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3.1: Leaves

  • Page ID
    93153
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    Learning objectives

    By the end of this lesson you will be able to:

    • Identify the parts of the angiosperm leaf.
    • Describe some of the ways in which leaf parts differ from plant to plant.
    • Recognize the basic patterns of leaf shape and orientation of the veins in the leaves.

    Leaves

    Leaves are shoot structures that attach to stems and branches at nodes. Leaves are made up of cells that usually contain a high concentration of chloroplasts (cell organelles unique to plants) and are specialized sites for photosynthesis. We will explore photosynthesis in greater detail later; for now, remember that photosynthesis is the process of capturing light energy and converting it into chemical energy that can be stored in plants (like starch and sugar). In some plants, leaves may be modified for nutrient storage (as with onions, where the bulb is made up of fleshy leaves), or for support (as with peas, where some leaves are modified into tendrils that wrap around a trellis).

    Leaf labeled with petiole and blade
    Photo by yooperann, CC BY-NC-ND 2.0.

    Leaves are also the surface where water that has moved from the soil into the roots and up through the plant finally evaporates back into the atmosphere in a process called transpiration.

    Angiosperms, which are flowering plants whose seeds develop inside an ovary, tend to have flattened leaves. Many perennial angiosperms (flowering plants that can grow for many years) have leaves that senesce, or die, at the end of each growing season and are replaced at the beginning of the next growing season. Gymnosperms, plants whose seeds are produced without the protection of an ovary, tend to have needle-like leaves. Perennial gymnosperms tend to hang on to their leaves for a number of years. This saves energy, since the plant doesn’t need to grow a whole set of new leaves every year. The needle-like form helps retain moisture in harsh, dry climates, including those that are very cold and dry.

    Leaf parts and venation

    Angiosperm leaves typically have a blade or lamina, a flattened part with high chloroplast concentration. They may also have a petiole, the stalk that attaches the blade to the stem at a node. Stipules, small leaf-like bracts at the point of attachment of the petiole to the stem, may also be present. Some leaves have no petiole at all, and are termed sessile.

    In contrast to the blade-petiole structure, grasses have a sheath-type structure in which the blade attaches to an envelope of leaf tissue that wraps around the shoot of the plant and then attaches to a lower node on the stem.

    Leaf blades also have characteristic patterns of venation. In grasses, the veins lie parallel to each other and to the long edges of the leaf. We call this parallel venation, and it is typical of monocots. Most other angiosperms have a strong major midrib with veins branching from the midrib, smaller veins branching from those, and so on to form a netted venation throughout the leaf. This type of venation is typical of dicots.

    Leaves may also have a palmate venation where several veins radiate from the point where the petiole attaches to the blade. Ginkgo tree leaves have palmate venation. The veins fork, then travel a bit, then fork again, travel, fork, and so on until the veins reach the margin (edge) of the leaf. Sugar maple leaves have a classic palmate venation with five lobes.

    Plant leaf labeled with stipule, petiole, and leaf blade
    Photo by Matt Lavin, CC BY-SA 2.0.
    Left: Corn leaves have parallel veins. Center: A leaf with netted veins. Right: Ginkgo tree leaves have palmate veins.
    Left: Corn leaves have parallel veins. Center: A leaf with netted veins. Right: Ginkgo tree leaves have palmate veins. L to R photo credits: Image by Hans Braxmeier, CC0; Thangaraj Kumaravel, a CC BY 2.0; Nate Hofer, CC BY-NC-SA 2.0.

    Watch this video on leaf veins:

    Leaf segmentation

    Simple leaves

    Simple leaves have uninterrupted leaf margins. The leaf may have lobes like the oak leaf, but the blade has one continuous margin. The venation differs in the two examples below. The oak leaf is pinnate, with a major vein heading down the midrib of the leaf. The maple leaf is palmate, with major veins that radiate from the point of attachment to the petiole.

    Oak leaves have pinnate venation (L), while maple leaves have palmate venation (R).
    Oak leaves have pinnate venation (L), while maple leaves have palmate venation (R). Photo credits L-R: Dendroica cerulea, CC BY-NC-SA 2.0; Richard Skiba, CC BY-NC-SA 2.0.

    Compound leaves

    The sumac leaf is a good example of how a compound leaf has a blade that is completely interrupted and segmented into separate leaflets. What you see in the picture — the entire thing — is one leaf. The leaf is divided or segmented into leaflets.

    Tree of heaven, (Ailanthus altissima) has pinnately compound leaves.
    Tree of heaven, (Ailanthus altissima) has pinnately compound leaves. Photo by Dr. Leslie J. Mehrhoff, CC BY 3.0.

    The petiole extends from the point of attachment at the node to the first leaflet. The central axis from that point on — from the first leaflet to the tip of the leaf — is called the rachis.

    Virginia creeper is an example of a palmately compound leaf. The stalk that connects the leaflet to the top of the petiole is called the petiolule. In this case there is no rachis; all leaflets are attached directly to the top of the petiole.

    Virginia creeper (Parthenocissus quinquefolia) has palmately compound leaves.
    Virginia creeper (Parthenocissus quinquefolia) has palmately compound leaves. Photo by Gavatron, CC BY-NC-SA 2.0.

    Below is a compound leaf with three leaflets, called a trifoliate leaf. Soybean, clover, and dry bean all have trifoliate leaves. In contrast to the palmately compound leaf above, there is a rachis to which the central leaflet is attached.

    Poison ivy (Toxicodendron radicans) has trifoliate leaves
    Poison ivy (Toxicodendron radicans) has trifoliate leaves. Photo by DaveSpier, CC BY-NC-SA 2.0.

    To tell whether a leaf is simple or compound, ook for attachment to a node. If the point of attachment doesn’t appear to be a node, it is likely a leaflet attached to the rachis of a compound leaf.

    Watch this video on compound leaves:

    Additional optional reading

    For more information about leaves, explore this Wikipedia page, starting about halfway down at the heading “Morphology (large-scale features)” and continuing through “Veins.”

    Review questions
    • What advantage do angiosperm leaves have because they are flattened?
    • What advantage do gymnosperm leaves have because they are needle-like?
    • What is the difference between a simple leaf and a compound leaf?
    • Is a petiolule found in a compound or simple leaf? To what structure does it attach?
    • What is the difference between a leaf with palmate venation and a palmately compound leaf?
    • Draw and label a picture of a leaf with these parts: rachis, petiole, petiolule, and leaflet.

    This page titled 3.1: Leaves is shared under a CC BY-NC 4.0 license and was authored, remixed, and/or curated by Tom Michaels, Matt Clark, Emily Hoover, Laura Irish, Alan Smith, and Emily Tepe (Minnesota Libraries Publishing Project) via source content that was edited to the style and standards of the LibreTexts platform; a detailed edit history is available upon request.