ARUM MACULATUM

Species

Sep 1

Written By

PROTOLOGUE: Arum maculatum , published in Species Plantarum 2: 966 (1753). The protologue is reproduced below.

SYNONYMS:

HOMOTYPIC SYNONYMS: Arisarum maculatum (L.) Raf.; Arum vernale Salisb.

HETEROTYPIC SYNONYMS: Arum alpinum subsp. pyrenaeum (Dufour) Nyman; Arum byzantinum Blume; Arum heldreichii Orph. ex Boiss.; Arum immaculatum (Rchb.) Rchb.; Arum italicum subsp. byzantinum (Blume) Nyman; Arum italicum var. amoenum Engl.; Arum italicum var. byzantinum (Blume) Engl.; Arum maculatum f. flavescens Melzer ex Riedl; Arum maculatum f. flavescens Riedl; Arum maculatum f. immaculatum (Mutel) Topa; Arum maculatum f. roseum (Grembl. ex Engl.) Riedl; Arum maculatum f. scolopendriforme Priszter ex Horvat; Arum maculatum f. spathulatum Terpó; Arum maculatum f. tetrelii (Corb.) Terpó; Arum maculatum subsp. pyrenaeum (Dufour) Nyman; Arum maculatum subvar. immaculatum Mutel; Arum maculatum subvar. malyi (Schott) Engl.; Arum maculatum subvar. pyrenaeum (Dufour) Engl.; Arum maculatum subvar. roseum Grembl. ex Engl.; Arum maculatum var. flavescens Melzer ex Janch.; Arum maculatum var. heldreichii (Orph. ex Boiss.) Nyman; Arum maculatum var. immaculatum Rchb.; Arum maculatum var. karpatii Terpó; Arum maculatum var. malyi (Schott) Nyman; Arum maculatum var. pyrenaeum (Dufour) P.Fourn.; Arum maculatum var. tetrelii Corb.; Arum maculatum var. tetrellii Corb.; Arum maculatum var. vulgare (Lam.) Engl.; Arum maculatum var. zeleborii (Schott) Nyman; Arum malyi Schott; Arum orientale subsp. amoenum (Engl.) R.R.Mill; Arum pyrenaeum Dufour; Arum trapezuntinum Schott ex Engl.; Arum vulgare Lam.; Arum vulgare var. albovenosum DC.; Arum vulgare var. immaculatum Gray; Arum zeleborii Schott

DISTRIBUTION: Europe to N. Türkiye and W. Caucasus: Albania, Armenia, Austria, Azerbaijan, Belgium, Bosnia and Herzegovina, Bulgaria, Croatia, Czechia, France, Georgia-GE, Germany, Great Britain, Greece, Hungary, Italy, Kosovo, Kriti, Krym, Montenegro, Netherlands, North Macedonia, Northern Ireland, Poland, Republic of Ireland, Romania, Serbia, Slovakia, Slovenia, Spain, Sweden, Switzerland, Türkiye, Türkiye-in-Europe, Ukraine.

DISTRIBUTION NOTE: iNaturalist: Baden-Württemberg, Bavaria, Bern, Clare, England, Hainaut, Hesse, Ille-et-Vilaine, Limburg, Liège, Lower Saxony, Namur, Neuchâtel, North Rhine-Westphalia, Rhineland-Palatinate, Saarland, Saxony, Schleswig-Holstein, Scotland, Seine-Maritime, Vaud, Wales, Zürich.

SPECIES DESCRIPTION:

Tuberous herb sprouting in late winter to early spring from a rhizomatous tuber 3–6 cm long, 2–2.5 cm thick. Petiole terete to D-shaped in cross-section, 9–53 cm long, 6–11 mm wide, mid-green to deep green. Leaf-blade sagittate-hastate to oblong-hastate or oblong-lanceolate-hastate, apex acute to obtuse, 7–27 cm long, 3.5–19 cm wide, mid-green to deep green, often lightly to heavily marked with jagged, purple-black spots and patches, these often bullate.

INFLORESCENCE:

Inflorescence smelling moderately of sour urine. Peduncle much shorter than to equalling the leaves, terete, 4–26 cm long, 6–8 mm wide, mid-green to deep green. Spathe 6.5–27 cm long, spathe-tube ovoid-oblong, 1.5–5 cm long, 1.2.5 cm wide, moderately to rather strongly constricted apically, exterior pale to mid-green, interior greenish white, usually with a c. 5 mm wide purple band around the middle, more rarely the tube apex may be suffused with purple within; spathe-limb elliptic-ovate to lanceolate-elliptic, or narrowly lanceolate, 5–22 cm long, 3–12 cm wide, erect, acuminate to acute, the tip tending to droop as the inflorescence ages, externally pale to mid-green, the margins occasionally suffused with purple, rarely the whole exterior mid-purple, internally greenish white to yellowish green, often similarly spotted to the foliage, more rarely pale yellow or flushed with pink to mid-purple, very rarely wholly mid-purple. Spadix ⅓–½ as long as the spathe-limb, 4–14 cm in total length; appendix slender-clavate to subcylindric, rarely broadly clavate or conical, long-stipitate, 2.5–9 cm long, 2.5–9 mm wide, deep purple to pale yellow, more rarely dull brown, stipe usually darker. Staminodes in 5 or 6 whorls forming a zone 3–9 mm long; bristles filiform, flexuous, 5–6 mm long, purple to violet; bases conic, weakly verrucate, pale yellow. Interstices: upper 0–3 mm long with some rudimentary staminode bases, pale cream; lower similar. Staminate flowers in an oblong to quadrate zone 1.5–7 mm long, 2.7–5 mm wide, anthers and connectives dark to mid-purple. Pistillodes in 1–3 whorls forming a zone 2.5–7 mm long; bristles filiform to aristate, flexuous, 2.5–3 mm long, purple to violet; bases bulbiform to compressed-triangular, ± smooth, pale yellow. Pistillate flowers in a cylindric to globose-cylindric cluster 8–21 mm long; ovaries ellipsoid-oblong, 2–3.5 mm long, cream to very pale greenish yellow, purple apically, stigma white. Fruiting spike cylindric to oblong-cylindric, 2.5–5 cm long, 1.5–2 cm wide; berries oblong-ovoid, 4–11 mm long, 3.5–5 mm wide.

ECOLOGY:

Open deciduous woodland, forest edges, hedgerows; wasteland in ruderal habitats; altitude: 0-1300 m

DISTRIBUTION MAPS:

ETYMOLOGY:

Latin maculatus, “spotted”, for the leaf-blade, which is often lightly to heavily marked with jagged purple-black spots and patches.

NOTES:

1. Pollinated by midges of the Psychodidae, in Britain effectively by female Psychoda phalaenoides alone.¹²

2. It has the largest lacunose area of the fifteen species examined, 9162.28 ± 804.51 µm² in the upper chamber, with 69.4 ± 8.8 upper and 17.2 ± 2.3 lower elongated sterile flowers — the midge-pollinated pattern in its most developed form.¹²

3. The female phase lasts only 12 to 18 hours, and most plants open a single inflorescence in a season, so the timing of that one opening largely decides whether the plant reproduces at all.¹³

4. Across four British populations, plants flowering early or late in the season were significantly less likely to set fruit than those flowering at the peak, and peak-flowering plants set heavier seed — evidence of stabilizing selection on flowering time. The number of female flowers per inflorescence did not vary through the season.¹³

5. Only about a third of the spadix is given over to flowers: 34 percent of its length is appendix, against 12 percent female flowers and 7 percent male. Unlike Arum italicum, larger inflorescences are not biased toward the female function, so the size-advantage model does not apply — the species may simply not vary enough in size for it to.¹⁴

6. Spontaneous selfing produced no seed in any bagged inflorescence, but self pollen moved between inflorescences of the same clone performed as well as outcross pollen, and germinated on the stigma as readily. The species has no mechanism to avoid geitonogamy.⁸

STORY:

STORY TITLE: The twelve-hour window

Arum maculatum gives itself one chance a year. Most plants in a population open a single inflorescence, and its female phase — the only interval in which the stigmas can be pollinated — lasts between twelve and eighteen hours. Everything the plant will set that season is decided inside that window.

Between 1992 and 1997 Jeff Ollerton and Anita Diaz followed four British populations to ask whether the timing of that single opening mattered. It did. In three of five population-years, plants that flowered early or late in the season were significantly less likely to set fruit than plants flowering at the peak. In one population the same pattern held for the proportion of flowers that set within an inflorescence, and seed from peak-flowering plants was significantly heavier. Plants at both ends of the season were being penalized, and plants in the middle were not: stabilizing selection, acting on flowering time, observable within a handful of seasons.

The mechanism is a counting problem. The inflorescence traps female psychodid midges, holds them overnight, and releases them dusted with pollen the following morning. For a plant to set seed, it must catch a midge that has already been trapped by another inflorescence and is already carrying pollen. Early in the season, and late, very few inflorescences are open, so very few midges in the population are carrying anything. The plant that opens at the peak is fishing in a pool that its neighbors have stocked.

What makes the result more than a local observation is where the constraint comes from. The number of female flowers per inflorescence did not vary across the season, so the plants were not adjusting their investment; the pressure falls entirely on timing. And the twelve-to-eighteen-hour female phase that creates the pressure is not a feature of Arum maculatum in Britain. It is shared across the Araceae — a phylogenetically conservative trait, inherited rather than adapted, that sets the terms every year and that no population can negotiate.

REFERENCES:

1. Plants of the World Online, Royal Botanic Gardens, Kew — Arum maculatum

2. Species Plantarum 2: 966 (1753) — protologue.

3. iNaturalist: Arum maculatum

4. Boyce, P.C. 1993. The Genus Arum: a Kew Magazine Monograph. Royal Botanic Gardens, Kew.

5. Boyce, P.C. 2006. Arum – a decade of change. Aroideana 29: 132–137.

6. Laina et al. 2022. Local Insect Availability Partly Explains Geographical Differences in Floral Visitor Assemblages of Arum maculatum L. (Araceae). Frontiers in Plant Science.

7. Szenteczki et al. 2022. Transcriptomic analysis of deceptively pollinated Arum maculatum (Araceae) reveals association between terpene synthase expression in floral trap chamber and species-specific pollinator attraction. G3 Genes|Genomes|Genetics.

8. Diaz et al. 2006. The effectiveness of some mechanisms of reproductive isolation in Arum maculatum and A. italicum (Araceae). Botanical Journal of the Linnean Society.

9. Leclerc et al. 2025. Subtle ecophysiological divergences in a deceptive strategy to attract the same pollinators in two sympatric Arum. Annals of Botany.

10. Gfrerer et al. 2023. Variation in scent amount but not in composition correlates with pollinator visits within populations of deceptive Arum maculatum L. (Araceae). Frontiers in Plant Science.

11. Marotz-Clausen et al. 2024. Where are volatiles produced in the highly synorganised inflorescence of Arum maculatum L.?. Biochemical Systematics and Ecology.