STIPULES AND STIPELS

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.

Which Plants Have Stipules?

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.

Forms and Functions

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.

1. Stipules that are green may be leaf-like (Examples: flowering quince, a geranium, and a sycamore) , linear, thread-like, or reduced to minute scales, thereby requiring a hand lens to see. Plant biologists have assumed that green stipules are photosynthetic, but ecophysiologists generally have not determined how significant green stipules may be as temporary or persistent photosynthetic structures. Anatomical reports on photosynthetic stipules are exceedingly rare, so that we know little about the presence of stomates and the nature of the mesophyll in stipules.
2. If hardened with lignin, a stipule may become modified as a spine, i.e., a stipular spine. In many species, one stipular spine or a pair of stipular spines may be observed at each node, e.g., among legumes, and for legumes, the stipular spine is the typical type of shoot armature (Examples: coral tree, Chloroleucon, and a mimosa). Obviously, stipular spines were effective in their specific native habitat for many of these cases as an adaptation to reduce predation by vertebrate herbivores. Very large, hollow stipular spines in certain species of bull acacia (Acacia, Family Mimosaceae) serve as homes for ants, which defend the plant from all other herbivores. Stipular spines are extremely important forms of shoot armature for the succulent euphorbs (Family Euphorbiaceae; examples: crown-of-thorns, young stipular spines, and old stipular spines), most species of which occur in thorny scrub and woodland as well as desert habitats in Africa and southern Asia. Stipular spines also are present on certain climbers.
3. In many groups of plants, stipules are short-lived, nonphotosynthetic structures that abscise before the leaf is fully expanded (a legume). Such stipules tend to be nongreen, often hyaline (an elm), off-white (ramie), yellow, or brownish, but there are some remarkable cases where the stipule is brilliantly colored, especially with anthocyanins, red to purple vacuolar pigments (Talauma hodgsonii). A nongreen stipule probably can be regarded as vestigial, lacking any function for the mature leaf and shoot but perhaps protecting the young leaf while it is at the growing tip.
4. Sometimes stipules and stipels have evolved as extrafloral nectaries, utilized to nourish mutualistic ants, or other types of glands.
5. In Smilax, a lianaceous monocotyledon, the stipule may develop as a tendril.
6. For whatever function, corky stipules are known from certain dryland shrubs, for example, the species of creosote bush (Larrea young stipules of L. tridentata and older stipules in L. nitida) in New World deserts and the tough-leaved evergreen species of Ceanothus in California chaparral.

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