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The outer envelope of a flower, composed of sepals.

Epigyny is the floral arrangement in which the perianth and stamens are inserted above an inferior ovary. It describes the attachment of floral structures relative to the ovary, not whether a flower points upward or downward.^[ref]

A useful observation is a lengthwise section through a flower. Trace the bases of the sepals, petals and stamens toward the ovary. In an epigynous arrangement, their attachment lies above it. Turning the specimen upside down does not change these connections, so it does not change epigyny.

The distinction becomes important when the hypanthium forms a cup. A cup surrounding an ovary does not alone establish epigyny: a free superior ovary can also stand inside such a cup. One must inspect attachment and fusion, rather than classify the flower from its outline alone.^[ref]

Floral organ identity is the developmental identity of a floral structure, such as a sepal, petal, stamen or carpel. It concerns what kind of organ develops, not merely its position or final size. The ABC model of flower development describes identities through combinations of regulatory functions in overlapping domains.

Genetic perturbation tests these assignments. Loss of the relevant SEPALLATA functions in Arabidopsis thaliana disrupted the normal B- and C-function specification of floral organs, showing that the simple letter combinations require additional molecular partners. ^[ref] An organ appearing in another position and an organ changing its developmental identity are different outcomes. Organ shape alone also need not reveal which regulatory interaction changed.

A flower is an angiosperm's reproductive structure. Stamens produce pollen; carpels enclose ovules. Petals and sepals surround these organs in a complete flower, but some flowers lack one or more groups, including either reproductive group.^[ref]

Read the drawing from outside inward, then trace two routes: where pollen is released, and where an ovule is enclosed. This reveals why catching pollen and forming a seed require different structures. A detached petal cannot substitute for a stigma simply because both belong to the same flower.

A hypanthium is a cup-like or tubular part of a flower whose rim bears the stamens and perianth, including sepals and petals when both are present. It forms around the region occupied by the ovary. The shape of the cup and its relationship to the ovary are separate features: a hypanthium need not be fused to the ovary along its sides.^[ref]

The apple provides a familiar consequence of this arrangement. During development of this pome, tissues around the ovary enlarge into much of the edible flesh, while the ovary contributes the central core. A longitudinal slice therefore reveals more than the wall of a swollen ovary: it contains tissues derived from different parts of the original flower. This distinction matters when asking which tissues undergo cell division and which increase in cell size during fruit growth.^[ref]

An inferior ovary is an ovary lying below the level where the perianth and stamens attach, with the surrounding floral tissue fused to it apart from, potentially, a free summit. Epigyny describes the corresponding position of the other floral parts above the ovary. “Inferior” specifies this anatomical relationship, rather than the direction in which a flower happens to point.^[ref]

To identify the arrangement, inspect a lengthwise section through a flower and trace the sepals, petals and stamens to their attachment around the ovary. Merely seeing a cup-shaped hypanthium is insufficient: a superior ovary can stand freely inside such a cup. The attachment and fusion must be checked together.^[ref]

An apple develops as a pome from an inferior ovary; its mature core and surrounding flesh retain the consequences of that floral construction.^[ref]

The following definitions use current terminology in plant morphology; the examples illustrate the structures named in the historical paper.

A rhizome is a persistent plant stem, usually growing horizontally at or beneath the ground surface. Like other plant stems, a rhizome has stem nodes separated by internodes, with buds and often reduced, scale-like leaves. It can produce upward-growing shoots and adventitious roots. These features identify it as a plant stem, even though it is underground: a root does not bear leaves at stem nodes. The fleshy, branched part of ginger used as a spice is a rhizome, rather than a root.^[ref][ref]

A spathe is an enlarged, sheathing bract that encloses, at least initially, a flower or inflorescence. A bract is a modified leaf associated with a flower or inflorescence. A familiar example is the pale green hood surrounding the spadix in Arum maculatum, or cuckoo-pint. The spadix is the thickened axis bearing the small flowers; the surrounding hood is the spathe. The spathe therefore belongs to the covering of the inflorescence, rather than being a large petal of one flower.^[ref][ref]

The perianth is the floral envelope: the calyx and corolla considered together, or whichever of these is present. The calyx comprises the sepals, and the corolla comprises the petals. When the two sets look alike and are not differentiated into a recognizable calyx and corolla, their individual members are called tepals. A lily provides an example: its six conspicuous tepals form the perianth. The word is especially useful for such an undifferentiated envelope, but its collective meaning also includes a distinct calyx and corolla. The stamens and carpels are separate reproductive organs and are excluded from the perianth.^[ref][ref]

An involucre is a ring, or several rings, of bracts surrounding a flower or inflorescence. In the common daisy, the small green bracts underneath and around the base of the flower head together form its involucre. That apparent single blossom is actually a flower head containing many small flowers, called florets. Its involucre surrounds this entire cluster; it is distinct from the calyx belonging to an individual flower.^[ref][ref]

A sepal is one of the parts making up the calyx, the outer floral envelope. Sepals are commonly green and protect the other parts of a flower while it is in bud. They may remain separate or be joined into a calyx with free lobes or teeth. In a typical buttercup, the five outer sepals surround the inner, usually yellow petals. Thus one part is a sepal, the whole outer set is the calyx, and the calyx plus corolla is the perianth.^[ref][ref]

An umbel is an inflorescence in which the flower stalks arise together from one point at the top of a common stalk. The stalk of each individual flower is a pedicel; the common supporting stalk is the peduncle. The pedicels spread out like umbrella ribs and are often of roughly equal length, producing a rounded or relatively level cluster. A flowering onion is an example of a simple umbel, with individual flowers at the ends of its pedicels. In a compound umbel, the primary branches instead end in smaller umbels, called umbellets, each carrying its own flowers. The defining feature is where the stalks join: a rounded outline alone does not establish that a cluster is an umbel.^{[ref][ref][ref]}

The floral structures surrounding the reproductive organs, often differentiated into sepals and petals. Protection and pollinator attraction can be combined in a shared display.

A taxonomic genus of angiosperms, typically with five sepals and five or more petals.^[ref]

plantnet.rbgsyd.nsw.gov.au/cgi-bin/NSWfl.pl?page=nswfl&lvl=gn&name=Ranunculus. Institutional description of Ranunculus, including sepals and petals.

A receptacle is the end of the floral plant stem on which the parts of a flower are borne. It can be a small platform or an enlarged structure. Its position explains the distinction from an ovary: the receptacle supports the floral organs, while an ovary encloses ovules within a carpel.^[ref]

The strawberry makes the distinction visible. Its enlarged fleshy receptacle carries many small achenes on the outside; each achene is a dry fruit containing a seed. Thus the red flesh and the individual seed-bearing fruits develop from different parts of the flower. In a rose, the small dry fruits instead lie inside the enlarged hypanthium of the rose hip.^[ref]

For a simple dissection, follow a flower's stalk upward and identify where sepals, petals, stamens and carpels attach. Comparing this arrangement before and after fruit development helps trace a mature structure back to its origin. A receptacle can contribute to an accessory fruit, but it is not itself a synonym for every fleshy fruit.

A part of the perianth when its members are not differentiated into sepals and petals.