The Description of Flowers

Terms describing flower anatomy

Classical angiosperm phylogeny is based mainly on flower anatomy, and in particular, the arrangement and form of the principle parts. The fruiting body is used as well, but its structure is often apparent in the flowers. More recently, genetic studies have been employed to determine the relationship of various botanical groups to each other. Many parts of the old classification system have proven more or less consistent with these studies, but there have also been a large number of changes, even to large and well studied groups. The changes to the classical system have been far more extensive than in the animal kingdom, where the classical, anatomy-based phylogeny has held up remarkably well. As a result, botanical nomenclature has been undergoing a bit of a revolution in the last two decades, and the process (as of 2009) is still ongoing. A community known as the Angiosperm Phylogeny Group or APG has provided some centrality to this process, but there are many areas where no consensus has been reached. Botany has always been prone to what might be termed "vanity species" with some genera (e.g. hieracium, the hawkweeds) having thousands of specific scientific names. The recent trend has been to recognize certain species as highly variable; but history dies hard, especially for those who discovered and described the "species".

A flower is the reproductive unit of an angiosperm plant. There is an enormous variety of flowers, but all have some characteristics in common. The definitive characteristic of the angiosperms is the enclosed ovary, which contains and protects the developing seeds. Floral reproduction is bisexual, and flowers have "male" and "female" parts. The "male" or pollen-bearing part is called the stamen, and is composed of the filament and the anther. The "female" or seed-bearing part is called the pistil, and is composed of the ovary, the stigma, and the style. A flower may have exclusively male parts, exclusively female parts, or commonly, both. When there are separate flower types, both may occur on the same plant; occasionally a plant may bear only male or female flowers.

Surrounding the reproductive parts is the perianth, a double envelope consisting of an outer portion, the calyx, which forms the sepals, and an inner portion, the corolla, which forms the familiar petals. There may also be leafy elements, termed bracts, surrounding a flower. Individual flowers are often organized into a larger group or cluster, termed an inflorescence. The stalk supporting a single flower is called a pedicel, that supporting an inflorescence, or an isolated flower, a peduncle.

Anther:

The pollen-bearing body of the stamen, usually relatively compact, and supported at the end of the narrow filament. Under a lens, anthers exhibit a wide variety of forms and means of attachment. These characteristics are often important in technical keys for flower identification.

Bract:

A leaf-like element below a flower or on an inflorescence. Bracts are typically shaped differently than other leaves on the plant. They are usually green, but occasionally are brightly colored and petal-like.

Calyx:

The outer perianth of a flower. The calyx surrounds the corolla, and is typically divided into lobes called sepals. These are frequently green, and reduced relative to the petals, but they can also be large, and brightly colored, resembling petals. In many flowers, the sepals enclose and protect the flower bud prior to opening.

Corolla:

The inner perianth of a flower. The corolla typically surrounds the reproductive parts of the flower. It may be continuous as in a petunia, lobed, or divided into distinct petals. In some cases, especially in cultivated varieties, the corolla may be doubled or even further multiplied, producing multiple layers of petals. In other cases, it may be lacking entirely.

Filament:

The usually narrow and often threadlike part of the stamen which supports the pollen-bearing anther.

Involucre:

A circle or cup of bracts that surrounds and supports the multiple florets of the head in the composite flowers of the family asteraceae. The shape and arrangement of the involucral bracts is important in describing the members of this family.

Ovary:

The part of the pistil that encloses the unfertilized seeds or ovules, and that typically develops into a dry or fleshy fruit once pollination takes place. The ovary is generally central to the flower, and supports the other principle parts. Whether they are attached at the top (ovary inferior) or the bottom (ovary superior) is an important anatomical characteristic for classification. Not all "fruits" are mature ovaries; some form from supporting parts of the flower, for example, strawberries develop from the receptacle - the enlarged top of the flower stalk.

Pedicel:

The footstalk supporting a single flower in an inflorescence.

Peduncle:

The stalk supporting an inflorescence or solitary flower.

Perianth:

The technical term for the envelope that surrounds the reproductive parts of a flower. This enclosure is composed of two concentric units, the outer perianth, or calyx which may be divided into sepals, and the inner perianth, or corolla, which may be divided into petals. Either the calyx or the corolla (or both) may be much reduced or lacking.

Petal:

A division or lobe of the corolla or inner perianth of a flower.

Pistil:

The seed-bearing or "female" reproductive part of a flower. The pistil is composed of the ovary, the style, and the stigma. The ovary contains the developing seeds, and is connected to the pollen-receiving stigma by the style. Flowers often contain a single pistil, but may contain several. Staminate or "male" flowers contain only stamens and lack pistils entirely.

Receptacle:

The generally enlarged top of the footstalk, which supports the other parts of the flower. Some "fruits" are enlarged receptacles rather than ovaries.

Sepal:

A division or lobe of the calyx or outer perianth of a flower. Sepals are often green, and/or reduced in size, but they can be colorful and petal-like as well.

Stamen:

The pollen-bearing or "male" reproductive part of a flower. The pollen is borne on a more or less compact body termed the anther, which is supported by the filament. A flower may have hundreds of stamens, or only a few. Pistillate or "female" flowers have pistils but no stamens.

Stigma:

The upper part of the pistil which receives the pollen. The stigma is often sticky, or covered with fine hairs or grooves, or other anatomical features that help the pollen to adhere. It may be cleft into several parts.

Style:

The usually elongated part of the pistil that connects the ovary to the stigma.

Terms describing the inflorescence

Inflorescence refers to the flowering body of a plant. These occur in an amazing variety of forms, from solitary flowers to enormously complex clusters, and there is an equally amazing variety of technical terminology used to describe them. Unfortunately, as with some other aspects of botany, this terminology is not universally standardized; different authors use different terms for the same structure, and sometimes the same terms in slightly or significantly different ways, and classify structures using different paradigms. We only brush the surface here, making use of a few of the most commonly used terms.

A basic and useful but somewhat non-immediately-obvious technical distinction is between determinate and indeterminate inflorescences. A determinate form has a fixed endpoint, usually a terminal flower that generally opens first. Lower side branches bear buds that open later. Indeterminate forms are often termed cymose or cymes. An indeterminate form is a shoot that can, at least theoretically, continue growing from the tip and producing more flowers indefinitely. The lower flowers open first, while upper buds may still be tiny and developing. Of course the whole process eventually comes to an end, but the distinction is frequently obvious. Indeterminate forms are termed racemose or racemes, though the latter term also has the more specific technical meaning described below. Of course in botany, nothing is ever simple. There are compound inflorescences that are mixtures of forms in every conceivable combination.

Solitary: show all

Flowers (or heads) borne singly on isolated stems or arising individually from leaf axils. Not part of a larger group.

Raceme: show all

A simple, indeterminate inflorescence consisting of stalked flowers attached to a central stem and forming a more or less elongated cluster. The stalk of a flower is termed a pedicle and pedicled flowers are implied by the term raceme when used alone in the specific sense.

Spike: show all

An indeterminate inflorescence consisting of stalkless flowers attached to a central stem, generally forming a highly elongated cluster. A raceme of stalkless flowers.

Corymb: show all

An indeterminate inflorescence forming a convex or flat-topped cluster, essentially a contracted raceme. Typically flowers arise from a central axis on stalks (pedicles) of different lengths that bring them all to near the same height. The term is also applied to racemes of similar shape with branching pedicles. The outermost flowers generally open first.

Umbel: show all

An branched inflorescence forming a convex or flat-topped cluster in which all the pedicles are nearly the same length, and appear to originate near a single point (like the spokes of an umbrella). Many umbels are actually highly contracted racemes, though the term is frequently used with determinate forms as well. Umbels can be compound, that is an umbel of umbels. The form is characteristic of the parsely family (apiaceae), which was formerly known as umbelliferae.

Head: show all

A compact inflorescence, with the flowers so tightly packed as to appear a single unit. The composite flowers of the sunflower family (asteraceae) are the most familiar examples, but there are many other instances. The term is applied to both determinate and indeterminate units.

Panicle: show all

An indeterminate inflorescence consisting of a compound raceme; that is, a central axis bearing racemes as secondary units, or a more general branching structure composed of racemes. The term is sometimes applied to any sort of a loose, branching cluster.

Cyme: show all

A determinate inflorescence, often one forming a convex or flat-topped cluster, though the term is applied to other determinate arrangements as well. In all forms, a terminal flower opens first, followed by flowers on lower branches. Multiple levels of branching are frequently involved.