Flowers
The organs of a plant serving sexual reproduction are flowers. Their final product is seed. A flower is a rather complex structure formed by the flower axis that is usually transformed into the receptacle and the perianth. The perianth consists of calyx and corolla. The calyx is made up by he sepals, the corolla by the petals, stamens and carpels. If calyx and corolla cannot be distinguished or if the calyx is missing, it is spoken of an incomplete flower or perigon. The androecium is the totality of the male reproductive organs. It comprises the stamens. Each stamen consists of a filament and an anther. The female reproductive organs, the gynoecium, include the carpels. Carpels are structures that are made up from an ovary and a stigma and that contain one or more ovules. One or more carpels may be combined to a pistil (ovary, style, stigma), the gynoecium as a whole. Taxonomical flower features are the flower symmetry, the way in which the carpels are fused and the resulting position of the ovules, also called placentation, and the position of the ovary. Flowers that contain both androecium and gynoecium are called androgynous or hermaphroditic. If both male and female flowers live on one plant it is called monoecious. Plants with male and female flowers borne on separate plants are termed dioecious.
Before we deal with the architectural design of flowers in more detail, it should be said that they are structures serving the sexual reproduction: their final product is seed. Flowering plants are thus also called spermatophytes (seed plants). They include two large groups, the gymnosperms and the angiosperms. The gymnosperms are the evolutionary older and more primitive group. They are, in a strictly systematical sense, no class on their own but enclose two real classes, the conifers and the cycadeans that do not have the same ancestors. The angiosperms that are usually seen as the original flowering plants do all belong to just one class. They represent the overwhelming majority of species and what you will learn in the following is principally true for their flowers.
It is taken for granted that everybody has through his own experience gained an impression of the diversity of flower shapes, colors and smells. It will be equally well known that these features developed in an evolutionary process of mutual adjustments of flowers and their individual pollinators (insects: bees, bumble-bees, butterflies, flies, etc.; birds: hummingbirds, etc.). Plants that are pollinated via wind have very unobtrusive flowers.
Many textbooks present the flower of the anemone (Anemone nemorosa) or that of tulip (Tulipa gesneriana) as typical examples. This is no bad choice, indeed, since their basic building plan is very easily demonstrated.
Diagram (cross section) of a flower green: receptacle and sepals, lilac: petals, orange: stamina, yellow: staminods, blue: carpels
Flowers are short shoots with limited growth. The leaf organs are arranged in several circles (whorls), one above the other at an often strongly compressed flower axis. These structures differ strongly both in their appearance and in their function from normal leaves.
Progressing from bottom to top and from the out- to the inside can it be distinguished between the usually unobtrusively colored calyx that consists of sepals, the conspicuously colored corolla that is built of petals, the stamens and finally the carpels. Calyx and corolla together are the perianth. But if calyx and corolla cannot be distinguished or if the calyx is missing it is spoken of an incomplete flower or perigon. Flowers with a missing perianth are typical for species with wind-mediated pollination. The totality of the stamens, the male reproductive organs, is called androecium that of the female reproductive organs is named gynoecium.
The flower axis has hardly any appearance in most angiosperms. It is usually shortened and broadened to the so-called receptacle that is sometimes slightly vaulted or disc-shaped. In gymnosperms, however, as well as in some primitive angiosperms (Magnolia, for example), it has the shape of a cone.
The flowers of angiosperms have originally only had an incomplete perianth that can very likely be traced back to leaflike bracts and occurs in an even more simplified version already in gymnosperms. The majority of angiosperm flowers has a double perianth with a clear separation of function between calyx and corolla. While the corolla has as a rule the function of attracting pollinators (it is, after all, designed mainly on insect aesthetics), the calyx is made to protect the growing bud. It loses its function as soon as the bud flowers; with some species it is even dropped off during flowering, though with most species it just stops growing or shrivels. Only rarely are the sepals reactivated after pollination. If they are, a new growth starts whereby an involucre of the fruit is formed.
A gallery of flower diagrams:
radial symmetryzygomorph flowersincomplete flowersunisexual flowers
© S. LIEDE
All flower structures have originally been organized in a tight screw (Anemone). These flowers are strictly spoken asymmetric but are nevertheless grouped together in books on classification with those of a radial symmetry. They are characterized by the arrangement of their flowers in whorls (flowering circles). All structures of one whorl are of the same appearance and every whorl has the same number of structures (three, four and five are very common).
Subsequent whorls are usually organized in a way that brings the structures of one whorl in the gap positions of the preceding (rule of alternation). Such flowers have two or more planes of symmetry; sometimes those with only two planes are called bilateral. Even if the gynoecium shows another number of organs, the flowers are grouped together with those with radial symmetry.
Flowers that have only one plane of symmetry due to either differing shapes of the structures of one whorl or due to incomplete whorls are widespread. They are thought to be of a very high developmental state and are called zygomorph (monosymmetric or dorsiventral). Examples are the pansy (Viola) or the labiates (Lamiaceae).
With most flowering plants, the petals are separate from each other (dialypetalae) while they are fused to a tubular corolla with sympetalae. The number of free points shows how many petals are involved.
Both symmetry and architecture of the flower are clearest represented via diagrams of the flowers. They show schematized outlines where the single structures, their numbers and positions are given. If such diagrams are compared to each other, it shows that five is a typical number for the whorls of dicots while monocots have mostly three organs per whorl.
Tulipa gesneriana (tulip). A. top view of flower, B. and C. pistil overall view, B. cross section through the ovary; D. generalized flower diagram. Flower diagrams were drawn in the last century by the German botanist A. EICHLER (1875, 1878) for most of the indigenous and foreign plant groups. They symbolize the position, number and the symmetry ratios of the flower's single components in every circle of petals (according to W. TROLL, 1975).
The petals can often be traced back to reorganized stamens. This interpretation is proven by the fact that links between both categories of structures exist. A classic example is the white waterlily (Nymphaea alba, a rather primitive angiosperm) but such metamorphosis can, too, be found in many other species. Cultivators use this feature to select filled flowers. The wild rose (Rosa canina), for example, has only five petals and many stamens while most of the cultured varieties are characterized by filled flowers with many petals and a reduced number of fertile stamens.
A normal stamen consists of a pollen-containing, fertile anther (a group of two to four microsporangia) borne at the tip of a blade stalk or filament. The anther has (usually) two theces that are combined via the connective. Each theca includes two pollen sacs where the production of the pollen takes place.
Sterile stamens are called staminodes. One variation are the nectar-secreting honey leaves that live mostly between perianth and stamens. They can adopt the size and color of petals and belong to the pollinator-attracting apparatus of the plant, like in buttercup (Ranunculus, Ranunculaceae).
The central part of a typical angiosperm flower is the gynoecium that is composed of one (simple pistil) or more (compound pistil) carpels. The carpels are, contrary to those of the gymnosperms, fused (see beneath). Their original structure that of a leaf, is hardly recognizable. The gynoecium contains one or more pistils.
A characteristic pistil has, from bottom to top, an enlarged basal ovary that includes the embryo sac, a columnar style and distal stigma, the organ that receives the pollen. Stigmata may be very varied in shape and structure. They are often button-shaped and equipped with papills. It is not rare that they are branched. It is distinguished between dry and moist stigmata. The surface of the dry ones is often studded with hair-shaped papills while moist ones are coated with a sticky film.
The style serves to bring the stigma into a favorable position for pollination. The leafy character of the carpels can easily be seen with a cross-section through the ovary. Both rims of a carpel seem to be rolled in and fused so that a tube-shaped structure is formed. On the basis of the thus built chamber (locule) are the seeds. This protection is missing in gymnosperms. Here the seeds lie openly on the carpels.
The suture that is formed by the fusion of the rims is called the ventral suture in angiosperm carpels while the midrib of the carpel is somewhat misleadingly called the dorsal suture.
Normally the gynoecium of angiosperms is composed of several carpels. A remarkable exception is made by the leguminoses where the flower contains just one carpel. Each carpel of a gynoecium may either form a pistil of its own (choricarpy or apocarpy) or several carpels may be fused together to one pistil (coenocarpy). The advantage of the latter solution is in the ability to distribute the pollen tubes among all carpels after successful pollination.
The way in which carpels are fused together and the resulting position of the ovules within the ovary, the so-called placentation, has been recognized to be an important taxonomical feature for the classification of angiosperms by A. ENGLER and E. PRANTL at the end of the last century. Some orders have been named after their moulding of ovary architecture, like for example centrospermae or parietales.
©: David T. Webb
An ovary that is built from free carpels is called apocarp and is supposed to be the most original form of an angiosperm gynoecium. It is often found in ranunculaceae. Ovaries with fused carpels are derived from it.
In a syncarp ovary, the carpels are fused laterally. Its number of locules fits that of its carpels that are separated from each other by a septum. This can be seen very clearly with the ovary of a tulip. The ovules are inserted at the inner angles of the locules (axial placentation). False septums may have been put in later on which could lead to the misapprehension of a syncarpous ovary at superficial examination. But the unchanged parietal placentation of the ovules enables a definite classification.
Scheme of evolution for the basic type of gynoecium. The syncarpous gynoecium developed from the apocarpous one (on top) which again differentiated into the paracarpous (right) and lysicarpous (left) gynoecium. The paracarpous and the lysicarpous types are depicted in two different stages of evolution to elucidate the different ways of development from the basic syncarpous type. The ovules are given in dark brown (according to A. TAKHTAJAN, 1942).
Scanning Electron Micropgraph: cross-section through immature flower bud. Detail: Bilocular ovary with immature ovules, anther with immature pollen (tetrad state) - Snapdragon (Antirrhinum majus))
from "Snapdragon" © K. STÜBER
The position of the ovary in relation to the other structures of a flower is an important taxonomical feature. It has to be distinguished between a hypogynous (the perianth is attached to the receptacle below the pistil), perigynous (perianth and stamens are borne on the rim of a concave structure in the depression of which the pistil is borne) or epigynous ovary (blossom seems to arise upon or above the ovary).
Flowers that contain both an androecium and a gynoecium are called androgynous or hermaphroditic. The flower is male, if just one androecium is present; if only one gynoecium is developed, then it is a female flower. A plant that has both male and female flowers is called monoecious while species where male and female flowers live on different plants, are called dioecious. In the extreme, the flower is reduced to only one stamen or carpel. An especially interesting case is the spurge (Euphorbia) and some of its closer relatives. With them several extremely reduced flowers are grouped together to a functionally hermaphroditic unity, a cyathium. By integration of a set of bracts that bear great, often yellowish nectaries and some additional colored bracts, the impression of a normal hermaphroditic flower is perfected (example: poinsettia; Poinsettia pulcherrima).
