A tepal is the term used to identify one of the outer parts of a flower (collectively the perianth) when these parts cannot easily be divided into two kinds, sepals and petals. This may be because the parts of the perianth are undifferentiated (as in Magnolia) or because, although it is possible to distinguish an outer whorl of sepals from an inner whorl of petals, the sepals and petals are similar in appearance to one another (as in Lilium). The term was first proposed by Augustin Pyramus de Candolle in 1827. (De Candolle used the term perigonium or perigone for the tepals collectively; today this term is used as a synonym for "perianth".)
Undifferentiated tepals are believed to be the ancestral condition in flowering plants. For example, Amborella, which is thought to have separated earliest in the evolution of flowering plants, has flowers with undifferentiated tepals. Distinct petals and sepals would therefore have arisen by differentiation, probably in response to animal pollination. In typical modern flowers, the outer or enclosing whorl of organs forms sepals, specialised for protection of the flower bud as it develops, while the inner whorl forms petals, which attract pollinators.
Tepals formed by similar sepals and petals are common in monocotyledons, particularly the "lilioid monocots". In tulips, for example, the first and second whorls both contain structures that look like petals. These are fused at the base to form one large, showy, six-parted structure (the perianth). In lilies the organs in the first whorl are separate from the second, but all look similar, thus all the showy parts are often called tepals. Where sepals and petals can in principle be distinguished, usage of the term "tepal" is not always consistent – some authors will refer to "sepals and petals" where others use "tepals" in the same context.
Terms used in the description of tepals include pubescent (with dense fine, short, soft hairs, downy), puberulent (minutely pubescent, hairs barely visible to the naked eye) and puberulous (dense covering of very short soft hairs).
Flowers of Magnolia × alba showing tepals in various stages of development
Tepals of Magnolia × wieseneri
A hellebore flower showing the petaloid sepals
- Augustin Pyramus de Candolle (1827). Organographie végétale, ou Description raisonnée des organes des plantes; pour servir de suite et de développement a la théorie élémentaire de la botanique, et d'introduction a la physiologie végétale et a la physiologie végétale et a la description des familles. Paris: Deterville. p. 503.
- Augustin Pyramus de Candolle (1841). Vegetable organography; or, An analytical description of the organs of plants 2. translated by Boughton Kingdon. London: Houlston & Stoneman. p. 90.
- Stearn, William Thomas (2004). Botanical Latin (p/b ed.). David & Charles/Timber Press. ISBN 978-0-7153-1643-6. p. 39.
- Ronse De Craene, L. P. (2007). "Are Petals Sterile Stamens or Bracts? The Origin and Evolution of Petals in the Core Eudicots". Annals of Botany 100 (3): 621–630. doi:10.1093/aob/mcm076. PMC 2533615. PMID 17513305.
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