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3.3.4: Stem Modifications

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    27721

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    Learning Objective

    Define and provide examples of the major stem and shoot modifications.

    Stems (or entire shoots) of some species deviate in structure and function from a typical stem. These are called stem modifications (shoot modifications). While leaves are part of the shoot, modifications involving just the leaf are discussed separately (see Leaf Modifications).

    Rhizomes and stolons are horizontal stems that function in propagation (Figure \(\PageIndex{1}\)). Rhizomes are belowground stems that burrow into the ground just below the soil surface. They have short internodes and usually have small, scale-like leaves that are not photosynthetic. Buds from the axils of the leaves make new branches that will grow to become aboveground shoots. Ginger (Zingiber) and Johnson grass (Sorghum halepense) form rhizomes. Stolons (runners) are aboveground horizontal shoots, which sprout and produce a new plants. Compared to rhizomes, stolons have long internodes. Examples include strawberry (Fragaria), spider plants (Chlorophytum), and Bermuda grass (Cynodon dactylon).

    Ginger shoots arise from the ginger rhizome, which is thick, has short internodes, and papery leaves.
    Several long, white stolons emerge from a spider plant. A plantlet is at the end of the stolon
    Figure \(\PageIndex{1}\): Examples of horizontal stems. Ginger rhizomes are thick and belowground (top), and stolons of this spider plant are aboveground with long internodes (bottom). Leaves and roots emerge from the end of a stolon, forming a smaller plant. Top image by Sengai Podhuvan (CC-BY-SA), and bottom image by Eptalon (CC-BY-SA).

    Tubers, corms, and bulbs are modified for starch storage. Tubers, such as potatoes, are thick, belowground stems found at the tips of rhizomes or stolons (figure \(\PageIndex{2}\)). The “eyes” of potato are actually lateral buds, and the tuber body is comprised of many parenchyma cells that contain amyloplasts with starch. Corms and bulbs are modified shoots. A corm is a short, thick underground storage stem with thin scaly leaves (for example, Gladiolus and Crocus; Figure \(\PageIndex{3}\); Video \(\PageIndex{1}\)). A bulb, such as an onion, differs from a corm in the fact that it stores its nutrients in its fleshy leaves (Figure \(\PageIndex{3}\)). Lilies and tulips also form bulbs.

    Potato tubers in the soil. They are oval, expanded structures.
    Figure \(\PageIndex{2}\): Potato tubers are expanded, belowground stems modified for starch storage. Image by pxhere (public domain).
    A corm cut longitudinally reveals a thick, white stem
    An onion bulb cut longitudinally has a small stem and layers of thick, fleshy leaves.
    Figure \(\PageIndex{3}\): Both corms (left) and bulbs (right) are shoots modified for starch storage. The Crocosmia corm has a thick, expanded stem stores starch, and the leaves that surround it are non-photosynthetic and papery. The onion bulb has a relatively small stem surrounded by thick, fleshy leaves. Left image by JonRichfield (CC-BY-SA), and right image by Amada44 (CC-BY-SA).

    Some plants have sharp, generally narrow projections that function in defense against herbivores. Such structures are called thorns when they arise from an entire stem. Hawthorn (Crataegus) and Bougainvillea produce thorns. In contrast, prickles form from the surface tissues (epidermis and cortex) of the stem rather than the whole organ (figure \(\PageIndex{4}\)). Rose (Rosa) and blackberry (Rubus) produce prickles. Spines are similar structures derived from leaves (see Leaf Modifications).

    A hawthorn stem with sharp, brown thorns arising from the axils above each leaf.
    Close-up of a rose stem with red, triangular prickles
    Figure \(\PageIndex{4}\): Thorns (left) and prickles (right). The thorns of the hawthorn are derived from the whole stem. They arise from axillary buds in the axils just above the leaves. The sharp projections on the rose stem are technically classified as prickles because they are derived from only the surface tissues of the stem. Left image by Rasbak (CC-BY-SA), and right image by Ingrid Taylar (CC-BY).

    Tendrils are thin, string-like structures that allow the shoot to attach to other surfaces to access light. Tendrils can be derived from stems, leaves, or leaflets, and they are common in vines. Morning glory and sweet potato (Ipomoea), grapes (Vitis), and many members of the cucumber family (Cucurbitaceae) such as cucumbers, pumpkin, and squash have tendrils that arise from stems (Figure \(\PageIndex{5}\)).

    A wild cucumber, with small, oval, spiky fruits and scattered tendrils.
    Figure \(\PageIndex{5}\): The thin, pale tendrils of the wild cucumber plant arise from stems and coil around surrounding structures. Image by erwin66as (public domain)

    Cladophylls (cladodes) are stems that resemble leaves in function and appearance, arise from the axils of a shoot, and have determinate growth (stop growth after reaching a certain size). They may be cylindrical (Asparagus; Figure \(\PageIndex{6}\)) or flattened (butcher's broom, Ruscus; figure \(\PageIndex{7}\)). Phylloclades are flattened stems that resemble leaves that can continue growing indeterminately (Figure \(\PageIndex{8}\)). They are subtended by reduced, scale-like leaves. Examples include prickly pear cactus (Opuntia), Christmas cactus (Schlumbergera), and ribbon plant, Homalocladium. Phyllodes are similar are similar to cladophylls and phylloclades, but they are modified petioles (see Leaf Modifications).

    Cladophylls of asparagus form a fluffy mass of thin, cylindrical stems.
    A young asparagus plant has not yet formed cladophylls. The actual leaves are reddish, triangular scales along the stem.
    Figure \(\PageIndex{6}\): Cladophylls in Asparagus (left) and actual leaves of young asparagus marked by arrows (right). Left image by DenesFeri (CC-BY-SA), and right image by Sam Saunders (CC-BY-SA).
    Cladophylls of butchers broom are flattened like leaves. They are rounded and come to a point at the end.
    Figure \(\PageIndex{7}\): Cladophylls in butchers broom (Ruscus) are flattened like leaves. A flower arises from a node within the cladophyll. Image by Rosser1954 (CC-BY-SA).
    Cladophylls of Christmas cactus are flattened stem segments.
    Figure \(\PageIndex{8}\): Phylloclades in Christmas cactus. Image by Mokkie (CC-BY-SA).

    Some shoots are modified as insect traps are used by some carnivorous plants, such as bladderwort (Utricularia; Video \(\PageIndex{1}\)).

    Video \(\PageIndex{1}\): This video shows a waterflea caught in a bladderwort insect trap. This video has no sound. Here is a description of what occurs during the video: Bladderwort and water fleas. Taken through a Leica microscope 60X objective- standard microscope lighting. The water fleas are tiny crustaceans, and they are swimming around in slow motion. At 3:50, one water flea is about to get caught. It swims into the cup-shaped leaf and escapes. A slow-motion replay of that moment shows the leaf close around the water flea temporarily before it escapes.

    Attributions

    Curated and authored by Melissa Ha using 7.4: Modified Shoot from Introduction to Botany by Alexey Shipunov (public domain)

    This page titled 3.3.4: Stem 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) .