Taxonomy of Flowering Plants - LECTURE NOTES
Hugh D. Wilson

Reproductive Morphology II

Review archaic floral 'ground plan' (modified shoot) and modern derivative

Types of Flowers - ovary position(overview)

The flowering plants are characterized by increased protection whereas the Ferns (free-living gametophytes, mostly) and Gymnsoperms (exposed ovules) show various levels of exposure.  Endosporic (development within the micro- and megaspore) gametophyte development, development of the embryo within the megagametophyte, and enclosure of these within sporophytic tissue (anther, ovary) reflects a 'protective' trend in flowering plant evolution that has extended one step further - to the ovary.  The most highly evolved angiosperms show an additional protective layer that is positioned beyond the ovary wall (pericarp) and, in most cases, this is manifested by a change, relative to the other three floral whorls, in ovary position.  Since relative position of the four floral whorls - with the gynoecium the uppermost element - is a fundamental feature of the flower, a change or deviation from this basic pattern is unusual and significant.  Thus, ovary position is a basic key characteristic that is often encountered in the initial couplets of keys to flowering plant families.

Terms denoting ovary position relative to the androperianth:

Hypogynous - the basic configuration - gynoecium the uppermost element of the flower and subtended by the androperianth, i.e., the calyx, corolla, and calyx appear to be attached to the receptacle BENEATH (hypo-) the gynoecium and the ovary is superior to the androperianth.  While connation can be involved, no adnation is evident.  This is the 'primitive' and most common condition. (photo, photo)

Perigynous - the androperianth appears to be attached AROUND the gynoecium, as opposed to beneath.  This is due to the presence of a 'floral cup' or hypanthium which emerges from the receptacle at the base of the ovary as a single unit and, along its margin, the androperianth parts.  While the hypanthium can represent an extension of the receptacle or other structural modifications, it is usually the product of basal adnation or fusion among whorls of the androperianth.  The ovary remains in a superior position relative to the ultimate point of androperianth attachment.  The free hypanthium, however, stands as a significant key characteristic and also as a structural 'pointer' to the next step in floral specialization via adnation, the inferior ovary. (Gentianaceae: Sabatia campestris - photo-top, photo-side)

Epigynous - the androperianth parts appear to be emerging from a position ABOVE the gynoecium, i.e., an expansion is present between the pedicel and the calyx and dissection of this expanded area reveals ovules (at anthesis or flower opening) or seeds (at maturity).  The ovary is structurally inferior to the point of attachment of the androperianth parts. (photo, photo, photo, photo)

Types of Flowers - Reduction/Sexual Expression

Again, the fundamental, archaic structural pattern for the basic angiosperm reproductive structure - the flower - is the complete set of four floral whorls, the upper two (gynoecium, androecium) with primary reproductive function and the lower two (corolla, calyx) with other, often supportive, functions.  The term 'incomplete' refers to the absence of one or more of this floral whorls.  The following terms relate to this type of variation:

Uniseriate perianth - only a single perianth whorl, usually the calyx which, when alone, can take a 'petaloid' or petal-like form
Perfect - both reproductive whorls present with the flower - in essence - bisexual (the flower, as sporophytic tissue, is - in reality - asexual)
Imperfect - one of the two reproductive whorls is missing, with the flower unisexual in terms of reproductive function
    Staminate - gynoecium lacking (bull nettle - photo)
    Pistillate - androecium lacking  (bull nettle - photo)
Monoecious - applied to taxa (not flowers) that feature production of both staminate and pistillate flowers on individual plants.  The flowers are imperfect but, in terms of sexual function, the individual plant is bisexual.(Indian Corn [Zea mays] - photo) and Cucumber [Cucumis sativus] - staminate/pistillate)
Dioecious - applied - again to taxa - that comprise populations of plants that produce onlystaminate or pistillate flowers on a given individual.  In this case, the individual plants are unisexual with regard to expression and, as a result, populations of dioecious taxa include both 'male' and 'female' individuals.

Types of Flowers - Symmetry

Archaic, shoot-like flowers are, like any shoot, radially symmetrical when viewed from above.  The floral appendages, spirally arranged along the floral axis, form an outline - in 'face' view - that can be cut along any axis to form two mirror images.  A flower that shows this radial symmetry is known as actinomorphic or 'regular'.

Actinomorphic (photo, photo, photo)
Floral specialization has included adaptive structural change to accommodate specific pollen vectors.  This is sometimes manifested by a departure from the 'primitive' expression of radial symmetry to produce landing platforms and other structural 'adjustments' for the pollen vector.  Flowers that lack radial symmetry, on 'face' view, are known as zygomorphic or 'irregular'.  They usually show bilateral symmetry, i.e., mirror images can be produced from a cut along a single axis.
Zygomorphic (photo, photo, photo)

The Floral Product - fruit types

As the 'new' sporophyte (embryo) matures within the megagametophyte the outer layers of the ovule (integuments) also harden and mature to form the seed coat or testa.  The flowering plants share this characteristic - the seed - with the gymnosperms.  Inclusion of seeds within the 'vessel' of the ovary is a feature unique to the angiosperms.  The mature ovary is known as a fruit. Basic fruit types are defined below:

Dry, indehiscent (pericarp remains intact), and single-seeded:

    achene - seed not adnate to the pericarp (example - Sunflower [Helianthus, Asteraceae])
        samara - a winged achene (example - Wafer Ash [Ptelea, Rutaceae)
    caryopsis - seed adnate to the pericarp (fruits produced by members of the grass family - Poaceae)
    nut - pericarp lignified (example - Oak [Quercus, Fagaceae])

Dry, dehiscent (pericarp dehisces or opens), and many-seeded:

    Formed from a single carpel or simple pistil (marginal placentation, one locule)
        follicle - dehiscent via only a single suture (example - Milkweed [Asclepias, Asclepiadaceae])
        legume - dehiscent along two sutures (example - Baptisia, Fabaceae)

     Formed from a multicarpellate, syncarpous gynoecium or compound pistil

        capsule - (overview) grouped by type of dehiscence ('suture' = line of weakened cells that opens)
         loculicidal - suture opens into the locule - (example - Mexican Buckeye [Ungnadia, Sapindaceae])
         septicidal - suture opens along the septum
         poricidal - seeds released via pores in the pericarp, no 'line' of dehiscence (example - Poppy [Papaver, Papaveraceae)

Fleshy (not dry at maturity), indehiscent

    berry -  multiseeded and derived from a compound pistil - a fleshy capsule (example - Tomato [Lycoperiscon, Solanaceae])
    drupe - single seed (usually) with pericarp differentiated into 3 layers (exocarp, mesocarp, and endocarp) with the innermost enclosing the seed to form a pyrene (stone, pit) - overview

Weird 'fruits' or 'fruiting structures'

If we define the term 'fruit' as "a mature ovary" then the structures listed below are not technically fruits.  They represent, however, true fruits as part of various 'packages' that represent units of dispersal.

Pome - product of an epigynous flower that features an 'overdeveloped' hypanthium that expands during development (overview, photo)

Aggregate 'fruit' - a cluster of fruits from the same (apocarpous) flower remaining on the receptacle at maturity (overview, photo, photo)

Multiple 'fruit' - gynoecia of separate flowers combined for form a single fruiting structure (photo, photo)

Inflorescence types

Primitive angiosperms often produce solitary flowers and the 'derived' condition is to aggregate or cluster the flowers into various configurations.  The most highly specialized and largest dicot family, the Asteraceae, has carried this trend full cycle in that the inflorescence resembles a solitary flower.  Some the more common inflorescence types are:

Determinate or Cymose

    Cyme - tending toward a 'flat topped' aspect because the uppermost flower opens first with anthesis (flower opening) moving toward the base - relatively rare (overview)

Indeterminate or Racemose (overview)

    Raceme - perhaps the most common type of inflorescence in that the developmental pattern is shoot-like with the flowers closest to the terminal meristem the youngest and anthesis moving from the base toward the tip.

    Spike - a raceme with the flowers sessile

    Corymb - a raceme with the lower pedicels longer and the upper pedicels shorter to produce a flat-topped aspect

    Panicle - a compound raceme

    Umbel - all pedicels about the same length and attached to the same point at the top of the peduncle (stalk of the inflorescence)

    Catkin or Ament - a spike or raceme with apetalous, unisexual flowers, often falling as a unit (photos)

    Capitulum or Head - a dense cluster of sessile flowers, can be envisioned as a flattened spike with the rachis expanding to a disk that holds the small flowers - inflorescence characteristic of the largest dicot family, the Asteraceae (Sunflower Family) - photos: Sunflower (Helianthus, Asteraceae)

Inflorescences often include foliar structures, often subtending each flower, that are different from the normal leaves with regard to size and shape.  These are bracts and, as a group, they are referred to as the involucre or involucral bracts.

The Androecium

The basic unit of the androecium, the stamen, represents a specialized reproductive leaf or microsporophyll.  Specialization has progressed toward reduction which has resulted in a relatively simple composition of stalk (filament), sporangia (anther), and tissue the lies between the cells or locules of the anther, the connective. (overview)

The anther of a typical stamen is attached at its base to the filament (basifixed) and dehiscence (opening to release pollen) is along its length (longitudinal dehiscence).  However, attachment to the filament can be at the center of the anther (versatile) and release of pollen can be through pores (poricidal dehiscence). (overview)

Stamens can be connate with all filaments fused to form a single, compound structure (monadelphous) or partial joining of the filaments to form two androecial structures (diadelphous).  Connation can also be limited to the anthers (synantherous), a feature that is common in the largest dicot family - the Asteraceae.  The stamens can also be adnate to the corolla (epipetalous) and present in two pairs of differing filament lengths (didynamous) or as a set of six with two shorter (tetradynamous) and either extend beyond the corolla (exserted) or not protrude from the corolla (included) - overview.

The Perianth

Overall symmetry of the flower, actinomorphic vs. zygomorphic, is defined by the perianth with the reproductive whorls conforming to the pattern.  The primitive or archaic type of flower, with numerous, separate perianth parts, is often described by the terms polypetalous or polysepalous whereas connation (and often reduction in number of parts) is denoted by the prefix 'syn-' (united); synsepalous and sympetalous ('m' must precede a 'p').  While the perianth can lack the corolla (apetalous), much of the terminology associated with the perianth deals with shapes of sympetalous corollas. (overview)

The sympetalous corolla can be viewed as two functional elements, an area of basal fusion (tube) and a terminal, flared or expanded portion (limb) that often produces corolla lobes that correspond to the number of connate petals.  General 'shape' terms include:

Actinomorphic corolla types:

Rotate - wheel-shaped with a short tube and large limb
Campanulate - bell-shaped with an extended, flaring tube
Funnelform - funnel-shaped with a continuously expanding tube and little flaring
Tubular - an elongated tube with minimal limb
Salverform - an elongated tube with a conspicuous limb
Urceolate - an inflated tube with a terminal constriction - urn-shaped

Zygomorphic corolla types:

Bilabiate - two-lipped, usually due to a 'landing platform' formed by basal lobes
Ligulate - petals connate at the margins to form a strap shaped corolla
Geniculate - with a knee-like 'bend'
Spurred - with an extension or 'spur' that often carries nectaries (secretory structures)
Papilionaceous - like a butterfly with a central 'standard' petal and lateral 'wing' petals

Describing the flower

We will be using a 'shorthand' descriptive notation to define the various patterns of floral variation to be encountered during our survey of the angiosperms.  See this overview to become acquainted with this 'floral formula' notation. (see also the 'virtual floral formula')

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  10 Sep 2010