ARUM NIGRUM
PROTOLOGUE: Arum nigrum , published in Österreichisches Botanisches Wochenblatt 7: 213 (1857). The protologue is reproduced below.
SYNONYMS:
HOMOTYPIC SYNONYMS: Arum nigrum var. schottii Engl.; Arum orientale subsp. nigrum (Schott) K.Richt.; Arum orientale var. nigrum (Schott) Engl.
HETEROTYPIC SYNONYMS: Arum neumayeri Vis. ex Beck; Arum nigrum var. variolatum (Schott) Nyman; Arum orientale f. variolatum (Schott) Engl.; Arum variolatum Schott
DISTRIBUTION: NW. Balkan Pen., N. Greece: Bosnia and Herzegovina, Croatia, Greece, Montenegro.
SPECIES DESCRIPTION:
Tuberous herb sprouting in spring from a discoid, vertical tuber 4–6 cm across, 2–2.5 cm thick. Petioles subterete, 11–22.5 cm long, 4–6 mm wide, deep green with slight purple staining basally. Leaf-blade sagittate-hastate to broadly sagittate, apex acute, 9.5–21 cm long, 7.5–18 cm wide, glossy deep green.
INFLORESCENCE:
Inflorescence smelling strongly of horse-dung. Peduncle much shorter than the leaves, terete, 5–9 cm long, 5–10 mm wide, bright mid-green. Spathe 12.5–20.5 cm long; spathe-tube oblong-cylindric, 4–5.5 cm long, 1.5–2.5 cm wide, constricted apically, exterior green basally, this colour extending c. ⅓ the way up the mid-vein, otherwise very dark purple, the margins darker still, interior pale green basally, darkening c. ½-way up to mid-green stained deep purple; spathe-limb broadly elliptic, 9–14 cm long, 3–8.5 cm wide, somewhat cucullate, minutely acuminate, externally glossy deep purple with greenish mottling towards the apex, interior deep purple. Spadix c. ½ as long as the spathe-limb, 8.5–13 cm in total length; appendix clavate to stoutly-cylindric, long-stipitate, 5.5–9.5 cm long, 5–10 mm wide, sooty purple, tinged with purple apically. Staminodes in 2 whorls forming a zone 2–4 mm long; bristles subulate, stiff, 5–7 mm long, violet to dark purple; bases bulbiform, verrucate, purplish yellow. Interstices: upper 2.5–3 mm long, ± smooth, dull yellow; lower 0.5–1 mm long, deeply and regularly longitudinally grooved, dull yellow, stained with purple. Staminate flowers in an oblong zone 8–9 mm long, 7–8 mm wide; anthers and connectives pale purple. Pistillodes in 1 whorl forming a zone 1–1.5 mm long; bristles subulate, stiff, 5–7 mm long, deep violet; bases massively conic, verrucate, dark yellow, stained with purple. Pistillate flowers in a cylindric cluster 13–15 mm long; ovaries oblong, 2.5–3 mm long, pale yellow, upper ⅓ violet; stigma pale grey-mauve. Fruiting spike oblong-cylindric, 3.5–5 cm long, 2–2.5 cm wide; berries pyriform, 3–9 mm long, 3–4 mm wide.
ECOLOGY:
Garrigue, amongst rocks in soil pockets, rocky hillsides; altitude: 250-800 m
DISTRIBUTION MAPS:
ETYMOLOGY:
Latin niger, “black”, for the spathe-limb, glossy deep purple within and without.
NOTES:
1. Pollinated by flies and beetles, with Sphaeroceridae and Staphylinidae recorded from the chamber.¹²
2. It has the largest trapping cells in the genus: papillae 69.80 ± 8.04 µm long in the upper chamber, on bases of 1779.30 ± 600.00 µm² — both the highest of the fifteen species measured — and correspondingly few lacunae.¹²
3. It is the species on which Knoll founded the modern understanding of the Arum trap in 1926. The spathe opens during a single night; the female state lasts less than 24 hours, and the anthers open before midnight of that first night, sometimes as early as 10 pm.¹²
STORY:
STORY TITLE: Knoll's nights in the Karst
The modern understanding of how an Arum works was built on Arum nigrum, in the karst of Herzegovina and Dalmatia, by Fritz Knoll, across several seasons before 1926.
Knoll watched the whole cycle rather than dissecting the end of it. The spathe of Arum nigrum unrolls and completes its business in a single night: the female state lasts less than 24 hours, and the anthers open before midnight of that first night, sometimes as early as ten in the evening. Whatever the inflorescence is going to catch, it catches in a few hours of darkness.
His central claim was negative, and it overturned the standing account. The inflorescences of Arum, he argued, are not Reusen — not fish-traps, not funnels of inward-pointing hairs that admit an insect and physically bar its return. They are something else, for which he coined a name: Gleitfallenblumen, slide-trap flowers. The insects do not push past a barrier and find it closed behind them. They lose their footing and fall in, and they stay in because they cannot climb out.
He identified three mechanisms holding them. The epidermis of the upper chamber and the spadix is slippery, so an insect cannot walk up. The chamber is too small to let an insect take off. And the insects are positively phototactic, drawn toward the light at the top of the chamber and away from the only exit.
Nearly ninety years later, scanning electron microscopy vindicated both of Knoll's corrections in detail. The downward-pointing papillae do make insects glide into the chamber, and the ring of elongated sterile flowers does not close the entrance at all — it produces a slippery oil so that insects cannot climb it. Arum nigrum turned out to be a good choice for the work: of the fifteen species later measured, it has the longest papillae in the genus and the broadest bases beneath them.
REFERENCES:
1. Plants of the World Online, Royal Botanic Gardens, Kew — Arum nigrum
2. Österreichisches Botanisches Wochenblatt 7: 213 (1857) — protologue.
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.
10. 2006. Studies on the Genus Arum (Araceae).
11. Delpino 1874. Ulteriori osservazioni e considerazioni.