EEB337H1 - Families of Vascular Plants

Reproductive morphology


Flower construction - Fruits - the payoff


Flower construction

Angiosperm flowers are interpreted as modified shoots, consisting of an axis and appendages that may be sterile (perianth) or fertile (stamens, pistils).

Recall that a whorled arrangement of leaves involves three or more leaves attached at the same node on a stem. Floral appendages are thought of as making up as many as four whorls of appendages, attached to a compressed axis, the receptacle. The outermost whorls (perianth) are sterile, and comprise the calyx (made up of sepals) and corolla (made up of petals). When sepals and petals cannot be distinguished from each other the perianth members may be referred to as tepals.

Showy perianth members are thought to function in attracting pollinators, and may be involved in complex patterns of coevolution in order to increase the frequency and fidelity of pollen movement between flowers of the same species. In groups where pollen is dispersed by wind or water the perianth members may be inconspicuous or absent.

Perianth whorls often develop as a tube, forming tubular calyces and corollas like those illlustrated at the right.

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Stamens and pistils make up the two central whorls of the flower, known as the androecium and gynoecium, respectively. The androecium comprises the stamens in which pollen is produced. The gynoecium comprises one or more pistils, within the ovary of which fertilization occurs and seeds are matured.

Flowers like the one illustrated at the right are described as hypogynous because the other floral whorls are inserted beneath the gynoecium (the flower thus has a superior ovary).

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Epigynous flowers are ones like those illustrated at the right in which the perianth and androecium are inserted on the sides or the summit of the ovary. In this case the flower has an inferior ovary.

Because of the way in which epigynous flowers develop, an inferior ovary may be referred to as a hypanthial ovary, referring to the hypanthium illustrated below.

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When growth occurs beneath the insertion of the perianth and androecium members, the resulting tube is called a "floral tube" or hypanthium. In some cases, e.g. some Fabaceae and Rosaceae, the hypanthium may be inconspicuous but nevertheless present. As illustrated at the right, hypanthia may be associated with either superior or inferior ovaries; in the former case the flower may be described as perigynous.

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  Illustrations above by T. Winterhalt, from The ROM Field Guide to Wildflowers of Ontario, pp. 20-23. © Royal Ontario Museum 2004.


Fruits

Fruits are matured ovaries containing seeds. These two illustrations are meant to reinforce the connection between the ovary (or ovaries) of a flower and the fruit (or fruitlets) into which it matures.

Fruits are classified according to their composition and the ways in which the ovary walls mature, as described HERE.

Note in the illustration above that in the flower of cherries and plums (Prunus) the pistil is surrounded by a hypanthium, and so the flower is perigynous, as explained above.

 

In apples and pears (now segregated as the genera Malus and Pyrus), however, the hypanthium and the ovary of the pistil have developed together, to form an inferior, or hypanthial, ovary. In this case the flower is referred to as being epigynous, as explained above.

Placentation, or the arrangement of the ovules (or seeds) within the ovary is often diagnostic at the family level and readily visible in cross-sections of fruits. In Momordica cochinchinensis (Cucurbitaceae) the pointing finger indicates the attachment of a seed to the fruit wall, and hence parietal placentation. In okra, Abelmoschus esculentus (synonym Hibiscus esculentus; Malvaceae) the seeds are attached to the center of the fruit. This is axile placentation. Several other types occur, as illustrated in Fig. 4.22 of Judd et al. 2002.


Photos S. Scharf, from the IDENTIFRUIT website. © Royal Ontario Museum 1999.

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Momordica cochinchinensis showing seeds enclosed by fleshy red arils. Scale in cm. Abelmoschus esculentus (okra). Scale in cm.

the payoff

As his predecessors discovered, and as Linnaeus embedded in his system for identifying plants, plants vary most conspicuously in the construction of their reproductive structures. Flower construction has been explained briefly above, and variation in floral construction is described in greater detail in Judd et al. 2002 (pp. 63-81, 87-90, 94-95). The terminology used to described flowers and fruits is outlined elsewhere on this site. In general, the lower the level in the taxonomic hierarchy, the more similar reproductive morphology will be between members. Even at the level of family there is usually a striking uniformity among genera in the basic plan of their flower and fruit construction.

What might be some of the factors that contribute to this pattern of great uniformity of reproductive morphology within groups of plants for which there is mounting evidence of their close phylogenetic relationships to each other? Why do you suppose that many such groups make their living in the tropics as woody plants, but do so in the temperate zones as herbaceous plants?

For these reasons it is usually extremely important to ensure that herbarium specimens of unknown plants include flowers and fruits whenever possible.

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Please send your comments to tim.dickinson@utoronto.ca; last updated 17-Mar-2014