Overview
A typical flower has four main parts or whorls referred to as the calyx, corolla, androecium, and gynoecium. The outermost whorl of the flower has green, leafy structures known as sepals. The sepals, collectively called the calyx, help to protect the unopened bud. The second whorl is comprised of petals, usually, brightly colored and are collectively called the corolla.
The number of sepals and petals varies depending on whether the plant is a monocot or dicot. In monocots, petals usually number three or multiples of three; in dicots, the number of petals is four or five, or multiples of four and five. Together, the calyx and corolla are known as the perianth.
The third whorl contains the male reproductive structures and is known as the androecium. The androecium has stamens with anthers that contain the microsporangia. The innermost group of structures in the flower is the gynoecium, or the female reproductive component(s). The carpel is the individual unit of the gynoecium and has a stigma, style, and ovary. A flower may have one or multiple carpels.
If all four whorls (the calyx, corolla, androecium, and gynoecium) are present, the flower is described as complete. If any of the four parts is missing, the flower is described as incomplete. Flowers that contain both an androecium and a gynoecium are referred to as perfect, androgynous or hermaphrodites.
There are two types of incomplete flowers: staminate flowers contain only an androecium, and carpellate flowers have only a gynoecium. If both male and female flowers are borne on the same plant, the species is called monoecious whereas species with male and female flowers borne on separate plants are termed dioecious.
Classification Of A flower
Based on the symmetry of the flowers can be described in to following types:
- Actinomorphic: Flowers can be divided into two halves through any vertical plane.
- Zygomorphic: Flowers can be divided into two equal halves only along one vertical plane.
- Asymmetrical: Flowers which cannot be divided into equal halves by any plane.
Based on the position of calyx, corolla, and androecium with respect of ovary, the flowers are described as following:
- Hypogynous (Superior ovary): Gynoecium occupies the highest position while the other parts are situated below it.
- Perigynous (Half inferior): If gynoecium is situated in the centre and other parts of the flower are located on the rim of the thalamus almost at the same level.
- Epigynous (Inferior ovary): The ovary situated in a flask shaped thalamus and other parts of flower arise above the ovary.
Parts of a Flower And Their Function
Sepals
Sepals are modified leaves that typically function as protection for the flower in bud and often as support for the petals when in bloom. They are sterile floral parts and may be either green or leaf-like or composed of petal-like tissue. Collectively, the sepals are referred to as the calyx, the outermost whorl of parts that form a flower.
Petals
Petals are modified leaves that surround the reproductive parts of flowers. They are often brightly colored or unusually shaped to attract pollinators. Together, all petals of a flower are called the corolla. Petals are usually accompanied by another set of modified leaves (sepals), that collectively form the calyx and lie beneath the corolla. Petal often consists of two parts: the upper, broad part, similar to leaf blade, also called the blade and the lower part, narrow, similar to leaf petiole, called the claw, separated from each other at the limb.
The calyx and the corolla together make up the perianth. When the petals and sepals of a flower are difficult to distinguish, they are collectively referred to as tepals. Although petals are usually the most conspicuous parts of animal-pollinated flowers, wind-pollinated species, such as the grasses, either have very small petals or lack them entirely (apetalous).
Petals can differ dramatically in different species. The number of petals in a flower may hold clues to a plant’s classification. For example, flowers on eudicots (the largest group of dicots) most frequently have four or five petals while flowers on monocots have three or six petals, although there are many exceptions to this rule.
The arrangements of sepals or petals in the floral bud, with respect to the members of the same whorls are called aestivation. Aestivation can of the following types:
- Valvate: When sepals or petals do not overlap one another at the margin, without overlapping.
- Twisted: Sepals or petals overlap with the next sepal or petal.
- Imbricate: If the margins of sepals or petals overlap one another but not in any particular direction are called imbricate estivation.
- Vexillary: Among five petals, the largest petal (standard) overlaps the two lateral petals (wings) which in turn overlap two smallest anterior petals (Keel).
Stamens
Stamen can be described as the male reproductive part of a flower. In all but a few extant angiosperms, the stamen consists of a long slender stalk, the filament, with a two-lobed anther at the tip. The anther consists of four saclike structures (microsporangia) that produce pollen for pollination. Small secretory structures, called nectaries, are often found at the base of the stamens; they provide food rewards for insect and bird pollinators. All the stamens of a flower are collectively called the androecium.
Depending on the species of plant, some or all of the stamens in a flower may be attached to the petals or to the floral axis. They also may be free-standing or fused to one another in many different ways, including fusion of some but not all stamens. The filaments may be fused and the anthers free, or the filaments free and the anthers fused. Rather than there being two locules, one locule of a stamen may fail to develop, or alternatively the two locules may merge late in development to give a single locule.
The stamens are variously fused among themselves. They can be of the following type:
- Monoadelphous: stamens may be united into one bunch or one bundle. e.g China rose
- Diadelphous: stamens may be united into two bundles. e.g pea
- Polyadelphous: stamens may be united into more than two bundles. e.g Citrus
Based on the attachment of filament to anther, it can be of the following type:
- Basifixed: Filament of stamen is attached to base of the anther.
- Adnate: Filament attached along the whole length of anther.
- Dorsifixed: Filaments attached to the back of anther.
- Versatile: Anther lobes attached with filament in the middle portion with both ends free.
Carpel
The pistils of a flower are considered to be composed of one or more carpels. A carpel is the female reproductive part of the flower —composed of ovary, style, and stigma— and usually interpreted as modified leaves that bear structures called ovules, inside which egg cells ultimately form. A pistil may consist of one carpel (with its ovary, style and stigma); or it may comprise several carpels joined together to form a single ovary, the whole unit called a pistil. Fertilization of an egg within a carpel by a pollen grain from another flower results in seed development within the carpel.
Carpels are of two types depending upon fusion:
- Apocarpous: Carpels free from each other e.g lotus
- Syncarpous: Carpels fused with each other e.g mustard.
Corolla
The corolla is made up of the petals of the flower, which are usually brightly colored in order to attract insects. Together, the corolla and the calyx make up the perianth, the nonreproductive portion of the flower. Corolla may be gamopetalous (petals united) or polypetalous (petals free).
Also Read: Difference Between Perigynous And Hypogynous Flowers
Filament
The filament of a flower is one of the male reproductive parts of the plant. When a flower opens, the filaments can be seen inside, thrusting up like stems within the flower itself. A filament is topped by the anther. Together the anther and filament make up the flower’s stamen.
Filaments exist to carry nutrients to the anther, where pollen develops. After the flower opens, the lengthening filaments facilitate access to the anthers and pollinating agents like bees. Anthers can top the filament in three ways. Sometimes the filament attaches to the anther at its base, but sometimes it is attached between two lobes of the anther, running up its back. The filament might also plug into one spot on the back of the anther.
Ovary
An ovary is a part of the female reproductive organ of the flower or gynoecium. Specifically, it is the part of the pistil which holds the ovule(s) and is located above or below or at the point of connection with the base of the petals and sepals. The ovary contains ovules, which develop into seeds upon fertilization. The ovary itself will mature into a fruit, either dry or fleshy, enclosing the seeds.
The arrangement of placentae bearing ovules inside the ovary is called placentation. It is of following types.
- Marginal: The placenta forms a ridge along the ventral suture of the ovary and the ovules are borne on this ridge e.g., pea.
- Axile: Margins of carpels fuse to form central axis. e.g. tomato
- Parietal: The ovules develop on the inner wall of the ovary. e.g mustard.
- Free central: Ovules borne from central axis and lacking septa. e.g chilly.
- Basal: Placenta develops at the base of the ovary. e.g sunflower.
Ovule
The ovule is the organ that forms the seeds when fertilized. It is borne in the ovary of the flower. A mature ovule consists of a food tissue covered by one or two future seed coats, known as integuments. A small opening (the micropyle) in the integuments permits the pollen tube to enter and discharge its sperm nuclei into the embryo sac, a large oval cell in which fertilization and development occur. Each ovule is attached by its base to the stalk (funiculus) that bears it.
Anther
Anthers are grainy pods on the ends of the filaments that range in color from light yellow to deep red. The anthers rise high from the flower’s center on long filaments. Different flower species have varying numbers of anthers, but most have five to six anthers that circle the center of the flower.
The anthers on the flowers are an integral part of the flower’s structure that creates the pollen. The transfer of pollen allows the plant to develop seeds that create a new generation of infant plants. Together the filaments and anthers form the stamen of a flower.
Anthers hold the pollen that contain the sperm necessary for reproduction. The long filaments hold the anthers up from the center of the flower to increase the chances that a visiting pollinator will brush against the anthers and collect the pollen. When the pollinator travels to the next plant, the pollen falls from its body onto the female organs of the flower. The pollen then sends sperm into ovary to fertilize the waiting egg. Without the anthers producing the sperm and the pollen, the flower cannot reproduce.
Bract
Bract is the modified, usually small, leaf-like structure often positioned beneath a flower or inflorescence. What are often taken to be the petals of flowers are sometimes bracts—for example, the large, colorful bracts of poinsettias or the showy white or pink bracts of dogwood blossoms.
Stigma
The stigma is at the top of the style and is a sticky platform where pollen is deposited. The stigma forms the distal portion of the style or stylodia. The stigma is composed of stigmatic papillae, the cells 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. The stigma receives pollen and it is on the stigma that the pollen grain germinates. Often sticky, the stigma is adapted in various ways to catch and trap pollen with various hairs, flaps, or sculpturing. Stigma can vary from long and slender to globe shaped to feathery.
Style
Style is a long tube like structure which connects the stigma to the ovary. After the germination of pollen grains on stigma, the pollen tubes containing male gametes penetrate the tissues of style to reach the ovule in the ovary. The stigma is at the top of the style and is a sticky platform where pollen is deposited. The ovary is located at the bottom of the style and houses the plant’s ovules, which contain the egg cells and supporting cells necessary for reproduction. Together the stigma, style, and ovary forms what is generally referred to as pistil.
Receptacle
The receptacle is the axis (stem) to which the floral organs are attached. Floral organs are attached either in a low continuous spiral, as is common among primitive angiosperms, or in alternating successive whorls, as is found among most angiosperms.
Peduncle
The peduncle is the stalk of a flower or an inflorescence. When a flower is borne singly, the internode between the receptacle and the bract (the last leaf, often modified and usually smaller than the other leaves) is the peduncle. When the flowers are borne in an inflorescence, the peduncle is the internode between the bract and the inflorescence; the internode between the receptacle of each flower and its underlying bracteole is called a pedicel. Thus, in inflorescences, bracteole is the equivalent of bract, and pedicel is the equivalent of peduncle.
Pedicel
A pedicel is a short flower stalk in an inflorescence or cluster of flowers. Pedicels hold individual flowers in place, but how they do that depends on the form of the inflorescence. The function of pedicels is to expose flowers to the sun and wind and put them in a position so their aroma and color attracts pollinating insects more easily.
In other words, a pedicel is a stem that attaches a single flower to the inflorescence. Such inflorescences are described as pedicellate. In the absence of a pedicel, the flowers are described as sessile. Pedicel is also applied to the stem of the infructescence.
Also Read: Difference Between True And False Fruit
Perianth
The perianth is a non-reproductive (accessory, sterile) part of the flower, consisting of floral leaves surrounding the androecium and gynoecium. It is differentiated into outer and inner whorls. The outer perianth whorl, calyx, is composed mostly of inconspicuous green sepals, and the inner whorl, corolla, is composed mostly of brightly colored petals.
A flower is dichlamydeous (dichlamydeus) if it has a perianth composed of 2 distinct whorls, the calyx and the corolla. A monochlamydeous (monochlamydeus) flower has a perianth composed of only one whorl. A flower without a perianth is called achlamydeous (achlamydeus). Such a flower is present in e.g. species of the Salix and Populus genera, as well as in some species of the genus Fraxinus.
Calyx
After flowering, most plants have no more use for the calyx which withers or becomes vestigial. Some plants retain a thorny calyx, either dried or live, as protection for the fruit or seeds. Examples include species of Acaena and some species of the Solanaceae (for example the Tomatillo, Physalis philadelphica). In some species, the calyx not only persists after flowering, but instead of withering, begins to grow until it forms a bladder-like enclosure around the fruit. This is an effective protection against some kind of birds and insects. In other species, the calyx grows into an accessory.
Reproduction In Flowering Plants
Reproduction in flowering plants begins with pollination, the transfer of pollen from anther to stigma on the same flower or to the stigma of another flower on the same plant (self-pollination) or from the anther on one plant to the stigma of another plant (cross-pollination). Once the pollen grain lodges on the stigma, a pollen tube grows from the pollen grain to an ovule. Two sperm nuclei then pass through the pollen tube. One of them unites with the egg nucleus and produces a zygote. The other sperm nucleus unites with two polar nuclei to produce an endosperm nucleus. The fertilized ovule develops into a seed.
Further References
- Parts of a Flower: https://www.researchgate.net/figure/The-parts-of-a-flower_fig1_339977625
- Structure of a Flower: https://www.amnh.org/learn-teach/curriculum-collections/biodiversity-counts/plant-identification/plant-morphology/parts-of-a-flower
- Flower Parts: https://web.extension.illinois.edu/gpe/case4/c4facts1a.html
- Flower Anatomy And Physiology: https://www.britannica.com/science/flower
- Flower Anatomy: https://cals.arizona.edu/pubs/insects/ahb/inf9.html