The word ligule comes from Latin “ligula” which means strap. It is a thin outgrowth at the junction of a leaf and leafstalk. Ligules occur only in some grass species. Basically, it acts in a passive way in protecting the culm and leaves that it encloses. It protects the leaf from the entry of water, dust and harmful spores. The ligule is the continuation of the leaf sheath and it encircles or clasps the stem as does the sheath.
Ligules take several forms. From a translucent membrane to a fringe of hairs. They can be very short 1–2 mm to very long 10–20 mm. Some have smooth edges, others ragged. A few species of grasses don’t even have a ligule!
The three basic types are: membranous, a fringe of hairs (ciliate), and absent. The shape, length, and appearance of the ligule is used to categorize and separate genera and species.
These are outgrowths borne on either side of the base of a leafstalk (petiole). A pair of them are considered part of the anatomy of the leaf of a typical flowering plant (Angiosperm). In many species, the stipules are inconspicuous or entirely absent. These of course are termed exstipulate !!!
Stipules are basically a modified leaf. Not very common in the recently evolved monocotyledons. They usually occur in pairs in the dicots.
They function to protect the emerging leaf or bud. Usually short-lived – they drop off after the leaves matures.
Very common in the Beech family – Stipules protect the emerging bud and leaves.
Stipules are morphologically variable and might appear as glands, scales, hairs, spines, or laminar (leaf-like) structures. If a single stipule goes all the way around the stem, it is known as an ochrea.
Types of stipules
The three types of stipules according to duration are caducous, deciduous and persistent. Caducous stipules fall off before the leaf unfolds. Deciduous stipules fall off immediately after the leaf unfolds. Persistent stipules remain attached to the plant.
Ficus (common houseplant – rubber plant) are distinguished by terminal conical stipules that cover the newly developing leaves. It gets shed as the new leaf expands. Called “interpetiolar” – it is located in between the petioles, as opposed to being attached to the petioles. Generally these stipules are fused together, so it appears that there’s just one between each leaf instead of two.
Just Plain Scary !
Sometimes the stipules evolve for protection – check out these scary stipules!
Common locust tree – Robinia
For grasses with ligules or beeches with stipules –
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Article written by our Staff Horticulturist, Peter B Morris, BSc, MSc, MBA
All photographs used with permission @SHUTTERSTOCK