bauer (public domain).

    This page titled 5: Leaves is shared under a CC BY-NC 4.0 license and was authored, remixed, and/or curated by Teresa Friedrich Finnern.