Identifying Wildflowers in Red Rocks Open Space
George Cameron, June 2008

These pages are excerpted from booklets written for the Biology of Plants and Plant Taxonomy courses at Pikes Peak Community College. In these courses, we learn flower structure after learning the reproductive cycles of plants, which involves a spore-making generation and a gamete-making generation. We will not cover this topic in the Red Rocks lecture, but the text below retains terms referring to it.

1. Terms Describing Flowers (go to part 2: Classification and Nomenclature)

Uncomplicated, complete flower

Distinct and free mean separate to the point of attachment on the receptacle. If the fusion is only at the base, a key might say “fused at the base,” to avoid a misunderstanding.

The prefix apo- means distinct. An apopetalous flower has distinct petals. The prefix syn- or sym- means connate. A sympetalous flower has connate petals.

Shapes

Some corollas are radially symmetrical; we call them actinomorphic, or regular.

Some corollas are bilaterally symmetrical; we call them zygomorphic, or irregular. (“Irregular” more generally refers to any asymmetry.)

The fused part of connate calyx or corolla is called a tube. The flaring part is a limb, which usually is made up of lobes. Sometimes we see a partly flared section between the tube and limb; this is a throat.

Corollas have many shapes, and each shape has a name. Look them up as needed – every plant manual has a glossary.

Incomplete flowers

An incomplete flower has one or more parts missing.

A flower without sepals is an asepalous flower.

A flower without petals is an apetalous flower.

A flower without stamens is a carpellate flower (= pistillate flower)

A flower without carpels is a staminate flower.

The sex parts are the essential parts (needless to say). A flower with carpels and stamens is a perfect flower. A carpellate or staminate flower is an imperfect flower. (These are not value judgments.)

A complete flower is a perfect flower, but a perfect flower can be complete or incomplete. For example, a flower with both stamens and carpels, but no petals, is perfect, but incomplete. (Got that?)

A species with imperfect flowers must have both staminate and carpellate flowers. The staminate and carpellate flowers can be on the same plant, or on separate plants. If staminate and carpellate flowers are on the same plant, the species is monoecious (“one house”). If staminate and carpellate flowers are on separate plants, the species is dioecious (“two houses”). (The terms monoecious and dioecious also describe the arrangement of male and female cones on gymnosperms.)

Variations to confuse us

Whorls of parts are always in the order calyx-corolla-androecium-gynoecium. However, fusion of parts can give an apparently different order.

In the flower diagram on page 31, the ovary is superior to the perianth and stamens (superior meaning “above,” not a value judgement). We can also say the insertion of the perianth and stamens is hypogynous (“below the gynoecium”). (An insertion is the apparent point of attachment of a floral part.)

Some flowers (for example, a cherry blossom) have this arrangement:

A flower is a modified stem with modified leaves attached spirally around it. The modified stem is called a receptacle. Some of the modified leaves are sporophylls: these are carpels and stamens. Some of the modified leaves are sterile: these are petals and sepals.

We can easily see the spiral arrangement of parts on primitive flowers, resembling the spiral arrangement of scales on a gymnosperm cone. But an evolutionary trend in flowers is compression along the longitudinal axis, so on most flowers the spiral arrangement is not obvious, and the parts seem arranged on the receptacle in whorls (pronounced, and with the same sense, as “whirl”).

Whorls of parts are always in the order shown, although sometimes this is not apparent.

The carpel is a megasporophyll rolled to form a tube. Inside the ovary of a carpel, megaspores become ovules, which later may become seeds. A carpel can be a distinct unit, or several can be fused. (The term “pistil” refers to the pestle-shaped female structure, whether a distinct carpel, or fused carpels. “Carpel” is a more precise term.) Several fused ovaries form a compound ovary. (Sometimes “ovary” refers to a compound ovary.)

The stamen is a microsporophyll, where microspores become pollen.

A pedicel is the stem below the receptacle of a single flower. A peduncle is the stem below a cluster of flowers.

The whorl of sepals is the calyx.

The whorl of petals is the corolla.

The whorl of stamens is the androecium (andros, man, oecos, house).

The whorl of carpels is the gynoecium (gyne, woman, oecos, house).

The calyx and corolla form the perianth (peri, around, anthos, flower).

We refer to number of parts by the number followed by the suffix -merous. For example, flower parts of an iris are 3-merous. If parts are many, we say “many,” or “numerous.”

Fusion of parts

Sepals, petals and stamens in this case are adnate, forming a hypanthium. The ovary is still superior, but the insertion of the perianth and stamens is called perigynous (peri = around).

Some flowers (for example, an apple blossom) have this arrangement:

The adnate perianth and stamens in this case are also adnate to the ovary. We say this flower has an inferior ovary (inferior meaning “below,” not a value judgement). The insertion of the perianth and stamens is epigynous (epi = above).

The (apparent) position of the ovary is an easy identification character. For example, both the mustard family and the evening-primrose family have flowers with four petals, and often, similar long thin ovaries, but are easily told apart by the superior ovary of the mustards and the inferior ovary of the evening-primroses.

Clusters of flowers

Some plants have flower clusters. A flower cluster is an inflorescence. Here are some inflorescences:

A rachis is the central axis of a spike, raceme or panicle. On a spike, the flowers have no pedicel – the flowers are sessile on the rachis. Sessile means “attached without a stalk.” (We also use rachis and sessile in reference to leaves.)

In many species, the flowers of an inflorescence bloom from the bottom up. We call this an indeterminate blooming pattern. New flowers form at the top – theoretically, this could continue indefinitely.

In some species, the flowers of an inflorescence bloom from the top down. We call this pattern determinate. Such inflorescences are cymes (pronounced “simes”).

Other terms

A leafless flowering stem is a scape.

On some flowers, sepals and petals look alike. In such a case we can call the unit of a perianth a tepal. This is a term of convenience: the flower has sepals and petals (look closely at the insertion of the perianth on a lily).

A structure with color and texture resembling petals we can call petaloid. A structure with the green, leaf-like appearance of a sepal we can call sepaloid.

In addition to normal stamens busy making pollen in their anthers, some plants have sterile, anther-less stamens. A sterile stamen is called a staminode (or staminodium). Staminodes have different functions in different species; in some cases the function is unknown. Some plants have petaloid staminodes.

Flowers that are animal pollinated may have nectaries. Nectaries are glands that make nectar, the sugar syrup plants make to attract pollinators. The origin and position of nectaries varies from species to species.

A hollow projection of a floral organ is called a spur. For example, columbine petals have spurs that contain nectar.

Some flowers have modified leaves just below the receptacle. These are bracts. Developmentally, they are not part of the flower, but may be closely associated with the flower, sometimes so closely that they look like sepals. Some bracts are petaloid, taking on the function of petals (as in poinsettias, for example). A whorl of bracts is an involucre.

Some plants have flowers so tightly packed into heads that each head looks like one flower. This is a notable feature of Asteraceae. Special terms describe flowers and heads of this family.

Flowering plants evolved to use insects to carry pollen, but some species reverted to throwing pollen into the wind. Wind pollinated plants do not need showy petals. The flowers of grasses are an example. Special terms describe the inflorescences of grasses.

Trends in Flower Evolution

We sometimes describe plant features as primitive or advanced. This is not a value judgment – advanced does not mean better or more complex. The terms refer to sequence in time.

single terminal flower

single axillary flower

pedicel

peduncle

spike

raceme

panicle

head

compound umbel

umbel

rachis

Annuals, Biennials, and Perennials

An annual plant flowers and sets seed once in its life, then dies.

A biennial plant extends this pattern over two growing seasons, growing but not flowering during one growing season, flowering and setting seed the next, and then dying. Monocarpic is a more general term for a plant that lives for more than a year, but flowers only once and then dies. Some monocarpic plants live for decades before flowering and dying.

A perennial plant lives to flower more than once. Perennials of some species may live for a few years, and others may live for centuries.

The concept of annual and perennial is easy to understand, but this character of a plant can be hard to determine in the field. Annuals do not plan for the future: they usually will not have extensive roots, or food storage organs. Sometimes we can recognize a perennial by the dried remains of last year’s flower still attached to a currently flowering plant.

Gardeners use “annual” loosely, sometimes in reference to perennials that are not winter hardy. When gardeners say “perennial,” they usually mean herbaceous perennials, but woody plants are perennials too.