A
petal (from
Ancient GreekAncient Greek is the historical stage in the development of the Greek language spanning across the Archaic , Classical , and Hellenistic periods of ancient Greece and the ancient world. It is predated in the 2nd millennium BC by Mycenaean Greek...
petalon "leaf", "thin plate") is one member or part of the
corolla of a
flowerA flower, sometimes known as a bloom or blossom, is the reproductive structure found in flowering plants . The biological function of a flower is to mediate the union of male sperm with female ovum in order to produce seeds...
. The corolla is the name for all of the petals of a flower; the inner perianth
whorlWhorl is a type of spiral pattern.Other meanings of whorl include:* Whorl , a single, complete 360° turn in the spiral growth of a mollusc shell...
, term used when this is not the same in appearance (color, shape) as the outermost whorl (the calyx) and is used to attract
pollinatorA pollinator is the biotic agent that moves pollen from the male anthers of a flower to the female stigma of a flower to accomplish fertilization or syngamy of the female gamete in the ovule of the flower by the male gamete from the pollen grain...
s based on its bright color. It is the inner part of the
perianth that comprises the sterile parts of a flower and consists of inner and outer
tepalTepals are elements of the perianth, or outer part of a flower, which include the petals or sepals. The term tepal is usually used when all segments of the perianth are of similar shape and color, or undifferentiated...
s. These tepals are usually differentiated into petals and
sepalA sepal is a part of the flower of angiosperms . Sepals in most flowers are green and lie under the more conspicuous petals. As a collective unit the sepals form a calyx, whereas the collection of petals is called the corolla...
s. The term "tepal" is usually applied when the petals and sepals are similar in shape and color. In a "typical" flower the petals are showy and colored and surround the reproductive parts. The number of petals in a flower (see
merosityMerosity is the number of component parts in each whorls of a plant structure. It is most commonly used in the context of flowers, in which case it refers to the number of sepals in the calyx, the number of petals in the corolla, and the number of stamens in each whorl of the androecium...
) is indicative of the plant's classification:
eudicotsEudicots and Eudicotyledons are terms introduced by Doyle & Hotton to refer to a group of flowering plants that had been called "tricolpates" or "non-Magnoliid dicots" by previous authors. The term means, literally, "true dicotyledons" as it contains the majority of plants that have been...
(the largest group of
dicotsDicotyledons, or "dicots", is a name for a group of flowering plants whose seed typically has two embryonic leaves or cotyledons. There are around 199,350 species within this group...
) having typically four or five petals and
monocotsMonocotyledons or monocots are one of two major groups of flowering plants that are traditionally recognized, the other being dicotyledons or dicots. Monocot seedlings typically have one cotyledon , in contrast to the two cotyledons typical of dicots. Monocots have been recognized at various...
and magnoliids having three, or some multiple of three, petals.
The genetics behind the formation of petals, in accordance with the ABC model of flower development, are that sepals, petals, stamens, and carpels are modified versions of each other. It seems that the mechanisms to form petals evolved a small number of times (perhaps only once), rather than evolving independently from stamens in a large number of plants.
Variation
There exists considerable variation in form of petals among the
flowering plantThe flowering plants or angiosperms are the most diverse group of land plants. The flowering plants and the gymnosperms are the only extant groups of seed plants...
s. The petals can be united towards the base, forming a
floral tube. In some flowers, the entire perianth forms a cup (called a
calyx tube) surrounding the
gynoecium, with the sepals, petals, and stamens attached to the rim of the cup.
The flowers of some species lack or have very much reduced petals. These are often referred to as
apetalous. Examples of flowers with much reduced perianths are found among the
grassesPoaceae or Gramineae is a family in the Class Liliopsida of the flowering plants. Plants of this family are usually called grasses, or, to distinguish them from other graminoids, true grasses; the shrub- or tree-like plants in this family are called bamboo...
.
The petals are usually the most conspicuous parts of a flower, and the petal whorl or corolla may be either radially or bilaterally symmetrical. If all of the petals are essentially identical in size and shape, the flower is said to be
regular or
actinomorphic (meaning 'ray-formed'). Many flowers are symmetrical in only one plane (i.e., symmetry is bilateral) and are termed
irregular or
zygomorphic (meaning yoke- or pair-formed). In
irregular flowers, other floral parts may be modified from the
regular form, but the petals show the greatest deviation from radial symmetry. Examples of zygomorphic flowers may be seen in orchids and members of the
pea familyFabaceae or Leguminosae is a large and economically important family of flowering plants, which is commonly known as the legume family, pea family, bean family or pulse family. The name 'Fabaceae' comes from the defunct genus Faba, now included into Vicia...
. The petal is the colorful, often showy part of a plant.
Similar structures
Some plants have petaloid stamens, in plants like
CannaCanna is a genus of nineteen species of flowering plants. The closest living relations to cannas are the other plant families of the order Zingiberales, that is the gingers, bananas, marantas, heliconias, strelitzias, etc.Canna is the only genus in the family Cannaceae...
that have true petals and
staminodeIn botany, a staminode is an often rudimentary, sterile or abortive stamen. This means that it does not produce pollen. Staminodes are frequently inconspicuous and stamen-like, usually occurring at the inner whorl of the flower, but are also sometimes long enough to protrude from the...
s, the stamen (staminodes) are modified to look like large showy petals.
A number of plants have
bractIn botany, a bract is a modified or specialized leaf, especially one associated with a reproductive structure such as a flower, inflorescence axis, or cone scale...
s that resemble petals for example in
BougainvilleaBougainvillea is a genus of flowering plants native to South America from Brazil west to Peru and south to southern Argentina . Different authors accept between four and 18 species in the genus...
and
Cornus florida (flowering dogwood). Petal-like bracts are common features in some plant families like
EuphorbiaceaeThe Spurge family is a large family of flowering plants with 300 genera and around 7,500 species. Most are herbs, but some, especially in the tropics, are also shrubs or trees. Some are succulent and resemble cacti....
.
In many plants of the
aster familyThe family Asteraceae or Compositae is the second largest family of flowering plants, in terms of number of species....
such as the sunflower,
Helianthus annuus, the circumference of the
flower headThe capitulum is considered the most derived form of inflorescence. Flower heads found outside Asteraceae show lesser degrees of specialization....
is composed of ray florets. Each ray floret is anatomically an individual flower with a single large petal.
Corolla
Corolla is the collective term for petals of a flower taken as a group within the calyx. Normally the corolla is the most conspicuous part of a flower and of a bright colour other than green. The concept of corolla description is widely used in
botanyBotany, plant science, phytology, or plant biology is a branch of biology and is the scientific study of plant life and development...
as a primary determinant of
vascular plantVascular plants are those plants that have lignified tissues for conducting water, minerals, and photosynthetic products through the plant. Vascular plants include the ferns, clubmosses, flowering plants, conifers and other gymnosperms...
identification. Alternatively the corolla may be considered as the inner
whorlWhorl is a type of spiral pattern.Other meanings of whorl include:* Whorl , a single, complete 360° turn in the spiral growth of a mollusc shell...
of the perianth structure. The role of the corolla in plant
evolutionIn biology, evolution is change in the genetic material of a population of organisms from one generation to the next. Though changes produced in any one generation are normally small, differences accumulate with each generation and can, over time, cause substantial changes in the population, a...
has been studied extensively since
DarwinCharles Robert Darwin FRS was an English naturalist who realised and presented compelling evidence that all species of life have evolved over time from common ancestors, through the process he called natural selection...
postulated a theory of the origin of elongated corollae.