Thestigma (pl.:stigmas orstigmata)[1] is the receptive tip of acarpel, or of several fused carpels, in thegynoecium of aflower.
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The stigma, along with thestyle andovary (typically called thestigma-style-ovary system) constitute thepistil, which is part of thegynoecium or the receptive female reproductive organ of a plant; in animals, this organ is equivalent to thevagina,cervix, andoviduct.
The stigma itself forms the distal portion of the style, or stylodia, and is composed ofstigmatic papillae, the cells of which are receptive to pollen. These may be restricted to the apex of the style or, especially in wind pollinated species, cover a wide surface.[2]
The stigma receivespollen and it is on the stigma that the pollen graingerminates. Often sticky, the stigma is adapted in various ways to catch and trap pollen with various hairs, flaps, or sculpturings.[3] The pollen may be captured from the air (wind-borne pollen,anemophily), from visiting insects or other animals (biotic pollination), or in rare cases from surrounding water (hydrophily). Stigma can vary from long and slender to globe shaped to feathery.[4]
Pollen is typically highly desiccated when it leaves an anther. Stigma have been shown to assist in the rehydration of pollen and in promoting germination of the pollen tube.[5] Stigma also ensure proper adhesion of the correct species of pollen. Stigma can play an active role in pollen discrimination and someself-incompatibility reactions, that reject pollen from the same or genetically similar plants, involve interaction between the stigma and the surface of the pollen grain.
The stigma is often split into lobes, e.g. trifid (three lobed), and may resemble the head of a pin (capitate), or come to a point (punctiform). The shape of the stigma may vary considerably:[2]
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The style is a narrow upward extension of the ovary, connecting it to the stigmatic papillae. Occasionally, it may be absent, in which case the stigma is described assessile. Styles are generally tube-like—either long or short.[6] The style can be open (containing few or no cells in the central portion) with a central canal which may be filled withmucilage. Alternatively the style may be closed (densely packed with cells throughout). Mostsyncarpousmonocots and someeudicots have open styles, while many syncarpous eudicots and grasses have closed (solid) styles containing specialised secretory transmitting tissue, linking the stigma to the centre of the ovary. This forms a nutrient rich tract for pollen tube growth.[4]
Where there are more than one carpel to the pistil, each may have a separate style-like stylodium, or share a common style. Inirises and others in the familyIridaceae, the style divides into three petal-like (petaloid)style branches (sometimes also referred to as 'stylodia'[7]), almost to the base of the style and is called a tribrachiate.[8] These are flaps of tissue, running from the perianth tube above the sepal. The stigma is a rim or edge on the underside of the branch, near the end lobes.[9] Style branches also appear onDietes,Pardanthopsis and most species ofMoraea.[10]
InCrocus, there are three divided style branches, creating a tube.[11]Hesperantha has a spreading style branch. Alternatively, the style may be lobed rather than branched.Gladiolus has a bi-lobed style branch (bilobate).Freesia,Lapeirousia,Romulea,Savannosiphon andWatsonia have bifurcated (two branched) and recurved style branches.[10][2]
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