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3.4.3: Leaf Modifications

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    37019

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    Learning Objective

    Identify common leaf modifications and their functions, including storage leaves, succulent leaves, spines, tendrils, phyllodes, showy bracts, plantlets, and insect traps.

    The structure and function of a leaf can be modified over the course of evolution as a plant adapts to a particular environment (Figure \(\PageIndex{1}\)). When function of the leaf blade is no longer primarily photosynthesis, some other plant part is usually modified to take its place. Storage leaves are thick leaves underground that store starch (as with a bulb; see Stem Modifications). Succulent leaves are also thickened leaves, but they are found above ground, still conduct photosynthesis, and function primarily in water storage (Figure \(\PageIndex{2}\)).

    Examples of leaf modifications. A single leaf has been filled in on each of the four examples.
    Figure \(\PageIndex{1}\): A diagram of leaf modifications. In each plant (A-D), one leaf has been shaded with orange. In A, the blade has been modified for a trap, the petiole is flattened into a phyllode. In B, the blade is a compound leaf and the petiole is flattened into a phyllode. In C, some of the leaves are thin, coiling tendrils (emerging below axillary buds). In D, the basal leaves are thick and fleshy with a short stem (a bulb). Diagram by Nikki Harris (CC-BY) with labels and color added by Maria Morrow.
    A plant viewed from the top with succulent leaves. The leaves are thicker than normal leaves.
    Succulent leaves are much thicker than regular leaves. They are nearly cylindrical and come to a dull point at the end.
    Figure \(\PageIndex{2}\): Succulent leaves are thickened for water storage. Left photo by Simon Burchell (CC-BY-SA), and right image by Melissa Ha (CC-BY).

    Spines are sharp projections derived from leaves that function in plant defense. Almost all cacti (in the plant family Cactaceae) have spines (Figure \(\PageIndex{3}\)). Other examples include barberry and some Acacia species, in which large spines house mutualist ants. Spines can also be formed from stipules (stipular spines) or bud scales. Recall from Stem Modifications that thorns and prickles have the same function, but they are derived from whole stems or the outer tissue layers of stems, respectively.

    Several spines from a cactus are pale white with orange tips. In the white area, small greenish dots can be distinguished.
    Figure \(\PageIndex{3}\): Cactus spines are not stems, but modified leaves. These spines have a thick cuticle and stomata can be seen as darker green areas within the pale white cuticle. Photo by André Karwath (CC-BY-SA).

    Tendrils are another structure that can originate from multiple structures, including stems, leaves, or leaflets. These are narrow, coiling structures that climbing plants attach to nearby structures for support (Figure \(\PageIndex{4}\)).

    A wide leaf blade with a tip that extends into a tendril curling around an adjacent stem
    Pinnately compound leaves of a pea plant. A few of the leaflets at the end are narrow and coiling, functioning as tendrils.
    Figure \(\PageIndex{4}\): Left: The tips of the leaves of glory lily are modified as tendrils. Right: some of the leaflets in the compound leaves of pea plants are modified as tendrils. Left image by Krishna satya 333 (CC BY-SA), and right image by Melissa Ha (CC-BY).

    A phyllode resembles cladodes and phylloclades but refers to a flattened petiole that resembles a leaf blade (rather than a narrow stalk). The Australian acacias (Acacia) have phyllodes (Figure \(\PageIndex{5}\)).

    An Acacia phyllode is a flattened, wide petiole. A compound leaf emerges from the phyllode.
    Figure \(\PageIndex{5}\): Acacia koa with a flat phyllode between the branch and the pinnately compound leaf blades. Image and caption from Wmpearl (public domain).

    Showy bracts are brightly colored leaves function in attracting pollinators. From a distance, showy bracts often appear like the petals of a flower. However, the actual flowers are typically small and in a cluster surrounded by the bracts (Figure \(\PageIndex{6}\)).

    A plant with regular green leaves and hot pink bracts that appear like flower petals.
    Closeup view of dogwood. Four pink bracts appear like petals. The actual flowers are in the center of the four bracts.
    Figure \(\PageIndex{6}\): Showy bracts are brightly colored leaves modified to attract pollinators in Bougainvillea (left) and dogwood (right). Although bracts superficially look like petals, the actual petals are part of tiny flowers at the central point where the bracts meet. Images by Melissa Ha (CC-BY).

    Plantlets are mini plants that grow on the main plant and then fall off and grow into new plants. In Kalanchoë , mitosis at meristems along the leaf margins produce tiny plantlets that fall off and grow independently into mature plants. This is a form of asexual reproduction (Figure \(\PageIndex{7}\)).

    Thick leaves have small rosettes on the margin that look like tiny plants
    Figure \(\PageIndex{7}\): Plantlets on the leaf margins of Kalanchoë. Plantlets that fall and land on the soil can grow into full-sized plants. Note that this species is also an example of succulent leaves. Image by Melissa Ha (CC-BY).

    Carnivorous plants grow in bogs where the soil is low in nitrogen, and their leaves are adapted to help them to survive this nutrient-poor environment. In these plants, leaves are modified as traps to capture insects. While they still conduct photosynthesis to capture energy and synthesize sugars (and are thus autotrophs), they rely on insect-capturing leaves as a supplementary source of much-needed nitrogen, similar to fertilizer. Several examples of carnivorous plants are the cobra lily (Darlingtonia), various pitcher plants (Nepenthes [Figure \(\PageIndex{8}\)], Cephalotus, Sarracenia), the butterwort (Utricularia), the sundew (Drosera; Figure \(\PageIndex{9}\)), and the best known, the Venus flytrap (Dionaea; Figure \(\PageIndex{h}\)).

    Dionaea (venus fly trap) leaf trap with teeth and flattened phyllode, labeled The Nepenthes leaf trap looks like a pitcher. A narrow tendril is attached to it. The flat phyllode is barely visible.

    The teeth of a Venus fly trap (left) and the deep chamber formed by the leaf of a pitcher plant (right).
    Figure \(\PageIndex{8}\): The (a) Venus flytrap has modified leaves that can capture insects. When an unlucky insect touches the trigger hairs inside the leaf, the trap suddenly closes. The opening of the (b) pitcher plant is lined with a slippery wax. Insects crawling on the lip slip and fall into a pool of water in the bottom of the pitcher, where they are digested by bacteria. The plant then absorbs the smaller molecules. Top two photos by Maria Morrow (CC-BY). Bottom photos are modification of work by Peter Shanks; credit b: modification of work by Tim Mansfield.
    Drosera (sundew) with glandular trichomes to trap insects
    Figure \(\PageIndex{9}\): The ends of sundew (Drosera sp.) leaves are covered with glandular trichomes. Each trichome has a blob of sticky fluid at the tip. Insects are attracted to the fluid, becoming trapped in it, as the plant slowly digests their bodies for the mineral nutrients that are lacking in their environment. Image by Maria Morrow (CC-BY).

    Attributions

    Modified by Melissa Ha from the following sources:

    This page titled 3.4.3: Leaf Modifications is shared under a CC BY-SA 4.0 license and was authored, remixed, and/or curated by Melissa Ha, Maria Morrow, & Kammy Algiers (ASCCC Open Educational Resources Initiative) .