Plant Classification

Plant Traits and Terminology

Plant Family Practice “Game”

As we go on field hikes in Biology Lab, you will be learning to identify many of the local plants and animals. For many of the plants we find, we will also discuss in what plant family those plants are classified. You will be expected to sight-identify these plants to family, based on knowing and recognizing key characteristics of the families to which they belong. You may also be expected to sight-identify new, unfamiliar plants to family based on their characteristics.

To a biologist, the words “plant” and “flower” have very specific and different meanings. A flower is a specialized reproductive structure which develops on plants in the Angiospermae. In most cases, it is technically incorrect to say that you have planted “flowers” in your garden (unless you buried the wilted, cut roses a friend gave you a month ago), and in general, the result of that would be compost. However, to say that you put “plants” into your garden so you could enjoy the flowers they produce is an appropriate statement. Note that both maple trees and marigolds are plants, and both produce flowers. Throughout this discussion, the word “flower” will be used to mean a flower, not a plant.

Plants are typically grouped into families based on similarities in their structures. The type(s) and organization of the flower parts are often key traits by which plant families may be determined. Thus, botanists, have come up with a scheme for representing the flower structure of the various families. Botanists represent the parts of flowers and arrangement of those parts using the following notation.

In this notation, a horizontal line is used to represent the receptacle, thereby indicating which parts are attached above or below the receptacle. The area of the stem just below the receptacle is called the pedicel. The following symbols are used to represent other flower parts.

• Ca = calyx of sepals
• Co = corolla of petals
• T = Occasionally when the sepals and petals are very similar, (like in lilies and tulips), they are collectively called tepals.
• Coz = irregular or zygomorphic corolla
• S = stamen consisting of anther and filament
• P = pistil consisting of stigma, style, and ovaries, with the terms carpels, locules, ovules, and/or placenta referring to parts of the ovary
• ∞ = many, possibly variable number of parts
• x = few, possibly variable number of parts
• = parts are united
• = parts are united below (like stamens with united filaments, but not anthers)
• = parts are united above (like stamens with united anthers, but not filaments)
• 4+4 = organs in two sets (S⁴⁺² would indicate two sets of stamens with 4 and 2 stamens in those sets)

• S

  Co

  = stamens attached to corolla, referred to as epipetalous

• S

  Ca  Co

  P

  = inferior ovary and epipetalous stamens

These symbols may be combined, as needed, to represent the structure of a given flower. For example, the formula to the right means that in this particular flower, there are five free sepals, five united petals, five free, epipetalous stamens, an inferior ovary with two united carpels, and the type of fruit is a capsule. Here, then, are descriptions of some of the main families of plants.

Selected Plant Families

• Monocots:
  • Corollifera: These have tepals and often bulbs, corms, and/or rhizomes.

    Araceae

    The type of inflorescence is a spathe and spadix.

    Liliaceae

    These have six stamens, a superior ovary, and regular flowers. The three petals + three sepals look very similar and so are called “tepals.”

  • Glumiflorae: These have no petals or sepals as such, but may often these modified into scales, bristles, etc. Runners are common, as well as rhizomes in perennial species.

    Gramineae

    Each leaf is divided into a blade and a sheath. The sheath is usually open on the opposite side. The stems are cylindrical or flattened, and frequently hollow. There are usually two rows (ranks) of leaves.

    Cyperaceae

    The sheathes are closed (united on the other side). The stems are usually triangular, and leaves are three-ranked. The stems frequently contain pith and are not hollow.

• Dicots:
  • Polypetalae: These exhibit the primitive condition of having non-united petals.
    • Hypogynous: The flower parts are located under the ovary, or as a botanist would say, each flower has a superior ovary.
      • Apocarpous (apo = off, from, away from, detached; carpo = fruit): These plants have several separate, un-united pistils in each flower.

        Ranunculaceae

        These plants are generally herbaceous, and have watery juice. The sepals are sometimes absent or similar to the petals, so often all called tepals instead. Sometimes there is an indefinite number of sepals. A spiral arrangement of flower parts is characteristic.

      • Syncarpous (syn = with, together): These plants have flowers with two or more pistils united together.

        Papaveraceae

        These plants are generally herbaceous. Almost all “bleed” milky or colored juice when injured. There are two to three sepals which usually are deciduous (fall off as the flower opens). The petals are crumpled in the bud. The seed capsule often has an apical lobed disc.

        Fumariaceae (some botanists consider this a subfamily within the Papaveraceae)

        These plants are herbaceous. Some species are annuals while others are perennial. They typically bear bi- or tripinnately, finely divided leaves which are smooth and glaucous. The flowers usually have one or two sac-like petals including a nectary spur.

        Cruciferae

        Co⁴S⁶ These have flower parts in sets of four. There are four petals. The stamens are diadelphous: the flowers have six stamens composed of a set of four plus a set of two. The type of fruit is called a silicle or silique. The inflorescence is a raceme. Many of these plants are edible and usually have a characteristic taste due to the presence of organic sulfur compounds.

        Caryophyllaceae

        The leaves of these plants are opposite or whorled, and typically there are swollen nodes where the leaves join the stem. Most species are herbaceous. The petals are frequently notched (“pinked,” like pinking shears). Many have united sepals with un-united petals inside the tube formed by the sepals. Many have two sets of stamens for a total of ten. The inflorescence is a cyme.

        Violaceae

        Most of these plants are herbaceous perennials. The leaves are simple, basal or alternate, and often cordate (cordis = heart: heart-shaped). The irregular flower often have a nectary spur (petal). In the formula, the “1:3” in a circle means a one-celled, tricarpelate (tri = three) pistil (the pistil originated from three fused carpels, but there is only one, not three chambers, and the seeds are attached in three batches around the edges).

    • Perigynous: Plants in these families possess an hypanthium, an outgrowth or enlargement of the receptacle which surrounds the ovary. The ovary is actually superior, but may appear to be inferior because of the hypanthium.

      Leguminosae

      There are both herbaceous and woody species. The irregular flowers are said to be papilionaceous in shape. The fruit is a legume (some are modified). Plants bear alternate leaves with stipules. The roots are frequently deep, and many have nodules containing nitrogen-fixing bacteria). While the fruits of many species are edible, others may have poisonous beans.

      Rosaceae

      These flowers possess an hypanthium (the fruit in an apple, which has grown around the ovary). These species are mostly perennial. Some are herbaceous while others are shrubs or trees. The leaves are alternate with stipules which sometimes fall off. The five-parted, regular flowers contain numerous stamens. It can be hard to tell Rosaceae from Ranunculaceae. While the hypanthium may be hard to see in some Rosaceae, in others it may be large enough to make the ovary appear to be inferior. However, it is not present in Ranunculaceae (which should have obviously superior ovaries).

    • Epigynous: The flower parts are located above the ovary, or as a botanist would say, the ovary is inferior.

      Umbelliferae

      Most species are herbaceous. The leaves are alternate, mostly pinnately-compound, and finely-divided. The leaf petioles sheath the stem. The inflorescence is an umbel. Some have reduced umbels that resemble heads. Many have a distinctive odor or flavor, and while some are edible or used as seasonings, others are poisonous.

  • Apetalae: The corolla is absent.
    • Floriferae: The sepals may or may not be colored and look like a false corolla.

      Polygonaceae

      These are mostly herbaceous species. At the base of each leaf, specially modified stipules sheath the stem and the nodes are frequently swollen into knots. The flowers are small.

  • Sympetalae: The petals are at least partially united in some way.
    • Hypogynous: These have a superior ovary with the other flower parts below the ovary.

      Asclepiadaceae

      These perennial herbs have milky juice. The leaves are opposite or whorled. In many species, the inflorescence is an umbel, but in a few others, may be a cyme. The flowers usually have a crown or corona, and the petals are reflexed. Often, only one flower per umbel produces fruit. Many have a strong, pleasant odor. The pollen is clustered in pollinia (sing. = pollinium, masses of pollen grains), rather than as single grains.

      Oleaceae

      Co⁴S² While these flowers also have four petals, they can be distinguished from the Cruciferae by the presence of only two, epipetalous stamens and a corolla which is united at the base. Many are woody plants.

      Labiatae

      Most species are herbaceous, and have square stems, opposite (or rarely, whorled) leaves, and a minty odor. There are two carpels per pistil, and these are deeply lobed because there are four nutlets per fruit (resulting in a four-lobed ovary as indicated by the “puffy” circle in the formula). They have a bilabiate corolla and two or four stamens.

      Scrophulariaceae

      While these also have a bilabiate corolla, the stem is not square, nor is there a minty odor. Most are herbaceous perennials. The leaves are opposite, exstipulate (having no stipules), and mostly simple. There are two carpels in the pistil, and the carpels are not deeply lobed. The fruit is a capsule.

      Solanaceae

      These herbaceous plants bear five-parted, regular flowers and have alternate leaves. The fruit is a pod or berry with numerous seeds. Most have a characteristic rank odor and contain many toxic chemicals. In the genus Solanum, the stamens are touching at the tips of anthers (the anthers come together, but are not united, giving the appearance of a “bird beak”) and dehisce by pores in the tips of the anthers. Some species have a totally-united, trumpet-like corolla, while others have the corolla united only at the base, frequently with reflexed petals.

    • Epigynous: These have an inferior ovary with the other flower parts above the ovary.

      Lobeliaceae (some botanists consider this a subfamily within the Campanulaceae)

      No information on Lobeliaceae as such was given in the notes I was consulting. For Campanulaceae, the stamens generally are not epipetalous and frequently have their bases enlarged. Campanulaceae plants are herbaceous with milky juice and alternate leaves. The formula given is that for Campanulaceae.

      Compositae

      The inflorescence is a head with an involucre of bracts (specially-modified leaves) subtending it. The fruit is called an achene. The anthers are united cylindrically, and the stigmas and style grow through them. The stamens are epipetalous. The calyx is modified into a pappus for dissemination. The heads of many species contain both ray (ligulate) and disc (tubular) flowers. Disc flowers are tubular and regular, while ray or ligulate flowers have a strap-like, fused corolla. The latter are irregular flowers with five, united petals. Dandelions have all ligulate flower, while thistles have only tubular flowers, and daisies have a ring of ligulate flowers surrounding a number of disc flowers. In many species with a border of ligulate flowers surrounding a number of disc flowers, the ligulate flowers are sterile and serve the purpose of attracting pollinators. Some species also have chaff, modified bracts that may subtend each of the disk flowers.

• A couple of non-Angiosperms:

  Pinaceae

  The leaves are modified as needles. The seeds form on the surface of a cone (= naked seeds), not in an ovary. There are no flowers.

  Polypodiaceae

  There are no flowers nor seeds, and reproduction is accomplished (in part) by the release of spores from chambers called sporangia on the backs of specially-modified fertile leaves.

Some Further Terminology:

• Complete vs. Incomplete: A complete flower has all four layers of parts while an incomplete flower lacks one or more of the layers.
• Perfect vs. Imperfect: A perfect flower has both sexes while an incomplete flower lacks one or the other. If a species has imperfect flowers, but both male and female flowers are on the same plant, that species is said to be monoecious. If the male and female flowers are on separate, male and female plants, that species is said to be dioecious.
• Regular vs. Irregular: A regular flower is radially symmetrical while an irregular flower has bilateral symmetry, and is also known as a zygomorphic flower.


• An hypogynous flower has a superior ovary and no hypanthium.
• A perigynous flower has an hypanthium around the ovary. The receptacle grows out to form a disc or cup around the ovary with the calyx and corolla at its edges. Thus, while the ovary may appear to be inferior, it really is superior.
• An epigynous flower has an inferior ovary.


• If all flower parts are attached to the receptacle and are free, the flower is termed polypetalous, and this is the primitive state.
• The more usual (and advanced) case is when the petals are united in some way, in which case the flower is termed sympetalous.
• If the stamens arise from out of the petal tissue, the flower is termed epipetalous.


• The pistil may be simple or compound, depending on the number of carpels which are united to form it. “Placentation” refers to where and/or how the seeds are attached.