Structure and function
All flower parts arise from the elongated or enlarged tip of a stem (the receptacle). Flowers for the most part consist of a whorl of colored petals (the corolla), which is surrounded by an outer whorl of green sepals (the calyx) which often look like leaves. The sepals look like the petals in most monocotyledons, and the two alternate around the flower rim. Both are called tepals, or in some genera called perianth segments.
In a bisexual flower's center, the male reproductive organs, known as stamens, surround female parts (or just part) known individually as carpels, collectively as pistils. Every stamen has a pollen producing anther at the end of the filament (a long, slender stalk). Flowers can have one or more carpels. Every carpel has a stigma that receives the pollen, sends it down the style (the stalk of the female organ), to the ovary, which contains one or more ovules. When the ovules become fertilized by the pollen, they develop into seeds, which will have their own nutrients to provide sustenance to an embryo plant until it develops a root system and shoots which will allow it to fuel its own growth.
There are countless flower forms, evolutionarily meant to aid pollination by either by insects, other animals, wind or even water. Flowers pollinated by insects and other animals tend to be brightly colored with a sweet scent, and usually having sugar-rich nectar. Some have evolved specialized forms to encourage particular pollinators - for example, the flowers of certain orchids can resemble female insects in order to attract the males. Flowers which are wind pollinated tend to be less easily visible and smaller, though in plants like grasses, they often are crowded in large inflorescences.
Flowers on some plants come as one flower on its own stem. Many other flowers come grouped into inflorescences. The types of inflorescences can be identified by the arrangement of flowers on the stem. Sometimes compound flowerheads resemble a single flower, such as in lantana.
There are two types of flower shapes; either regular or radially symmetrical and rounded in outline; or long and irregular or symmetrical along one axis only. Petals can be separate (free) or else partly fused, forming a flower which is tubular or funnel-shaped. Composite flowers may have elongated florets, but usually the flowerhead is rounded.
Almost all flowers in the wild have a single whorl or fused group of 2-6 petals. A few do have more, but this is more a tendency far more often seen in cultivated plants, which occurred as a mutation and was perpetuated through selective breeding. Semi-double flowers tend to have 2 or 3 whorls of petals, while double flowers have 3 or more whorls and no stamens (or just a few), sometimes they don't even have carpels. Many times stamens have been modified into structures like petals called staminodes. Doubling occurs in a good number of members of the Asteraceae, like Dahlia and Chrysanthemums, but when this happens the number of ray florets goes up, and partially or fully replaces the disk florets.
HabitsThe habit of a flower or an inflorescence is a description of its orientation on its stalk at maturity. This can change on some plants during the development of the flower.
Both colors and markings on a flower evolved originally as a means of attracting pollinators. For cultivated plants, they are modified to enhance their decorative value.
A flower, (<Old French flo(u)r<Latin florem<flos), also known as a bloom or blossom, is the reproductive structure found in Flowering plants (Plants of the division Magnoliophyta, also called angiosperms). The flower's structure contains the plant's reproductive organs, and its function is to produce Seeds. After Fertilization, portions of the flower develop into a Fruit containing the seeds. For the higher plants, seeds are the next generation, and serve as the primary means by which individuals of a species are dispersed across the landscape. The grouping of flowers on a plant is called the Inflorescence.
In addition to serving as the reproductive organs of flowering plants, flowers have long been admired and used by Humans, mainly to beautify their environment but also as a source of food.
The biological function of a flower is to mediate the union of male and female Gametes in order to produce Seeds. The process begins with Pollination, is followed by Fertilization, and continues with the formation and dispersal of the seed.
Flowering plants heterosporangiate (producing two types of reproductive Spores). The Pollen (male spores) and Ovules (female spores) are produced in different organs, but the typical flower is a bisporangiate strobilus in that it contains both organs.
A flower is regarded as a modified stem with shortened internodes and bearing, at its nodes, structures that may be highly modified leaves. In essence, a flower structure forms on a modified shoot or axis with an apical Meristem that does not grow continuously (growth is determinate). The stem is called a Pedicel, the end of which is the torus or Receptacle. The parts of a flower are arranged in Whorls on the torus. The four main parts or whorls (starting from the base of the flower or lowest node and working upwards) are as follows:
- Calyx – the outer whorl of Sepals; typically these are green, but are petal-like in some species.
- Corolla – the whorl of Petals, which are usually thin, soft and colored to attract insects that help the process of Pollination.
- Androecium (from Greek andros oikia: man's house) – one or two whorls of Stamens, each a Filament topped by an Anther where Pollen is produced. Pollen contains the male Gametes.
- Gynoecium (from Greek gynaikos oikia: woman's house) – one or more Pistils. The female reproductive organ is the Carpel: this contains an ovary with ovules (which contain female gametes). A pistil may consist of a number of carpels merged together, in which case there is only one pistil to each flower, or of a single individual carpel (the flower is then called apocarpous). The sticky tip of the pistil, the Stigma, is the receptor of pollen. The supportive stalk, the style becomes the pathway for Pollen tubes to grow from pollen grains adhering to the stigma, to the ovules, carrying the reproductive material.
Although the floral structure described above is considered the "typical" structural plan, plant species show a wide variety of modifications from this plan. These modifications have significance in the evolution of flowering plants and are used extensively by botanists to establish relationships among plant species. For example, the two subclasses of flowering pla nts may be distinguished by the number of floral organs in each whorl: Dicotyledons typically having 4 or 5 organs (or a multiple of 4 or 5) in each whorl and Monocotyledons having three or some multiple of three. The number of carpels in a compound pistil may be only two, or otherwise not related to the above generalization for monocots and dicots.Hermaphrodite. However, in some species of plants the flowers are imperfect or unisexual: having only either male (stamens) or female (pistil) parts. In the latter case, if an individual plant is either male or female the species is regarded as dioecious. However, where unisexual male and female flowers appear on the same plant, the species is considered monoecious.
Additional discussions on floral modifications from the basic plan are presented in the articles on each of the basic parts of the flower. In those species that have more than one flower on an axis—so-called composite flowers— the collection of flowers is termed an Inflorescence; this term can also refer to the specific arrangements of flowers on a stem. In this regard, care must be exercised in considering what a ‘‘flower’’ is. In botanical terminology, a single Daisy or Sunflower for example, is not a flower but a flower head—an inflorescence composed of numerous tiny flowers (sometimes called florets). Each of these flowers may be anatomically as described above. Many flowers have a symmetry, if the perianth is bisected through the central axis from any point, symmetrical halves are produced - the flower is called regular or actinomorphic e.g. rose or trillium. When flowers are bisected and produce only one line that produces symmetrical halves the flower is said to be irregular or zygomorphic. e.g. snapdragon or most orchids.
A floral formula is a way to represent the structure of a flower using specific letters, numbers, and symbols. Typically, a general formula will be used to represent the flower structure of a plant family rather than a particular species. The following representations are used:
Ca = calyx (sepal whorl; e.g. Ca5 = 5 sepals)
Co = corolla (petal whorl; e.g., Co3(x) = petals some multiple of three )
Z = add if zygomorphic (e.g., CoZ6 = zygomorphic with 6 petals)
A = androecium (whorl of stamens; e.g., A∞ = many stamens)
G = gynoecium (carpel or carpels; e.g., G1 = monocarpous)
x - to represent a "variable number"
∞ - to represent "many"
A floral formula would appear something like this:
- Ca5Co5A10 - ∞G1
Several additional symbols are sometimes used (see ).
- Main article: pollination The primary purpose of a flower is to join the pollen of one plant with the ovules of another (or in some cases its own ovules) in order to form seed which is genetically unique, allowing for Adaptation to occur. As such, each flower has a specific design which best encourages the transfer of this pollen. Many flowers are dependent upon the wind to move pollen between flowers of the same species. Others rely on animals (especially Insects) to accomplish this feat. Even large animals such as birds, bats, and Pygmy possums can be employed. The period of time during which this pr ocess can take place (the flower is fully expanded and functional) is called anthesis.
Attraction methodsBirds and Bees are common Pollinators: both having color vision, thus opting for "colorful" flowers. Some flowers have patterns, called Nectar guides, that show pollinators where to look for nectar; they may be visible to us or only under Ultraviolet light, which is visible to bees and some other insects. Flowers also attract pollinators by scent. Many of their scents are pleasant to our sense of smell, but not all. Some plants, such as Rafflesia, the Titan arum, and the North American Pawpaw (Asimina triloba), are pollinated by flies, so produce a scent imitating rotting meat. Flowers pollinated by night visitors such as bats or moths are especially likely to concentrate on scent - which can attract pollinators in the dark - rather than color: most such flowers are white.
Still other flowers use mimicry to attract pollinators. Some species of orchids, for example, produce flowers resembling female bees in color, shape, and scent. Male bees move from one such flower to another in search of a mate.
The pollination mechanism employed by a plant depends on what method of pollination is desired.
Entomophilous flowers (those which employ insects to transfer pollen) have an arrangement of the stamens that ensures that pollen grains are transferred to the bodies of the pollinator when it lands in search of its attractant (such as nectar, pollen, or a mate). In pursuing this attractant from many flowers of the same species, the pollinator transfers pollen to the stigmas - arranged with equally pointed precision - of all of the flowers it visits. Many flower rely on simple proximity between flower parts to ensure pollination. Others, such as the Sarracenia or lady-slipper orchids, have elaborate designs to ensure pollination while preventing Self-pollination.
The flowers of other species are pollinated by the wind (for example, grasses); they have no need to attract pollinators and therefore tend not to be "showy". Wind-pollinated flowers are referred to as anemophilous. Whereas the pollen of entomophilous flowers tends to be large-grained, sticky, and rich in Protein (another "reward" for pollinators), anemophilous flower pollen is usually small-grained, very light, and of little nutritional value to Insects, though it may still be gathered in times of dearth. Honeybees and bumblebees actively gather anemophilous corn (Maize) pollen, though it is of little value to them.
Many flowers have close relationships with one or a few specific pollinating organisms. Many flowers, for example, attract only one specific species of insect, and therefore rely on that insect for successful reproduction. This close relationship is often given as an example of Coevolution, as the flower and polli nator are thought to have developed together over a long period of time to match each other's needs.
This close relationship compounds the negative effects of Extinction. The extinction of either member in such a relationship would mean almost certain extinction of the other member as well. Some endangered plant species are so because of shrinking pollinator populations.
Fertilization and dispersal
- Main article: biological dispersal Dandelions. Conversely, many species of plants have ways of preventing self-fertilization. Unisexual male and female flowers on the same plant may not appear or mature at the same time, or pollen from the same plant may be incapable of fertilizing its ovules. The latter flower types, which have chemical barriers to their own pollen, are referred to as self-sterile or self-incompatible (see also: Plant sexuality).
Flowers provide less food than other major plants parts (Seeds, Fruits, Roots, stems and leaves) but they provide several important foods and Spices. Flower vegetables include Broccoli, Cauliflower and Artichoke. The most expensive spice, Saffron, consists of dried stigmas of a Crocus. Other flower spices are Cloves and Capers. Hops flowers are used to flavor Beer. Marigold flowers are fed to Chickens to give their egg yolks a golden yellow color, which consumers find more desirable. Dandelion flowers are often made into wine. Bee Pollen, pollen collected from bees, is considered a health food by some people. Honey consists of bee-processed flower nectar and is often named for the type of flower, e.g. orange blossom honey, Clover honey and Tupelo honey.
Hundreds of fresh flowers are edible but few are widely marketed as food. They are often used to add color and flavor to salads. Squash flowers are dipped in breadcrumbs and fried. Edible flowers include Nasturtium, Chrysanthemum, Carnation, Cattail, Honeysuckle, Chicory, Cornflower, Canna, and Sunflower. Some edible flowers are sometimes candied such as Daisy and Rose (you may also come across a candied Pansy).
Main and related articles at: Floristry, Flower garden, Gardening, and List of flowers Flowers can also be made into tea. Dried flowers such as chrysanthemum, rose, jasmine are infused into tea by the oriental people both for their fragrance and medical properties. Sometimes, they are also mixed with tea leaves for the added fragrance.
Standard Cyclopedia of Horticulture
|Standard Cyclopedia of Horticulture|
Flower is a popular or semi-technical term for the aggregate of structures having to do with sexual reproduction in the higher plants. The concept usually includes color, and a definite organization as outlined below; therefore, gymnosperms, ferns, and the lower plants are said not to have true flowers. As ordinarily understood, the flower is a showy structure useful for esthetic purposes, gratifying in color and often in odor, and in some way intimately connected with the production of seed; but analogous although inconspicuous structures are sometimes popularly recognized as "flowers." To the layman, many of our common herbs, shrubs and trees are said not to bear flowers at all, although the botanist recognizes that at least inconspicuous greenish flowers are borne by all of these plants unless they be ferns or gymnosperms.
Botanically considered, the flower when complete consists of four sets of organs from the center outward: the gynoecium, androecium, corolla, and calyx, to which may possibly be added a fifth, the disk.
The gynoecium.—In the center are one or more small flask-like or pouch-like organs (pistils) which are hollow and contain tiny bud-like growths (ovules). The pistils collectively are termed the gynoecium (female household). The hollow ovule-bearing part of the pistil is the ovary. At the summit of the ovary is a more or less sticky or roughened surface, the stigma, which may rest directly on the ovary (sessile) or may be raised aloft on a stalk (the style). From the ovules seeds are developed (see Fertilization).
The fundamental or unit foliar organ of the gynoecium is termed a carpel. In the simplest case there is but one carpel, folded to form a pouch with the upper ventral leaf- surface within, and the margins forming a suture down one side. The structure thus formed is a simple pistil. The suture bears the ovules and is termed the placenta, and is normally ovuliferous throughout, but frequently only the uppermost or basal ovule of the row is present (apical and suspended, or basal and erect). In other cases there are several or many carpels but these remain distinct, then forming many simple pistils. In most cases, however, the carpels are more or less fusea, at least below, and the resulting pistil is said to be compound. The sutures are axially placed and the midribs are outward (anterior), the ventral surface of each carpel lining the ovarian cavity. There are, therefore, normally as many cells or locules in a compound ovary as there are carpels. Through the partical opening-out of each carpel while the margins of adjacent carpels still remain united, the ovary may become one-celled though still compound, as in the violet. The placenta will in this case be parietal (on the walls). In certain families (Caryophyllaceae;, Primulaceae) the compound ovaries are one-celled but have a basal placenta, or this basal placenta may project upward into the single chamber of the ovary as a central post on which the ovules are borne (free-central placenta) (Fig. 1515). To determine the number of carpels in a given pistil is often difficult. If there are several separate stigmas or styles, it is usually safe to infer that each represents a carpel. If the ovary is several-celled, each cell usually denotes a carpel and in one-celled ovaries the placenta; if parietal, denote the number of carpels. In the case of a pistil with a one-celled ovary, basal placenta, one style and one stigma, only developmental or phylogene tic studies will show how many carpels are present.
Ovaries are sometimes raised on a stalk within the flower, as in the caper family (gynophore) and in Coptis (thecophore). The styles and stigmas are frequently much modified for pollination purposes, as in the orchids and in the pitcher plant (Sarracenia). The androecium (Figs. 1520- 1522).—Surrounding the pistils are found one or more whorls of organs called stamens, collectively termed the androecium (male household). A stamen normally consists of a slender stalk (filament) capped by an enlarged part (anther), although this stalk is often wanting. The anther contains one, two or four cavities (locules or "cells") in which a powdery mass (pollen) is located. The so-called cells are not to be confused with the cells of the plant tissue. The gynoecium and androecium, both necessary for the production of good seed, are termed the essential organs of the flower. Ordinarily each stamen represents one foliar unit. When many stamens are present, this increase in number is brought about in one of three ways: by an increase in the number of whorls of stamens (Caryophyllaceae, Rosaceae) or an increase in length of the spiral (Ranunculus), by the conversion of petals into stamens, or by a breaking up of each individual stamen into many (St. John's-wort). The first method is by far the most common. In the last method, the origin is usually betrayed by the aggregation of the stamens in fascicles. Normally both filament and anther of each stamen is free from its neighbors, but in some cases the filaments are all joined into a tube around the pistil (monadelphous) as in the hollyhock, or into two groups (diadelphous) as in the pea family. These two groups are usually very unequal in the pea tribes, nine stamens being united while the tenth is free. In other cases the anthers may be coherent while the filaments are free (syngenecious), as in the Composite. In the Sterculiaceae, the filaments or tube of filaments are variously toothed, crested or otherwise modified; while in the Orchidaceae they are fused with the style to form the so-called column or gynandrium of the flower. In the milkweeds, each stamen bears a cornucopia-like appendage which together form the crown. In Viola, two of the filaments bear nectar-spurs.
The anthers are usually oval or oblong bodies fixed to the filament by the base (basal), or by the center (versatile). At maturity they contain normally two 1518. Head of simple pollen-sacs separated pistils in hepatica. by a sterile tissue (connective) which is a prolongation of the filament. The anther-sacs are sometimes four in number, sometimes reduced to one through fusion. The walls of the sacs contain a peculiar fibrous layer by the hygroscopic properties of which they are enabled to curve back, thus opening the pollen-chamber along definite prearranged lines and allowing the pollen to escape. The dehiscence is usually by a longitudinal slit, but it is frequently by terminal pores as in the Ericaceae, or rarely by transverse slits. In Vaccinium, the pores are carried aloft on long tube-like extensions of the anther, while in Berberis the pores are provided with an uplifting trap-door.
The pollen-grains are normally spherical or oval cells in which the two or three nuclei representing the male gametophyte are found. The wall consists of a delicate inner layer (inline), surrounded by a thicker cutinized layer (exine) which is either smooth or externally sculptured in various ways. Specialized places in the extine serve as germ-pores through which the pollen-tubes easily emerge. These pores are sometimes provided with actual lids (pumpkin and squash) which pop off at the proper time. The pollen in the Orchidaceae and Asclepiadaceae is more or less waxy and coheres into one or several masses (pollinia). The pollinia are in many cases produced into minute stalks which connect with a sticky gland that is designed to become attached to visiting insects. On the departure of the insect the gland, together with the attached pollinia, is carried away to the next flower. The pollen-grains of orchids, heaths and a few other plants are composed of two to four cells (compound).
Corolla.—-Outside the stamens is found a whorl of flat leaf-like usually colored organs termed petals or collectively the corolla. The petals are usually in one whorl and follow the numerical plan of the flower closely; rarely are they fewer or numerous. They are normally flat or concave colored bodies distinct from one another (polypetalous) and regularly spreading from the receptacle. But in many plants the petals are connate (gamopetalous) into one structure for a greater or less distance toward the apices. The united part is the tube, the lobed border the limb of the gamo- petalous corolla. The lobes or segments are either all alike and equally placed (regular corolla) or they vary much among themselves (irregular corolla). If the lobes are united higher up into groups of two and three, as in many mints, the upper more or less erect, the lower spreading, the corolla is bilabiate (Fig. 1526). A particular type of irregular polypetalous corolla is the so called papilionaceous corolla (Fig. 1527) found in the pea family and consisting of a standard, two lateral wings, and a ked. A regular corolla is radially symmetrical, possessing an infinite number of planes of symmetry (actinomorphic), while most irregular flowers possess but one plane of symmetry (zygomorphic). A few possess no such plane (as Canna). Gamopetalous corollas fall into certain types based on the shape of the tube and limb. The more common types are rotate, salver-form, funnelform, bell-shaped, tubular, and urceolate.
The corolla may be variously colored. White flowers owe their color to light reflected from air which is between the cells of the petals, as shown by the fact that when waterlogged these petals become transparent. Yellows and oranges are usually due to abundant minute color bodies (chromoplasts) located within the cells of the petal. Reds and blues are due to colored cell-sap.
Calyx.—Surrounding the corolla is another set or whorl of organs, the calyx, the individual organs of which are sepals. The calyx is usually composed of as many sepals as there are petals, but in the Portulacaceae there are but two sepals, while in some plants there are many. In many of the Ranunculaceae and other families they are colored like petals and replace these organs. In the Easter lily and tulip they are similar to the petals. In the Composite the calyx is reduced to scales or bristles or is absent entirely. The sepals are frequently connate (gamosepalous), and the resulting structure is often irregular. The calyx and corolla are together termed the floral envelopes. If they are similar in appearance, and, therefore, difficult to recognize, as in the Easter lily, they are collectively termed perianth.
Disk.—In many plants a glandular disk, or series of glands corresponding to such a disk, is found. When present, this disk may lie either between the stamens and pistil (intrastaminal) as is the common case, or more rarely be- i525 R0tate co_ tween the stamens and petals roiia and connivent (extrastaminal). The genus Acer is peculiar in having some species with an intrastaminal disk while in others it is extrastaminal. By some morphologists this disk is considered a fifth set of organs in the flower, while by others it is considered merely as an outgrowth of the floral axis or receptacle on which all other parts of the flower are inserted. The disk is in many cases characteristic of whole families, which led Bentham and Hooker to place these families together in the series Disciflorae. The disk also occurs in other families not obviously related. It forms a ring about the styles in some Rubiaceae. The glandular cup of Populus and the finger-like gland of Salix are probably to be referred here, although by some they have been interpreted as a reduced perianth. The disk usually functions as a nectary. In shape and structure it is very diverse. It may be cup-shaped, saucer-shaped, annular, regular, or irregular; or it may be of separate glands, either simple or variously lobed. It may line the cup of the perigynous flower or it may be adnate to the surface of the ovary.
Receptacle.—The apex of the stem on which the various floral organs are inserted is termed the receptacle or torus. This is normally a simple club-shaped thickening of the summit of the stem. In the strawberry it is much enlarged and fleshy, forming the greater part of the fruit. In the raspberry it remains on the plant when the "fruit" is removed. In the Composite there is a common receptacle for all the flowers of the head, as well as for each individual flower. In the caper family the receptacle is often prolonged upward, forming a stalk for the ovary within the flower (gynophore).
In the Rosaceae:, Onagraceae, Saxifragaceae, and in various other plants, the stamens, petals and sepals are perigynous, that is they are inserted on the edge of a cup-shaped organ which springs either from below the ovary or from its summit. The view has been held that the gamosepalous calyx here bears the stamens and petals on its tube. Another early proposed view has in recent years gained ground rapidly and is now widely accepted. This view interprets the cup as a hollowed receptacle likened to a glove-finger when the apex is slightly pushed in. The ovary at the bottom of the cup is really apical as usual, while the sepals, petals and stamens,located at the higher margin of the cup, are as usual inserted morphologically lower on the receptacle. While in most flowers the ovary is inserted on the summit of the receptacle (superior ovary), in others, as in the Orchidaceae, Onagraceae, Umbelliferae, Rubiaceae, and Composite, the ovary appears to occupy the center of the club-shaped structure (inferior ovary) below the insertion of the calyx, corolla, and stamens which seem to spring from the summit of the ovary (epigynous). The view has been held that in such cases a gamosepalous calyx similar to that described above in the perigynous flower has grown fast to the surface of the ovary, and that the other organs are borne on the calyx-tube at the summit of the ovary. The opinion is now becoming general that the true explanation of the phenomenon is that the cup-shaped receptacle of the perigynous flower, and not the calyx, has grown fast to the surface of the ovary. In the Onagraceae, and some other plants, the hollow receptacle has not only grown fast to the whole surface of the ovary but projects beyond it so that such flowers have an inferior ovary and are also perigynous.
Bracts.—The leaves on the peduncles and upper parts of the stem adjacent to the flower deserve a word. They are often much modified in size, shape and color from the normal foliage leaves, being often much reduced. They sometimes form an involucre around the flower, and are calyx-like, as in hepatica and strawberry. In other cases, they form a showy corolla-like involucre, as in Cornus and Poinsettia, and are then often mistaken for a corolla. In the Arum, a single huge bract (spathe) envelopes the entire flower-cluster (spadix).
Incomplete flowers.—Not all of the floral sets described above are always present. The flowers may be incomplete. Thus the corolla may be wanting (flower apetalous) as in hepatica and anemone, or both calyx and corolla may be absent (naked or achlamydeous) as in willow and pepper, or the stamens may be wanting (imperfect or unisexual, pistillate flower) as in willows and oaks, or the pistils may be absent (staminate flowers of willows and oaks). At least one set of essential organs is necessary for a functional flower, but in some cases, through specialization for other purposes, both sets may be absent. Thus the marginal flowers of the hydrangea are enlarged and showy for insect attraction, but are neutral. In the case of unisexual flowers, the stamens and pistils may be borne in different flowers on the same plant (monoecious) as in the oak and birch, or on separate plants (dioecious) as in the willow and poplar. In some plants, as in the maple, certain flowers are unisexual while others are perfect, a condition termed polygamous.
The plan of the flower.—If the numbers of parts in each set are counted, a certain number will be found to be common to many or all of the sets of the same flower. This is the numerical plan of the flower (Fig. 1534). Thus in geranium there are five sepals, five petals, ten stamens, and five parts to the pistil. The stamens, when numerous, are often in multiples of this numerical plan. The parts of the pistil, on the other hand, frequently show a reduction from the numerical plan as exhibited by other parts of the flower. The number of parts in some flowers is so irregular that a numerical plan can be made out only with difficulty, while in some flowers such a plan is apparently wanting. The members of each floral set are usually inserted all at the same height on the floral axis (receptacle), and are therefore in whorls, although frequently more than one whorl occurs in the androecium and rarely in other sets. The parts of one set normally fall between those of the set next outside and next inside, and are said to alternate with these. In some families, as for example in the Ranunculaceae and Magnoliaceae, some or all of the organs of the flower are inserted spirally on the receptacle like scales on a pine cone. In such cases there is often a marked intergrading between the organs of the adjacent sets at the boundary line. The relative position of parts of the flower may be graphically indicated by means of a diagramatic cross- sectional plan, called the floral diagram. Information in regard to the number and union of parts may also be indicated by so-called floral formula; as follows:
K C A G
5 5 5+5 2
In this formula, the letters from left to right indicate calyx, corolla, androecium, and gynoecium respectively. The brackets over the letters indicate a fusion of parts in the same set, while the bracket underneath indicates a fusion of different sets. The above flower would be polysepalous with five sepals, gamopetalous of five fused petals, have ten stamens in two whorls all inserted on the corolla, and two carpels united into one pistil with a superior ovary.
Double flowers.—Occasionally in nature and very frequently in cultivation, the number of petals becomes very greatly increased, often to the exclusion of the stamens and pistils, so that the flower presents a full rosette-like appearance. Such flowers are popularly said to be "full" or "double." The increase in petals is apparently a mutation, but is stimulated by changes in nutrition due to cultivation. Most double-flowered varieties tend strongly to run out. The origin of the extra petals is not always the same. In most cases, as in double hollyhocks and carnations, the stamens and even carpels have been transformed into petals; in rarer cases the extra structures are interpolated organs. Double "flowers" in the sunflower, golden glow, and the like, are simply heads in which all disk-flowers are converted into ray-flowers (see next paragraph).
False flowers of the Composite.—The so-called flowers of such plants as the white daisy, sunflower, aster, goldenrod, and dandelion are found on close study not to be flowers at all, but flower-clusters of the type termed heads. These heads are remarkably specialized for economy and division of labor. This community of flowers functions as does one individual flower in other cases, and the whole make-up of the head simulates a flower to a remarkable degree. Around the head is a calyx-like involucre of bracts, functioning like a calyx as a protection in the bud. In daisy, sunflower and others there is a corolla-like part consisting of highly modified ray-flowers or ligulate flowers. The central part of the head in these plants is occupied by disk-flowers. The aster, goldenrod, cone- flower and many others are like the daisy, while in the dandelion, chicory, hawkweed and sow thistle the head consists of ligulate flowers only, and in the thistle, bone- set and iron weed the head contains only disk-flowers. The morphology of the less specialized disk-flower is as follows: A one-celled, one-seeded inferior ovary is surmounted by a variously modified calyx, which is often wanting, and a tubular five-toothed gamopetalous corolla. On the corolla-tube are borne five syngenesious stamens, and from the summit of the ovary projects a single style which is two-branched above. The ray flowers have been developed from the disk type in the course of evolution by greatly increasing the size of such a tubular corolla, and by splitting the tube down one side, at the same time flattening out the slit portion. In the sunflower, there was no great change in color as the ray-flowers evolved, while in the daisy and the asters the rays are of a different color from the disk-flowers. Since the involucre performs for the whole head the same function that the individual calyx does normally for each flower, there is no longer any necessity for the calyx. Therefore, following the general rule that a useless structure tends either to disappear or take on a new function, the calyx has become obsolete in some cases while in others it has become modified into scales, awns or bristles (pappus) which aid the fruit in dissemination. In many cases the ray flowers have been sacrificed entirely for insect attraction and have become sterile. By this massing of the flowers, more flowers may be pollinated by one insect visitor, and more easily pollinated. Efficiency and economy run through the whole organization of the composite head to a remarkable degree.
The biology of the flower.—The flower is a structure developed by plants to promote and safeguard sexual reproduction, primarily in land plants, and to bring about cross-pollination in these plants. The three definite agents of cross - pollination with which the flower is concerned are water, wind and insects. The agent for which the flower is adapted exerts a profound influence on the structure of the flower. Only insect - pollinated flowers are normally showy. Water- and wind- pollinated flowers are usually green and small, with often a total loss of corolla or of both corolla and calyx. The pollen in such plants is produced in abundance to make up for great loss, as it is wafted indiscriminately through the air. Water plants usually flower at the surface and are wind- or insect-pollinated. The true water-pollinated or hydrophilous plants are few in number. Naias, Zannichellia, Zostera and Ruppia may be mentioned, all of which belong to the Naiadaceae. In Zostera, the pollen-grains are long and spiral as a further adaptation to water-pollination.
Wind-pollinated or anemophilous flowers are very numerous. Elodes and Vallisneria (eel-grass) among aquatic plants may be mentioned. Vallisneria is remarkable because the staminate flowers break off before anthesis, rise to the surface, expand, and are floated about by the wind, the three reflexed sepals acting as floats which cannot be upset. The pistillate flowers are attached to long peduncles which extend to the surface of the water, whether it is shallow or deep. The pistillate and staminate flowers are so shaped that when the two float together the stamens are in exactly the right place to touch the stigmas. After pollination, the peduncle coils up and the fruit matures under water. The catkin-bearing trees are all anemophilous and have very much reduced flowers. The willows are both wind- and insect-pollinated. Among herbs the grasses, sedges, rushes, and sorrels (Rumex) are wind-pollinated. Interesting in this respect is Thalictrum (meadow-rue) of the Ranunculaceae, the flowers of which are wholly green and insignificant with large exserted anthers and abundant pollen and feathery stigmas. It thus exhibits perfectly the various adaptations to wind-pollination in a family that is normally insect-pollinated and has showy flowers. The time of flowering of wind-pollinated flowers often shows a distinct relation to efficiency. The wind-pollinated trees and shrubs bloom in early spring before the leaves interfere with the passage of pollen through the air. The grasses and other herbaceous anemophilous plants bloom before the tall growth of late summer has matured, at which time plants are mostly insect-pollinated. The pollen-grains of anemophilous plants are nearly always smooth and very light, and usually contain starch as a reserve food instead of oil. This pollen is capable of withstanding greater desiccation than is the pollen of most insect-pollinated flowers. In the pines, each grain is provided with two air-sacs to increase the buoyancy and to expose greater surface to the wind.
Insect-pollinated or entomophilous flowers must meet two distinct problems: they must entice the insect to the flower; and they must guide the insect in such a way that cross-pollination will be assured. The attractive agents are four in number, color, honey, scent, and abundant pollen (for pollen-eating insects), but they are not usually all found in one species. Color is provided mainly by the corolla, but the calyx (in Anemone) or even the bracts around the flowers (in Cornus and Poinsettia) may function thus instead. Attempts have been made to show that certain colors are more attractive than others to certain groups of insects. Yellow has been designated as the color for flies and beetles, blue and red for hymenoptera, browns for carrion insects and wasps, and whites for night-flying insects especially. Honey (nectar) is produced in a great variety of flowers and it is a reward for the insect visit. The honey-secreting glands (nectaries) are borne either on the disk or on the petals, but more rarely are they staminal or ovarian. In order that the honey may not be appropriated by undesirable insects which would not effect cross-pollination, it is frequently placed at the end of spurs or grooves which are adapted to the proboscis of the insects for which the flower is adapted. Various markings of the corolla, such as bright eye- spots and dark converging lines, called honey-guides, often direct the insect accurately to the honey, and in such a way that cross-pollination will be accomplished. An interesting case is the violet, where the honey is produced by staminal nectaries but is collected and stored in the spur of the lower petal. To this storehouse honey-guides in the form of purple lines lead. The beard in the throat of the violet flower protects the pollen from rain and also discourages the insect from entering the flower on the wrong side. Scent as a means of attracting insects is very general, and is especially frequent in nocturnal and crepuscular (twilight) flowers. The scent is due to volatile oils produced mainly by the petals. These oily compounds are comparatively few in number and often re-occur in plants that are wholly unrelated. Thus the clove scent is found also in some orchids, and the violet scent is found with slight modification in the flowers of several plants. Flowers that attract pollen - eating insects are often yellow, as buttercups and dandelion, but flowers of other colors are frequently visited at least by bees that carry away quantities of pollen in their femoral pollen-pockets. Most pollen is injured by exposure to rain and dew. The grams tend to swell and burst owing to the excessive osmotic pressure. It is for this reason that pollen when studied or germinated in the laboratory must be mounted in a sugar solution approximating the density of the stigmatic fluid. It is not a surprise, therefore, to find that nature has protected the pollen of many flowers from rain, by structural means. Thus, bell- shaped hanging flowers, salverform corollas with a small eye which requires pressure to force a drop of water in, closed corollas of the snapdragon type, beard in the throat, flowers that droop only in wet weather, flowers that close up during rain, and many other contrivances, are adaptations, in part at least, for the protection of the pollen.
The protection of the honey and pollen from unbidden insect guests and the safeguarding of the flower from self-pollination by such insects, has led to various protective devices. The closed throat of the toadflax and snapdragon, the small eye of the salverform corolla, the beard in the violet, setose peduncles and stems over which insects can walk with difficulty, glandular peduncles and bands of viscid matter which serve as a sort of sticky fly-paper to prevent wingless insects from reaching the flower, are all adaptations of this nature. Remarkable in this respect is the teasel, which has connate-perfoliate leaves. These leaves form a basin around the stem at each node. The basins fill with water during each shower, and, as the water will not evaporate for several days, there is a veritable moat around the stem at each node which climbing insects cannot pass.
Cross-pollination is frequently rendered more certain by various mechanical devices. Thus a device of great efficiency found in many plants is the separation of stamens and pistils in different flowers (diclinism) which renders self-pollination impossible. In this respect, the dioecious plant is the most perfect type. Diclinism is especially common in anemophilous plants, in which the pollen is blown about indiscriminately. Another efficient device consists in the early maturation of the stigmas (proterogyny) or of the stamens (proterandry) before the other sex in the same flower (condition of dichogamy). Still another, although much less common device, is the production of two or three types of flowers in the same species in which the styles and stamens are of different lengths (heteromorphism). Thus in the primrose (Fig. 1539) one flower may have long stamens and short style, and another flower short stamens and long style (dimorphic), so that an insect coming from a long-stamened flower will have pollen on his proboscis at exactly the right height to brush the stigma of the long-styled flower. In Lythrum salicaria, the various combinations between the length of style and of each of the two sets of stamens furnish three types of flowers (trimorphic). Other devices are often found. Thus in some flowers the pollen of another plant is prepotent in fertilization over that of the same plant if both are placed on the stigma at the same time. There are also many special structural mechanisms in individual species, a study of which forms one of the most interesting chapters in biology. Here may be mentioned the wonderful adaptations of the orchid flowers, the catapulting of the pollen of the orchid Catasetum against the insect, the lever-hammering stamens of Salvia, the deliberate stuffing of the Yucca stigma with pollen by the Pronuba moth as she deposits eggs in the ovary, the gall flowers and caprification of the fig, and many other equally extraordinary eases.
Although most plants seem to need cross-pollination and to have structures adapted to this end, there are some in which definite preparation is made for close- or self-pollination. Thus certain plants, as violet, barley, Polygala, Dalibarda and others, produce cleistogamous flowers, which are small green apetalous structures often hidden by the leaves or are even subterranean. The calyx of these flowers never opens. The anthers lie against the stigma, and on opening, the pollen is immediately applied to the stigma of that same flower. Seeds produced by such flowers are often much in excess of those produced by the showy flowers of the same species. In the violet, cleistogamous flowers are produced in abundance through the summer after the showy flowers have disappeared. Incidentally it is interesting that these flowers in violets are more important in classification than are the showy ones.
Evolution of the flower.—In the Thallophyta, Bryophyta and Pteridophyta there is no flower as that term is here used. The sporophyte shows an increasing complexity through these groups, but there is no differentiation into an organ that could popularly or even technically be called a flower. Among the Gymnosperms, the cones of the Pinaceae have been likened to a flower with many carpels but with no calyx or corolla, while those of the Gnetaceae are still more flower-like. The true flower, however, is a structure characteristic of the Angiosperms.
There are two prominent theories in regard to the origin of the flower. First, the foliar theory holds that sepals, petals, stamens and carpels are real leaves modified in the course of evolution from the foliage-leaves of their ancestors. Floral parts are, therefore, metamorphosed leaves. The evolution in this case would have been from below toward the apex of the floral shoot, or from the foliage leaves toward the carpels. Certain teratological conditions have been cited in support of this theory, especially when petals, stamens and sometimes carpels have been replaced by green leaves. This has been considered merely a reversion to ancestral conditions. Trillium grandiflorum frequently furnishes cases of this sort. This theory has been exclusively held in the past. Recently another wholly different theory has been proposed by Bower, and is now accepted by very many botanists. This has been termed Bower's sterilization hypothesis. It holds that the foliage-leaves together with the sepals and petals are sterilized sporophylls and that evolution has been from above downward. Specifically it holds that although the simple sporophyte of the mosses consisted as at present of a capsule and seta undifferentiated into stem and leaves, in some special groups of mosses, however, the spore-bearing region around the columella of the capsule became segmented into transverse belts separated by sterile belts. Coincident with this, the exterior of the capsule became lobed in such a way that each fertile belt came to lie in the axil of a lobe. From this it is easy to postulate an increase in size of the lobes to form the scale-leaves of the club-mosses and selaginellas, and an increase in specialization of the fertile belt to form the axillary sporangium of these plants. It is but a step now to the angiospermous flower, in which some of the sterile sporophylls have become modified into petals and sepals instead of leaves. The demand for a large independently growing sporophyte is thought to have led to the sterilization of the sporophylls. According to this theory, leaves are recent rather than primitive structures. The sterilization theory has the advantage of being more in accord with modern knowledge of the evolution of organs in these groups.
Floral evolution within the angiosperms is also difficult to follow and botanists differ as to its course. It is by many held that the most ancient type is the acyclic type as represented by the Ranunculaceae, Magnoliaceae and the like. Another although gradually diminishing school holds that the simple flowers of the Gramineae among the monocotyledons and the Amentiferae among the dicotyledons are the most primitive. The high specialization of other parts of these plants and the likelihood that the flowers have been simplified because of the adoption of the wind method of pollination, strongly suggests that these flowers are not primitive but specialized.
The flower from standpoint of comparative morphology.—The newer evolutionary morphology has brought about changes in viewpoint in regard to floral parts, and a new terminology has arisen. According to present knowledge, there is in some algae and in all bryophytes, pteridophytes and spermophytes a definite alternation of two generations or phases in the life history of each plant, separated by a unicellular condition of the organism. One of these, the more primitive, bears only sex- cells (eggs and sperms) called gametes and is termed the gametophyte, while the other bears spores only and is termed the sporophyte. These generations have exactly reversed their relative size, complexity and degree of independence as evolution has progressed. The originally independent carbon-assimilating gametophyte of the mosses has become in the higher plants wholly parasitic on the sporophyte and is entirely lacking in green color. On the other hand the sporophyte, represented in the mosses and liverworts by the dependent capsule and seta stalk, has become the real plant, bearing leaves and flowers in the higher group. The thalloid reduced gametophyte of the ferns is termed a prothallium, bearing sperm-cells in antheridia and an egg-cell in an archegonium. This prothallium has become differentiated in the more specialized family Selaginellaceae into two types differing in size and complexity of structure, and originating from spores of different size. The large type of spore (megaspore or macrospore) gives rise to the large female prothallium which bears the archegonia; and the small spore (microspore) gives rise to the small male prothallium bearing only a single antheridium. The prothallia of both sexes are very much reduced and permanently inclosed within the spore wall. In the flower-bearing plants, the reduction and dependence of the gametophyte have been carried much farther. The male gametophyte or male prothallium is inclosed in the pollen-grain and the female prothallium within the embryo-sac. The spore-bearing chamber or chambers (sporangia) corresponding to the capsule in the mosses are borne on leaves (sporophylls) in the ferns and fern allies. If these terms used for the mosses and ferns are now applied to the organs of the higher plants the terminology will be as follows: Stamens, microsparophytes; anther-chambers, microsporangia; pollen-grain, microspore; nuclei within pollen-grain, male prothallium (male gametophyte); carpel, megasporophyte; ovule, megasporangium; embryo-sac, megaspore; cells within embryo-sac except embryo, female prothallium (female gametophyte); the embryo growing from the fertilized egg is the daughter sporophyte. A mature seed, therefore, contains parts of three generations; seed-coats and nucellus, if present equals sporophyte; endosperm (according to one interpretation) equals gametophyte; and embryo equals daughter sporophyte This terminology is now gaining ground over the old in morphological circles for it shows the relation of the flower to organs in the lower groups.
- ↑ Eames, A. J. (1961) Morphology of the Angiosperms McGraw-Hill Book Co., New York.
- Eames, A. J. (1961) Morphology of the Angiosperms McGraw-Hill Book Co., New York.
- Esau, Katherine (1965) Plant Anatomy (2nd ed.) John Wiley & Sons, New York.
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