Stipule

stipule [ STIP-yool ] noun: small leaflike structure typically found at the base of a leaf in some plants; adjective, stipulate [ STIP-yoo-leyt ], stipular [ STIP-yoo-lur ]

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Stipules are modified leaves that originate from the bases of leaf primordia (rudiments). Their size and shape vary widely among species, but they are typically much smaller than and distinct from the primary leaves, often leading casual observers to overlook them. However, rose enthusiasts likely recognize stipules even if they do not know them by name. In roses, a prominent pair of stipules fuses along the petiole at its base, creating a distinctive wing-like appearance.

Stipules occur in approximately 150 of the more than 400 angiosperm families, including about 60% of rosid families and about 30% of asterid families (Ye & De Craene, 2024). While stipules can be found in certain dicot families, they are rarely seen in monocots. A notable exception is Smilax (greenbrier), which possesses the only recognized stipule-modified tendril. There remains controversy, however, over whether leaf sheaths and ligules in monocots are equivalent to stipules.   

Depending on the plant species, stipules may develop simultaneously with the leaf, or at an earlier or later stage. They mostly occur in pairs, either fused together or unattached to each other, on both sides of the leaf; unattached is the most common type. Some may change their form during development. For example, in some species, stipules can start in an annular form, encircling the stem at the leaf/bud base, and then may separate into pairs as the plant matures. Stipules can be caducous (dropping off early as the leaf expands), deciduous, or persistent. When a stipule is foliaceous (leaflike), it can be differentiated from a true leaf by the absence of a bud in its axil. 

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Different Types of Stipules

• Adnate: Stipules that are fused to the petiole base in a pair, e.g., Aronia spp., Duchesnea indica, Fragaria virginiana, Rosa spp.
• Annular: Stipules that are “single and surround the node, leaf bud, or petiole base without any separation” (Ye & De Craene, 2024), e.g., Liriodendron tulipifera, Magnolia grandiflora, Persicaria spp., Polygonum spp. They occur in less than 10% of stipulate families.
• Foliaceous: Stipules that are leaflike in appearance, e.g., Agrimonia gryposepala, Baptisia australis, Liriodendron tulipifera, Viola rafinesquei.
• Free Lateral: Stipules that are attached to the sides of the petiole and are not fused together, e.g., Baptisia australis, Liriodendron tulipifera, Magnolia grandiflora, Melilotus officinalis, Viola rafinesquei. 
• Interpetiolar: Stipules located between the petioles of adjacent leaves, e.g., Cephalanthus occidentalis, Diodia virginiana, Psychotria capensis. Only 20% of stipulate families have interpetiolar and intrapetiolar stipules.
• Intrapetiolar: Stipules that occur between the petiole and the stem itself, against alternate or opposite leaves, e.g., Myriocarpa stipitata. 

• Ocreate: Stipules that form a sheath around the stem at the base of the leaf petiole encircling the stem node, e.g., Persicaria spp., Polygonum spp.
• Spinous: Stipules that are modified into thorn-like structures, e.g., some Euphorbia spp., Vachellia spp.
• Tendrillar: Stipules that are modified into tendril-like (thin, twining/climbing) structures, e.g., Smilax.

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What are the functions of stipules? Stipules may provide protection, help in moisture retention, and offer support to the developing leaf and bud. Some stipules can contribute to photosynthesis. Sharp, spinous stipules can discourage herbivores. Spiraling, tendrillar stipules can aid in climbing or cling to structures to add extra support. Pollinators may be attracted to stipules “transformed into nectary tissue (e.g. the glands on the cyathium (false flower) of Euphorbia) [or] glandular tissue (e.g. Rubiaceae…)” (Ye & De Craene, 2024). For botanists and gardeners, the presence and characteristics of stipules provide another way to distinguish among and help identify plant species.

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References

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Ye T, De Craene LR. July 2024. Stipules in angiosperms. Botanical Journal of the Linnean Society. 205(3): 191–242. doi: 10.1093/botlinnean/boad076.