2.6.3.1: Flowers

Learning Objectives

• Identify the components of a flower and to which whorl each belongs.
• Write and interpret floral formulas.
• Differentiate between flowers and inflorescences.
• Explain the difference between raceme-based and cymose inflorescences.

Flowers are sets of highly modified leaves that function to attract a pollinator or, if no animal pollinator is used, to optimize spore dispersal in some way. Over the course of evolutionary history and coevolution, this has lead to an incredible diversity of shape, size, color, smell, and just about any other characteristic you can think of. Because most plants are angiosperms and because flowers are often so diverse, learning the terminology to describe flowers is a major step in learning to identify plants.

The modified leaves in flowers are called sepals, petals, stamens, and carpels (Figure \(\PageIndex{1}\)). These components are arranged in whorls and attach to an area called the receptacle, which is at the end of the stem that leads to the flower. This stem is called the peduncle. In the case of an inflorescence, where multiple florets are produced in place of a single flower, the stems leading to the florets are called pedicels (Figure \(\PageIndex{2}\)).

Figure \(\PageIndex{1}\): This diagram shows a long section through a flower. Starting from the bottom, there is a stem called the peduncle. The peduncle terminates in a region called the receptacle, where all of the parts of the flower are attached. Sepals are found on the outside of the flower, two are visible here, with petals located just within the ring of sepals. There are five petals visible. Inside the petals, six stamens encircle a central pistil (composed of fused carpels). Diagram by Nikki Harris CC BY-NC with labels added.

Figure \(\PageIndex{2}\): An image of an Allium inflorescence. Many small florets on stalks (pedicels) emerge from a central point at the tip of the peduncle. At this junction, papery leaves (bracts) can be seen. These features are labeled in a sketch (second image). Photo by ramazan_murtazaliev, CC-BY-NC. Sketch by Maria Morrow, Public Domain.

The general characters that a flower has are whorl morphology, sex, merosity, symmetry, and the position of the gynoecium. Merosity is simply the number of parts in each whorl of a plant structure, whether it is the number of sepals, petals in a corolla, or the number of stamens. Each of these characters will be discussed below.

Whorls

The outermost whorl of a flower is called the calyx and is composed of sepals. Inside the calyx is the corolla, which is composed of petals. The sepals are often smaller and less colorful than the petals, but this general rule can be misleading. For example, lilies and tulips have identical sepals and petals (called tepals, these can be seen in the florets in Figure \(\PageIndex{2}\)). The only way you can distinguish between them is by location: Which whorl is on the outside?

The Perianth: Calyx and Corolla

Together, the calyx and corolla are called the perianth (peri- meaning around, anth- meaning flower; Figure \(\PageIndex{3}\)). The calyx is the outermost whorl of the flower. In most coses, the sepals are not showy--lacking bright colors and typically smaller--and instead serve a protective function in the developing flower. The corolla is the whorl just within the calyx. Petals tend to be the showy part of the flower, specialized for attracting animal pollinators.

Figure \(\PageIndex{3}\): A closed buttercup (Ranunculus sp.) flower shows the entire perianth. The sepals are smaller and covered in long trichomes. There are four sepals visible, though a fifth hides on the other side. Inside the sepal whorl (calyx), there are five overlapping, yellow petals. These petals form the corolla. Photo by Maria Morrow, CC BY-NC.

Reproductive Whorls: Androecium and Gynoecium

Inside the perianth is the androecium (house of man), a whorl composed of stamens. Each stamen has a long filament holding up pollen sacs called anthers (Figure \(\PageIndex{4-5}\)). Each lobed anther contains microsporangia, within which meiosis of the diploid microspore mother cells in the anther produces four haploid microspores. Each of these develops into a pollen grain consisting of two cells: a larger vegetative cell (the tube cell), inside of which is a a smaller germ cell (also called the generative cell). At some point, depending on the species, the germ cell divides by mitosis to produce 2 spermatia.

Figure \(\PageIndex{4}\): The four main parts of the flower are the calyx, corolla, androecium, and gynoecium. The androecium is the sum of all the male reproductive organs, and the gynoecium is the sum of the female reproductive organs. Descriptive text: The calyx (outermost whorl) is composed of sepals and the corolla (inside the calyx) is composed of petals. These two whorls form the perianth. At the center of the perianth is a vase-like structure called the carpel. A flower may have one or more carpels, but the example shown has only one. The narrow neck of the carpel, called the style, widens into a flat stigma at the top. The ovary is the wide part of the carpel. Ovules, or megasporangia, are clusters of pods in the middle of the ovary. The androecium is composed of stamens which cluster around the carpel. The stamen consists a long, stalk-like filament with an anther at the end. The anther shown is tri-lobed. Each lobe, called a microsporangium, is filled with pollen. (credit: modification of work by Mariana Ruiz Villareal)

Figure \(\PageIndex{5}\): A single stamen that has been removed from a flower. The filament is long and flattened. At the end of the filament, there is an anther that has split partially open, releasing pollen grains. Photo by Melissa Ha, CC BY-NC .

Inside the androecium is the gynoecium (house of woman), which is composed of carpels. Each carpel has an ovary at the base where ovules are housed. A style emerges from the ovary and is topped by the stigma (Figure \(\PageIndex{6}\)), where pollen is deposited. A carpel consists of a single ovary, style and stigma. Often several carpels are fused into a single structure, referred to as a pistil.

Within the ovule, a megasporangium produces a megaspore mother cell. Meiosis of the megaspore mother cell in each ovule produces 4 haploid cells, a large megaspore and 3 smaller cells that disintegrate. This megaspore develops into the megagametophyte, all within the ovule. Pollen grains land on the stigma and must grow a tube down the style to reach the ovule and complete fertilization.

Figure \(\PageIndex{6}\): All of the whorls of this Trillium flower are visible in this picture. The sepals are on the exterior and green. The three petals are a light pink. There are six stamens, each with much longer anthers than filaments. The stamens encircle the enlarged ovary of this fertilized flower, which is grooved and beaked into an odd shape. A short style connects the ovary to three long, curling stigma lobes. Is this flower a monocot or eudicot? Photo by Maria Morrow, CC BY-NC.

Incomplete Flowers

Most flowers are composed of four whorls. If all whorls are present, a flower is said to be both complete and perfect. If any whorl is missing, the flower is incomplete (Figure \(\PageIndex{7}\)). If one of those missing whorls is either the androecium (pollen-producing) or gynoecium (seed-producing), the flower is also imperfect (Figure \(\PageIndex{8}\)).

Figure \(\PageIndex{7}\): This wild ginger flower (Asarum caudatum) has no corolla. It is incomplete, but it is still perfect. The hairy sepals are fused together at the base, then split into long, pointed tips that extend out like tendrils. These flowers are normally a deep wine red. Photo by mhays, CC-BY-NC.

Figure \(\PageIndex{8}\): Drawing of Begonia ynesiae, which has imperfect flowers. Flowers either have an androecium or gynoecium but not both. A shows the overall palnt struture, B shows staminate flower (which has an androecium but no gynoecium), C and D show pistillate flowers (which has a gynoecium but no androecium). This species is named after Ynés Mexía, the first female Mexican-American botanist. She collected over 145,000 specimens and discovered more than 500 new species. You can learn more about her life and work here. Image by Ynés E. J. Mexía (public domain).

Evolution seems to favor (and be favored by) genetic variability. Genetic variability is promoted by outbreeding - sexual reproduction between genetically dissimilar parents. Just why sexual reproduction is so popular throughout the world of living things is still a hotly-debated question, but the fact remains.

Plants, being anchored in position, have a special problem in this regard. Many employ the services of animals (e.g., insects, birds, bats) to transfer pollen from plant to plant. But if the flowers have both sex organs, what is to prevent the pollen from fertilizing its own eggs? Plants have evolved a variety of solutions. One of these is to produce imperfect flowers.

There are two types of imperfect flowers: staminate flowers contain only an androecium (Figure \(\PageIndex{9}\)), and carpellate (or pistillate) flowers have only a gynoecium. Monoecious plants have both types of imperfect flower on the same plant. Dioecious plants have imperfect flowers on separate plants; that is, some plants are male, some female. Examples include willows, poplars, and the date palm. Most dioecious plants use an X-Y system of of sex determination like that in mammals. However, a few species use an X-to-autosome ratio system like that of Drosophila, and a very few use a ZW system like that of birds and lepidopterans.

Figure \(\PageIndex{9}\): This Sirdavidia solannona flower has no gynoecium, only tightly packed stamens at the center (a staminate flower). This flower is both imperfect and incomplete. Photo by Thomas L.P. Couvreur (Institut de Recherche pour le Développement , Montpellier, France), CC BY 4.0, via Wikimedia Commons.

But the vast majority of angiosperms have perfect flowers; that is containing both male and female sex organs. So how do they avoid self-fertilization? Many plants have self-incompatibility genes to prevent successful pollination between two of their own gametes. However, there is also a morphological solution: heteromorphic flowers. The flowers are perfect but come in two structural types; for example

• long stamens with a short style
• short stamens with a long style

A single plant has one type or the other. If the pollinator has a short tongue, pollination is favored from the first type to the second - but not the reverse. Heteromorphic flowers are not common, and even in the angiosperm families that favor them (e.g., primroses, flax), the same biochemical mechanisms of self-incompatibility that we will find in homomorphic flowers are usually present as well.

Fusion Within and Between Whorls

Determining the merosity of flowers is complicated by floral fusion. In many flowers, parts of a single floral whorl will be partially or completely fused together. When the fusion is between parts of the same whorl, such as the petals fusing together to form a tubular structure (a sympetalous flower), it is called connation. When there is fusion of parts between whorls, such as the stamen fusing the the petals, it is called adnation.

A frequent form of connation occurs within the gynoecium. In an apocarpous gynoecium, the carpels are free. In a syncarpous gynoecium, some or all parts of the carpels are fused (see Figure \(\PageIndex{10}\))

Figure \(\PageIndex{10}\): In an apocarpous gynoecium, the carpels are not fused together. The illustrated gynoecium on the left has three free carpels. A cross section of the ovaries shows the ovules inside. The gynoecium shown in the center is syncarpous, with fusion of the ovaries. A cross section of the ovary shows three sets of ovules. The gynoecium on the right is fully syncarpous with fusion of all parts (stigma, style, and ovary). The ovary cross section is divided into five distinct locules. Images by Michael G. Simpson. Redrawn and color: User:RoRo, Public domain, via Wikimedia Commons.

Floral Symmetry

Flowers that have multiple lines of symmetry (like a starfish) are radially symmetrical, also called actinomorphic or regular. Flowers with only a single line of symmetry (like you) are bilaterally symmetrical, also called zygomorphic or irregular (Figure \(\PageIndex{11}\)).

Figure \(\PageIndex{11}\): On the left is a picture of a radially symmetrical flower. It has five petals that all radiate out from a central point, like a star. We can draw many lines of symmetry through this flower, where if we folded the flower along that line, the two sides would line up with each other. (Note: There may be a line out of place here. Can you find it?) On the right is a bilaterally symmetrical flower. Only one line of symmetry can be drawn through this flower. Much like for humans, this flower is mirrored from side to side, but not top to bottom or any other direction. Diagram by Nikki Harris, CC BY-NC.

Ovary Position

We can use the location of the ovary to further distinguish between flowers (Figure \(\PageIndex{12}\)). If the other whorls of the flower meet below the ovary (the ovary or ovaries look a bit like an egg or eggs in a nest; Figure \(\PageIndex{13}\)), the ovary is superior (on top of the rest of the flower). This means that the rest of the flower parts are below the gynoecium, so we can also call this flower hypogynous (below the gynoecium). The two terms both describe the same situation, but superior refers only to the ovary while hypogynous refers to the flower, in general.

In the opposite situation, the other floral whorls join at a point above the ovary. In this case, the ovary is inferior and the flower is epigynous (on top of the gynoecium). As always, there are less clear situations. In some flowers, as in the rose family, the floral whorls join together and fuse at a point above the ovary, then travel down, around, and below the ovary as a fused unit. This fused unit is called a hypanthium. The ovary is termed semi-inferior, as it is located below the unfused parts of the floral whorls. Because the floral whorls travel around the ovary as the hypanthium, the flower is perigynous (peri- meaning around).

Figure \(\PageIndex{12}\): The first flower (a) is a drawing of a hypogynous flower. The calyx, corolla, and androecium fuse to the receptacle below the gynoecium. The ovary sits on top of these other whorls, a superior ovary. The central flower (b) is a perigynous flower. The calyx, corolla, and androecium fuse together to form a layer of tissue (hypanthium) that surrounds and fuses with the ovary. The ovary appears to sit below the other whorls, but is surrounded by the hypanthium (fused or not), a semi-inferior ovary. The last flower (c) is an epiogynous flower. The calyx, corolla, and androecium fuse together above the gynoecium. The ovary sits below these other whorls, an inferior ovary. Images by Gustav Hegi (1876–1932), Gustav Dunzinger, Public domain, via Wikimedia Commons.

Figure \(\PageIndex{13}\): This flower has a superior ovary and is considered hypogynous. In the image above, we see two flowers (actually two florets of an underground umbel!) of a fetid adderstongue, Scoliopus bigelovii. The one on the left has shed all whorls except the gynoecium. This allows you to see what the gynoecium looks like and where it is positioned in the one on the right. In the complete flower, the sepals, petals, and stamens all meet below the ovary, which sits in the middle like one giant egg in an alien nest. Photo by Maria Morrow, CC BY-NC.

Inflorescences

An inflorescence is when, in the place of a single flower, multiple florets are formed. Florets can be sessile or attached by a stem called a pedicel. When leaf-like structures are found within the inflorescence, they are called bracts. There are a wide range of possibilities for the structure and development of inflorescences, though most can be split into four models: raceme-based, cymose, panicle, and intercalate (Figure \(\PageIndex{14}\)). Two models are most widespread.

Raceme-based inflorescences are mostly monopodial, having indeterminate growth. This means that the terminal bud continues producing lateral florets, never truly forming a terminal floret. The inflorescence can be simple or compound (Figure \(\PageIndex{15}\)).

Figure \(\PageIndex{14}\): Four kinds of inflorescences (left to right): Model I (raceme-based) has single florets emerging on pedicels from the main peduncle. Model II (thyrsoid or cymose) has racemes emerging alternately from the main pedicel; the terminal florets are the oldest. Model III (panicle) has branched racemes emerging from the main peduncle, but unlike model II, the peduncle terminates in a floret. and Model IV (intercalate) produces lateral florets along the peduncle, separated by bracts, and continues to grow.

Figure \(\PageIndex{15}\): Different Model I inflorescences and their evolutionary connections. Circles represent florets, larger circles are older. Digits represent the simple encoding system: first position is main axis, second position are secondary axes (flower pedicels), 1 means developed, 0 reduced. Compound inflorescences have four digit positions, for the first and second orders of branching. Some names for these inflorescences: 11 raceme, 11/11 compound raceme, 10 spike (left) and spadix (right), 01 umbel, 01/01 compound umbel, 00 head. All of these models are shown diverging from a central 11 raceme model.

Cymose inflorescences are sympodial, having determinate growth. This means there is a terminal floret that forms first, then other florets are produced laterally (Figure \(\PageIndex{16}\)). The order of lateral floret maturation can be a useful identification feature.

Figure \(\PageIndex{16}\): Model II inflorescences. Circles represent florets, larger circles are older. Thyrse (left), cyme (top right), and dichasium (bottom right). The dichasium has a stunted central flower flanked by two long pedicels that each have a compound inflorescence with a stunted central floret flanked by two longer pedicels that end in a cluster of florets. It is fractal. The thyrse has a large terminal floret; each lateral floret has the same structure as the lateral florets in the dichasium. The cyme has a terminal flower that opens first, then a later floret that produces another later floret, which produces another lateral floret, and so on, forming a spiral.

Attribution

Curated and authored by Maria Morrow, CC BY-NC, using the following sources:

• 16.3E Self incompatibility – How Plants Avoid Inbreeding, 16.3D Angiosperm Life Cycle, and 19.1.5 Diversity and Evolutionary Relationships of the Plants from Biology by John. W. Kimball (licensed CC-BY)
• 26.2 Gymnosperms and 32.1 Reproductive Development and Structure from Biology 2e by OpenStax (licensed CC-BY). Access for free at openstax.org.
• 8.2 The Flower and the Fruit from Introduction to Botany by Alexey Shipunov (public domain)

Figure \(\PageIndex{8}\) caption by Melissa Ha.

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