Flowers

Flowers

An enormous diversity of size, shape, and complexity exists among the flowers of the quarter-million species of angiosperms . Flower size varies over a thousandfold, with Rafflesia (Rafflesiaceae) flowers as large as 1 meter in diameter dwarfing the minuscule flowers of Wolffia (Lemnaceae), which measure less than 1 millimeter across. The number of floral organs also varies, with the complex flowers of the Tambourissa (Monimiaceae) species having more than one thousand organs while the simple flowers of the Chloranthaceae may consist of just a few. The coevolution of angiosperms with their animal pollinators is a driving force in the generation of flower diversity. The end product of pollination is the formation of a viable seed, therefore ensuring that the species will be perpetuated. Exclusive pollinator-flower relationships ensure that pollen will not be wasted by delivery to flowers of a different species.

Definition and Flower Parts

Despite the enormous diversity in the number, size, and shape of floral organs within the angiosperms, they all are built of four basic organ types (sepals , petals, stamens, and carpels ) whose relative positions are invariant. The flower is an assemblage of sterile and fertile (reproductive) parts borne on a shoot or axis called the receptacle. The sterile parts include the sepals (collectively called the calyx) and the petals (collectively called the corolla). The sepals and petals together constitute the perianth. In a typical flower the sepals are green, and they enclose and protect the young flower before it opens. The petals, whose function is to attract pollinators, exhibit an assortment of colors, shapes, and sizes. In flowers in which the sepals and petals are indistinguishable from each other, such as tulips (Liliaceae), the perianth parts are called tepals .

The reproductive parts can be divided into the androecium and gynoecium . The androecium is composed of stamens, the male floral organs. Stamens usually have an apical anther, in which pollen develops, and a basal filament connecting the anther to the receptacle. One or more carpels constitute the gynoecium. Carpels are made of several functional tissues that facilitate pollination and protect developing ovules and seeds. The stigma on which the pollen germinates, and the style, through which the pollen tube grows toward the ovules, are examples of tissues that are intimately associated with pollination. The ovary, which houses the ovules, provides protection for both the developing ovules and seeds. In addition, the ovary often develops into a fruit that facilitates seed dispersal. The formation of a protected chamber in which the ovules and seeds develop is one of the defining features of angiosperms. The term angiosperm is derived from the Greek angio (a capsule-like covering) and sperm (seed).

MAJOR ANIMAL POLLINATORS AND TARGETED FLOWERS
Animal	Flower Characteristics
Beetle	Open flower, white or dull coloring with strong odor (usually fruity, spicy, or similar to the foul odors of fermentation).
Bee	Any color but red; flower has nectar at the base of the flower that forces the bee to pass by the stigma and anthers on its way to the nectar.
Butterfly and some moths	Flowers tubular in shape, which precludes large insects from crawling into them but allows the long proboscis of the butterfly or moth to enter. Nectar contains amino acids that butterflies require; nectar is their sole food source.
Bird	Usually bright and showy flowers, the colors of which are red, orange, or yellow. Because of the bird's high rate of metabolism, bird-pollinated flowers usually produce large quantities of thin nectar. In the Western Hemisphere hummingbirds are the main bird pollinator; in other parts of the world representatives of other specialized bird families (e.g., sunbirds and honeyeaters) act as pollinators.
Bat	Large white flowers such as those of the saguaro cactus (Cactaceae), which arevisible in dim light. Bats also require large amounts of nectar.

Function of Flowers

The function of the flower is to facilitate the reproduction of the organism. Cells within the pollen and embryo sac are haploid and are derived from the diploid cells that develop within the anthers and ovules, respectively. In angiosperms, pollination results in double fertilization. The egg cell nucleus fuses with a sperm nucleus to produce the zygote while the other sperm cell nucleus fuses with the two polar nuclei to form the triploid endosperm . The endosperm acts as a food supply for the developing embryo. After fertilization, the ovule with the developing embryo becomes a seed and the ovary becomes the fruit that houses the seed(s).

Diversity of Flowers

Among the quarter-million species of angiosperms, there are many variations of the generalized flower yielding an immense diversity of floral patterns. The diversity is due to variations in the number, symmetry, size, and fusion of floral parts. While most flowers contain both stamens and carpels (and are referred to as hermaphroditic), other flowers are unisexual. These may be either staminate flowers (missing the carpels) or carpellate flowers (missing the stamens). Species bearing both carpellate and staminate flowers on a single plant are referred to as monoecious ("one house"; maize [Gramineae] and oak trees [Fagaceae], for example). In contrast, dioecious ("two houses") species bear staminate and carpellate flowers on different plants (willow [Salicaceae], for example). Monoecism and dioecism, both of which have evolved multiple times within the angiosperms, provide a mechanism to promote outbreeding. Many other species have evolved more subtle mechanisms to promote cross-pollination. For example, plants may be self-incompatible; that is, they discriminate between self and nonself pollen and consequently reject their own pollen. Alternatively, a difference in the timing of maturation of the androecium and gynoecium in hermaphroditic flowers favors outbreeding.

Two conspicuous characters that contribute to floral diversity are the number and size of floral organs. Angiosperms are often divided into two groups based on their cotyledon number: monocotyledons (monocots, meaning one cotyledon) and dicotyledons (dicots, meaning two cotyledons). One of the characters that distinguish monocots from dicots is the number of appendages within a whorl . Monocots usually have flowers with floral parts in multiples of three, whereas the dicots often have floral parts in multiples of four or five. Organ size can also vary enormously. For example, in the species Lepidium (Brassicaeae), the petals are microscopic, but in the genus Camellia (Theaceae), some species have petals 15 centimeters long.

Variations in fusion, arrangement, and symmetry of floral organs provide further diversity. Fusion between organs of the same whorl (coalescence) and fusion of organs from separate whorls (adnation) is common among many families of angiosperms. For instance, coalescence is seen in snapdragon (Antirrhinum, Scrophulariaceae) with the petals fused at the base, and adnation in tobacco (Nicotiana, Solanaceae) with stamens fused to the petals. Position of organ attachment to the receptacle also influences flower architecture. If the corolla and stamens attach to the receptacle below the ovary, the flower is referred to as having a superior ovary (e.g., Liriodendron tulipifera, Magnoliaceae). In contrast, having the corolla and stamens attach to the receptacle above the ovary produces a flower with an inferior ovary (e.g., Iris, Iridaceae). In radially symmetric flowers, termed actinomorphic, all organs within any particular whorl are identical and positioned equidistant from other organs within the whorl (e.g., California poppy Eschscholzia californica, Papaveraceae). Flowers in which organs in a particular whorl differ from other organs in the same whorl are referred to as zygomorphic (e.g., most orchids, Orchidaceae).

While all of the mechanisms generating diversity contribute to their interactions with pollinators, the fusion and asymmetry of floral organs has allowed the evolution of fascinating and often bizarre plant-insect interactions. For example, some species of orchids attract potential pollinators with insect pheromones , luring the insect into a maze constructed of fused petals and stamens in which there is one entrance and one exit. In navigating the maze, the insect both delivers to the stigma of the gynoecium pollen from another flower and picks up a load of pollen to be distributed to another flower.

Evolution of Flowers

While angiosperms are prevalent in the fossil record from the mid-Cretaceous (approximately one hundred million years ago), it is thought that they may have evolved substantially earlier, perhaps as far back as two hundred million years ago. The closest extant relatives of the angiosperms are the gymnosperms, of which conifers are members. Conifers do not have flowers but rather produce female and male cones consisting of scales bearing exposed ovules and pollen sacs, respectively. It's intriguing to consider what evolutionary processes occurred to produce the complex assemblage of floral organs of extant angiosperms. Charles Darwin, upon thinking about this question, stated that flowers are an "abominable mystery." The answer to this question, however, may be found in comparative genetic studies. For example, some of the important regulatory genes promoting floral organ development are also found in conifers. Understanding their function in the cones of conifers may allow scientists to model evolutionary changes that occurred resulting in the formation of early flowers.

It is not clear which features were present in the flowers of the earliest angiosperms. Although by the mid-Cretaceous period angiosperm flowers were already quite diverse, a number of key features of extant flowers had not yet appeared. Fossil flowers from the Cretaceous often have organs that are spirally arranged, a perianth that does not have a distinct calyx and corolla, relatively few stamens, and multiple carpels that are not fused together. The attractiveness of the mainly fly- and beetle-pollinated flowers was due to the androceium, which was composed of anthers attached to showy, leaflike structures. The stigma and style of the early individually fused carpels ran down along the side of the ovary instead of being at the top, as seen in most extant carpels.

There are a number of major evolutionary trends when comparing these Cretaceous flowers to their modern counterparts. For example, in many modern flowers the perianth is differentiated into a distinct calyx and corolla. The evolution of the corolla facilitated the reduction in stature of the androecium such that the stamens are composed of anthers attached to slender filaments rather than large, leaflike appendages. In addition, fusion of organs occurred along with the establishment of zygomorphic flowers, creating flowers with deep, open, funnel-shaped flowers, both innovations allowing the evolution of elaborate pollination strategies. Another evolutionary trend is the formation of a gynoecium made up of multiple fused carpels that have only one stigma and style situated at the apical end of the ovary. This has been hypothesized to provide selection at the level of the male gametophyte , which must grow through these structures to effect fertilization.

Coevolution with Pollinators

The early seed-producing plants (such as conifers) utilize wind to move pollen from the staminate cones to the female ovule-bearing cones. To ensure that enough viable seeds are generated, copious quantities of pollen need to be produced. This process requires the expenditure of large amounts of stored resources and is not very efficient. Utilizing insects to transfer pollen to other flowers enables angiosperms to produce less pollen and still maintain a high fecundity in comparison with wind-pollinated plants. In contrast to flowers of early angiosperms, extant flowers have evolved highly attractive characters to ensure that a specific pollinator continues to visit flowers from a specific species of plant. Flowers provide special sources of food for their pollinators to induce them to visit similar flowers. In addition to pollen and other edible floral parts, nectaries provide nectar, a high-energy food source for animals that can sometimes contain amino acids, proteins, lipids, antioxidants , and alkaloids.

Many of the modifications that have evolved in angiosperm flowers are adaptations to promote constancy in pollinator visitation. However, not all angiosperm flowers require animal pollinators. The grasses, which evolved from insect-pollinated flowers, are wind-pollinated. The flowers of grasses are small with reduced or absent petals, and they produce large amounts of pollen.

The Development of Flowers

A basic floral ground plan exists that defines the relationship between organ type and position in all angiosperm species. Because of the constancy in the relative positions of floral organ types, it is hypothesized that a common genetic program to specify floral organ identity is utilized during the development of all flowers. Floral organs are ultimately derived from primordia that arise from the flanks of the flower meristem . It is thought that cells within the flower meristem assess their position relative to other cells and differentiate into the appropriate floral organ based on this positional information.

To clarify how the identity of flower organ primordia is specified, researchers have taken a genetic approach. Mutations affecting flowers and their organs provide a powerful means for studying the genetic interactions involved in their development. Differences in the development of mutant versus normal (wild-type) plants reveal the function of the mutated gene. To carry out this work, researchers use two model plant systems, Arabidopsis (Brassicaceae) and Antirrhinum (Scrophulariaceae), and screen plants that have induced mutations in their genome . Studies have focused on a particular set of homeotic mutations. In homeotic mutants, normal organs develop in the positions where organs of another type are typically found. Specifically, the Arabidopsis floral homeotic mutations result in transformations of one floral organ type into another floral organ type.

It is of interest to note that several homeotic mutants exist in commonly cultivated garden plants. For example, a wild rose flower has five petals, and yet some of the hybrid tea roses have many times that number. Similar situations exist for camellias and carnations. These hybrid varieties represent changes in the genetic constitution of the plant to yield alterations in the floral architecture, which some people find attractive.

see also Angiosperms; Genetic Mechanisms and Development; Identification of Plants; Inflorescence; Interactions, Plant-Insect; Pollination Biology.

Stuart F. Baum

John L. Bowman

Bibliography

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