Botany online: Features of Flowering Plants (2024)

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 arearranged in several circles (whorls), one above the other at an oftenstrongly compressed flower axis. These structures differ stronglyboth in their appearance and in their function from normalleaves.

Progressing from bottom to top and from the out- to the inside canit be distinguished between the usually unobtrusively coloredcalyx that consists of sepals,the conspicuously colored corollathat is built of petals, the stamens and finally the carpels. Calyxand corolla together are theperianth. But if calyx andcorolla cannot be distinguished or if the calyx is missing it isspoken of an incomplete flower orperigon. Flowers with a missingperianth are typical for species with wind-mediated pollination. Thetotality of the stamens, the male reproductive organs, is calledandroecium that of the femalereproductive organs is namedgynoecium.

The flower axis has hardly any appearance in most angiosperms. Itis usually shortened and broadened to the so-calledreceptacle that is sometimesslightly vaulted or disc-shaped. In gymnosperms, however, as well asin some primitive angiosperms (Magnolia,for example), it has the shape of a cone.

The flowers of angiosperms have originally only had an incompleteperianth that can very likely be traced back to leaflike bracts andoccurs in an even more simplified version already in gymnosperms. Themajority of angiosperm flowers has a double perianth with a clearseparation of function between calyx and corolla. While the corollahas as a rule the function of attracting pollinators (it is, afterall, designed mainly on insect aesthetics), the calyx is made toprotect the growing bud. It loses its function as soon as the budflowers; with some species it is even dropped off during flowering,though with most species it just stops growing or shrivels. Onlyrarely are the sepals reactivated after pollination. If they are, anew growth starts whereby an involucre of the fruit is formed.

Allflower structures have originally been organized in a tight screw(Anemone). These flowers are strictly spoken asymmetric butare nevertheless grouped together in books on classification withthose of a radial symmetry. They arecharacterized by the arrangement of their flowers in whorls(flowering circles). All structures of one whorl are of the sameappearance 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 thestructures of one whorl in the gap positions of the preceding (ruleof alternation). Such flowers have two or more planes of symmetry;sometimes those with only two planes are called bilateral. Even ifthe gynoecium shows another number of organs, the flowers are groupedtogether with those with radial symmetry.

Flowers that have only one plane of symmetry due to eitherdiffering shapes of the structures of one whorl or due to incompletewhorls are widespread. They are thought to be of a very highdevelopmental 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 eachother (dialypetalae) while they are fused to a tubular corolla withsympetalae. The number of free points shows how many petals areinvolved.

Both symmetry and architecture of the flower are clearestrepresented via diagrams of the flowers. They show schematizedoutlines where the single structures, their numbers and positions aregiven. If such diagrams are compared to each other, it shows thatfive is a typical number for the whorls of dicots while monocots havemostly 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. Thisinterpretation is proven by the fact that links between bothcategories of structures exist. A classic example is the whitewaterlily (Nymphaea alba, a rather primitive angiosperm) butsuch 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 manystamens while most of the cultured varieties are characterized byfilled flowers with many petals and a reduced number of fertilestamens.

A normal stamen consists of a pollen-containing, fertile anther (agroup of two to four microsporangia) borne at the tip of a bladestalk or filament. The anther has (usually) two theces that arecombined via the connective. Each theca includes two pollensacs where the production of thepollen takes place.

Sterile stamens are calledstaminodes. One variation are thenectar-secreting honey leaves that live mostly between perianth andstamens. They can adopt the size and color of petals and belong tothe pollinator-attracting apparatus of the plant, like in buttercup(Ranunculus, Ranunculaceae).

The central part of a typical angiosperm flower is thegynoecium that is composed of one(simple pistil) or more (compound pistil)carpels. The carpels are,contrary to those of the gymnosperms, fused (see beneath). Theiroriginal structure that of a leaf, is hardly recognizable. Thegynoecium contains one or more pistils.

A characteristic pistil has,from bottom to top, an enlarged basalovary that includes theembryo sac, a columnarstyle and distalstigma, the organ that receivesthe pollen. Stigmata may be very varied in shape and structure. Theyare often button-shaped and equipped with papills. It is not rarethat they are branched. It is distinguished between dry and moiststigmata. The surface of the dry ones is often studded withhair-shaped papills while moist ones are coated with a sticky film.

The style serves to bring the stigma into a favorable position forpollination. The leafy character of the carpels can easily be seenwith a cross-section through the ovary. Both rims of a carpel seem tobe rolled in and fused so that a tube-shaped structure is formed. Onthe basis of the thus built chamber (locule) are the seeds. Thisprotection is missing in gymnosperms. Here the seeds lie openly onthe carpels.

The suture that is formed by the fusion of the rims is called theventral suture in angiospermcarpels while the midrib of the carpel is somewhat misleadinglycalled the dorsal suture.

Normally the gynoecium of angiosperms is composed of severalcarpels. A remarkable exception is made by the leguminoses where theflower contains just one carpel. Each carpel of a gynoecium mayeither form a pistil of its own(choricarpy orapocarpy) or several carpels maybe fused together to one pistil(coenocarpy). The advantage ofthe latter solution is in the ability to distribute the pollen tubesamong all carpels after successful pollination.

The way in which carpels are fused together and the resultingposition of the ovules within the ovary, the so-calledplacentation, has been recognizedto be an important taxonomical feature for the classification ofangiosperms by A. ENGLER and E. PRANTL at the end of the lastcentury. Some orders have been named after their moulding of ovaryarchitecture, like for example centrospermaeor parietales.

©: David T. Webb

An ovary that is built from free carpels is calledapocarp and is supposed to be themost original form of an angiosperm gynoecium. It is often found inranunculaceae. Ovaries with fused carpels are derived from it.

In a syncarp ovary, the carpels are fused laterally. Its number oflocules fits that of its carpels that are separated from each otherby 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 (axialplacentation). False septums may have been put in later on whichcould lead to the misapprehension of a syncarpous ovary atsuperficial examination. But the unchanged parietal placentation ofthe 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

Lysicarpous ovaries display a unified cavity formed throughdisappearance of the lateral rims. Their ovules are inserted at anaxial cone in the centre of the ovary (free central placenta).

The position of the ovary in relation to the other structures of aflower is an important taxonomical feature. It has to bedistinguished between a hypogynous (the perianth is attached to thereceptacle below the pistil), perigynous (perianth and stamensare borne on the rim of a concave structure in the depression ofwhich the pistil is borne) or epigynous ovary (blossom seemsto arise upon or above the ovary).

Flowers that contain both an androecium and a gynoecium are calledandrogynous orhermaphroditic. The flower ismale, if just one androecium is present; if only one gynoecium isdeveloped, then it is a female flower. A plant that has both male andfemale flowers is calledmonoecious while species wheremale and female flowers live on different plants, are calleddioecious. In the extreme, theflower is reduced to only one stamen or carpel. An especiallyinteresting case is the spurge (Euphorbia)and some of its closer relatives. With them several extremely reducedflowers are grouped together to a functionally hermaphroditic unity,a cyathium. By integration of aset of bracts that bear great, often yellowish nectaries and someadditional colored bracts, the impression of a normal hermaphroditicflower is perfected (example: poinsettia;Poinsettia pulcherrima).