Inbotany, thepetiole (/ˈpiːti.oʊl/) is the stalk that attaches theleaf blade to thestem.[1]: 87 [2]: 171 It is able to twist the leaf to face the sun, producing a characteristic foliage arrangement (spacing of blades), and also optimizing its exposure to sunlight.[3][4] Outgrowths appearing on each side of the petiole in some species are calledstipules. The termspetiolate andapetiolate are applied respectively to leaves with and without petioles.
The petiole is a stalk that attaches a leaf to the plant stem. In petiolate leaves the leaf stalk may be long (as in the leaves ofcelery andrhubarb), or short (for examplebasil). When completely absent, the blade attaches directly to the stem and is said to besessile. Subpetiolate leaves have an extremely short petiole, and may appear sessile.[2]: 157 The broomrape familyOrobanchaceae is an example of a family in which the leaves are always sessile.[5]: 639 In some other plant groups, such as the speedwell genusVeronica, petiolate and sessile leaves may occur in different species.[5]: 584
In the grasses (Poaceae), the leaves are apetiolate, but the leaf blade may be narrowed at the junction with the leaf sheath to form apseudopetiole, as inPseudosasa japonica.[6]: 391
In plants withcompound leaves, the leaflets are attached to a continuation of the petiole called therachis.[1]: 98 Each leaflet may be attached to the rachis by a short stalk called the petiolule.[1]: 87 There may be swollen regions at either end of the petiole known as pulvina (singular =pulvinus)[1]: 97 that are composed of a flexible tissue that allows leaf movement. Pulvina are common in the bean familyFabaceae and the prayer plant familyMarantaceae. A pulvinus on a petiolule is called a pulvinulus.
In some plants, the petioles are flattened and widened to becomephyllodes (also known as phyllodia or cladophylls) and the true leaves may be reduced or absent. Thus, the phyllode comes to serve the functions of the leaf. Phyllodes are common in the genusAcacia, especially the Australian species, at one time put inAcacia subgenusPhyllodineae.
InAcacia koa, the phyllodes are leathery and thick, allowing the tree to survive stressful environments. The petiole allows partially submergedhydrophytes to have leaves floating at different depths, the petiole being between the node and the stem.
In plants such as rhubarb (Rheum rhabarbarum), celery (Apium graveolens),artichokes, andcardoons (Cynara cardunculus), the petioles ('stalks' or 'ribs') are cultivated as edible crops. The petiole of rhubarb grows directly from therhizome and produces the leaf at its end. Botanically, it is categorized as a vegetable but, culinarily, it is more often used as a fruit.[7][8]
The longest known petiole is that of the royal waterlily or iaupê jaçanãVictoria amazonica which is up to 23 ft (7.0 m) in length.[9]
'Petiole' comes fromLatinpetiolus, or 'little foot', 'stem', an alternativediminutive of 'pes', 'foot'. The regular diminutive 'pediculus' is also used for 'foot stalk'.
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