In many orders of angiosperms, mainly among dicotyledons, the leaf may possess one or, more commonly, two stipules. A stipule is an outgrowth of the lower zone (Unterblatt) of a young leaf, part of the leaf base.
Stipules are relatively easy to observe and diagnose when these appendages are attached to the base of the petiole (i.e., the proximal end), but often the stipules deceptively appear to arise from the stem (interpetiolar stipules; Examples: Tithonia diversifolia and a geranium), although technically they are not part of the stem. Always the best place to look for stipules is among the immature, developing leaves at the shoot tip, because in many plant species the stipules die, shrivel, and are abscised relatively soon after the leaf has finished it growth (Example: a lupine).
The ochrea is described as fused stipules that form a sheath around the stem. This is especially easy to observe in the broad-leaved species of the genus Rumex, which often has a persistent, membranous ochrea, and Family Magnoliaceae, in which they are deciduous (Examples in Magnoliaceae: Magnolia grandiflora, M. sprengeri, Talauma, and a tulip tree).
Stipule-like outgrowths may also be found at the base of leaflets on a compound leaf, in addition to stipules at the base of the petiole (rachis). Leaflet stipules are termed stipels or, less commonly, secondary stipules.
When stipules abscise, a stipular scar develops. In certain lineages, e.g., Family Magnoliaceae (Example: Talauma hodgsonii) or the genus Ficus (Family Moraceae), the stipule encircles the stem and thereby forms an annular scar, diagnostic on young woody stems.
Although nobody has counted, probably less than a quarter of all known angiosperm species have stipules. Among the dicotyledons, stipules have been observed in approximately 140 families, although in nearly half of these only relatively few species possess stipules. There are many groups of angiosperms in which stipules have never been reported, so that when stipules are observed, they often help an investigator to guess its home family.
In certain families or orders, such as legumes (Examples: Melilotus, Bauhinia, and Calliandra) and roses (Examples: mountain mahogany and Photinia), stipules are characteristic and nearly always present in some form. Most species of the exceeding large family Rubiaceae have interpetiolar stipules (A typical example: Galium). The large stipule of sycamore species (Platanus) appears to surround the stem and is very leaf-like. Families of the wideranging orders Malvales (including mallow, hibiscus, and Chinese bellflower) and related Urticales (ramie) and Euphorbiales have stipules.
Stipules are uncommon among the monocotyledons, where they tend to be small and vestigial.
The original function of stipules is obscure, but may have been involved as protection for the emerging leaves (stipules enclosing young bud and young leaf emerging from stipule pair in Exbucklandia). Conspicuous stipules covering buds also can be observed in Cunonia capensis. In some plant groups, e.g., figs (Ficus), relatives of teak (Family Dipterocarpaceae), and certain mangroves (Family Rhizophoraceae), the stipule is well developed and appears to serve in bud protection. However, the majority of angiosperms living today show no disadvantage by lacking them, and in many the stipule is small and vestigial, without any obvious function.
There are a number of interesting functions demonstrated for stipules of certain plant species.
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