Inflorescence

inflorescence [ in-flaw-RES-uhns ] noun: 1. the flowering structure consisting of more than one flower, usually comprising distinct individual flowers 2. the way flowers are arranged or develop on an axis 3. the budding and flowering of a plant

Inflorescence Determinate Diagram — Diagram by Mary Free including some individual figures by Amada44

Inflorescence Indeterminate Diagram — Diagram by Mary Free including some individual figures by Amada44

Some plants produce solitary flowers–one flower per stem. Others produce a cluster of flowers or inflorescences. They are grouped by the arrangement of flowers on an axis and by the sequence of anthesis, or opening of the flower bud, which occurs in two forms, determinate and indeterminate. Determinate (or cymose) flowering plants have the oldest flower at the top of the main axis (rachis), meaning the terminal bud stops growing and lateral flowers are produced from the lower axillary buds. Indeterminate (or racemose) plants flower with the youngest bud at the top of the main axis.

These inflorescences can take many forms, including cymes, spikes, racemes, panicles, thyrses, catkins or aments, spadices, corymbs, umbels, and capitulums or heads. Forms can be simple, compound, or mixed (such as verticillasters).

Can you identify the types of determinate inflorescence below?

Flat-topped cymes of Viburnum sargentii ‘Onondaga’ (sargent viburnum) in April.
Photo © Mary Free

Axillary cymes of Jasminum nudiflorum (winter jasmine) in February.
Photo © Elaine Mills

Helicoid cymes of southeastern U.S. native Spigelia marilandica (Indian pink) in June.
Photo © Mary Free

Scorpiod cymes of Mertensia virginica (Virginia bluebells)

Scorpioid cymes (cincinnus) of native
Mertensia virginica (Virginia bluebells) in March.
Photo © Mary Free

The verticillasters of Salvia guaranitica 'Black and Blue'

Dichasial cymes (dichasiums) arise from a single point in nodes of
opposite leaf axils of Salvia guaranitica ‘Black and Blue’ verticillasters in July.
Photo © Mary Free

Can you identify the types of indeterminate inflorescence below?

Panicled spikes of native Verbena hastata (blue vervain) buds and flowers in July.
Photo © Mary Free

Racemes of native Actaea racemosa (common black cohosh) buds in June.
Photo © Mary Free

Umbellate panicles of native Aralia racemosa (spikenard) flowers and buds in July.
Photo © Eaine Mills

Conical panicles of Hydrangea paniculata ‘Limelight” (panicle hydrangea) flowers in July.
Photo © Eaine Mills

A thyrse of Syringa vulgaris (common lilac) buds in April. The central axis
is indeterminate and the lateral branches are determinate cymes.
Photo © Elaine Mills

A thyrse of midwestern U.S. native Penstemon digitalis fruit

A thyrse of fruit of midwestern U.S. native Penstemon digitalis ‘Husker Red’
(foxglove beard-tongue) in August. The central axis is indeterminate and
the lateral branches are determinate cymes. Photo © Elaine Mills

Catkins of native Quercus montana (chestnut oak) in April.
Photo © Mary Free

Small racemose clusters of male catkins of native Betula nigra

Small racemose clusters of male catkins bearing scaly bracts of native Betula nigra (river birch) in May.
Photo © Mary Free

A waxy, bright red, palette-shaped spathe of tropical Anthurium.
Photo © Robert M. Kline

A rounded umbel of Allium cristophii (star of Persia) blooming in May
can have up to 100 star-shaped flowers.
Photo © Christa Watters

Flat-topped, compound umbel of native Zizia aurea (golden-alexanders) in April.
Photo © Mary Free

Flattened, compound corymb of hybrid Achillea

The capitulum of central and southern U.S. native
Rudbeckia maxima (large coneflower) with a leafcutter bee in June.
Photo © Mary Free

Some taxonomy scholars, particularly of grasses, note that there is not always consistency in application of the various terms such as spike, raceme, panicle, and thyrse. For example, various botanical references either describe the inflorescence of Aesculus pavia (red buckeye) as a panicle or as a thyrse. Also, compare two plants with the Latin epithet “spicata” or spiked. An example of a determinate inflorescence in spike form is the familiar native Liatris spicata (blazing star or gayfeather) shown below with spent flowers at the top of the spike. An indeterminate plant of a similar form is annual Celosia spicata (wheat celosia), but it blooms from the bottom up with anticipation of new buds forming until frost.

Indeterminate plants include certain tomato plants and some grasses; and because the new flowers occur at the tops of stems, they can continue to produce new growth and new flowers, and at least in theory, remain visibly attractive for longer seasons.

For home gardeners, it is good to know whether the tomato plants you are growing are determinate or indeterminate, as this affects fruit production and timing. Indeterminate tomatoes will generally have a longer bearing season, while determinate crops tend to ripen around the same time.

A skipper butterfly feeds just below the spent flowers at the top of the determinate spike of native Liatris spicata — A skipper butterfly feeds below the spent flowers atop the determinate spike of *Liatris spicata* (blazingstar) in July. The buds at the spike’s bottom will open but no new buds will be produced. Photo © Mary Free

Indeterminate Celosia spicata (wheat celosia) with flowers open at the bottom of the spike in September. — A few flowers are open at the bottom of the indeterminate spike of *Celosia spicata* (wheat celosia) in September. This spike will continue to elongate adding buds at its top until frost.
Photo © Mary Free

A mason wasp feeds on the upper third of an elongated Celosia spicata (wheat celosia) spike — A mason wasp feeds on the upper third of an elongated *Celosia spicata* (wheat celosia) spike in September.
Photo © Mary Free

Moneymaker heirloom tomatoes — Because it is Indeterminate, a ‘Moneymaker’ heirloom tomato plant can continue to produce flowers and set fruit until cool weather or frost inhibits their growth. Regular harvesting increases fruiting chances later in the season making this a reliably high-yielding plant. Photo © Mary Free

References

Armstrong WP. 2001. Botany 115 Terminology: Inflorescence Terminology Part 1. Wayne’s World: 9 May 2001. (accessed April 5, 2021).

The Flower: Vocabulary-o-Rama! BIL 226 – Lecture 17. College of Art and Sciences. University of Miami. (accessed April 5, 2021).

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