Illustration: Hill 1996, fig. 17. Historical notes: [C. media forma inermis Miq., Arch. Néerl. Sci. Exact. Nat. 3(5): 235 (1868), in syn. (although Miquel refers to previous publications of forma inermis (Miquel 1842: 7, 17; 1847: 412), this combination was not actually published in these papers, he merely discussed the unusual thornless specimen).] Distinguishing features: distinguished by the glossy green mature new growth and the discolorous leaflets with the midrib equally prominent above and below. This species also has the smallest overall stature of all Australian taxa. Originally distinguished on the basis of the thornless petiole, but petioles are frequently at least partly spinose, and wholly unarmed only on about 10--15% of individuals in a population. This contrasts with C. conferta, which has unarmed petioles in about 90% of individuals in a population. The holotype specimen does not have a complete petiole, but Armstrong 379 does. The petiole of the latter is unarmed, and about 35 cm long, which is outside the usual range for C. armstrongii. The leaflets, however, are well-spaced and match C. armstrongii rather than C. conferta (the spacing is 10--12 mm, whereas that of C. conferta is usually less than 8 mm). Distribution and habitat: an abundant species, in dense and extensive populations on sand over Tertiary laterites in Eucalyptus miniata - E. tetrodonta forests around Darwin. It extends from the Adelaide River west to the Finniss River and south to the township of Adelaide River, with sporadic occurrences farther south almost to Hayes Creek and in a limited area on Melville Island. Intergrading populations with C. conferta occur at Mt Bundey and near Hayes Creek, and apparent hybrids are known with C. maconochiei subsp. maconochiei. This species is facultatively deciduous late in the dry season of the monsoonal climatic cycle experienced in its natural habitat. Plants in cultivation or in locally wetter sites can retain leaves for longer, but become dormant, and it appears that the strong seasonal stress is required to rejuvenate plants. Regular (almost annual) dry-season fires experienced over much of the range also promote leaf-drop, and fertile plants are frequently seen with reproductive structure only, and no leaves. The regular fires can also kill the above-ground parts of shorter plants, promoting below-ground branching from an often massive swollen subterranean caudex. Typification: The type was cited as `In Nova Hollandia boreali ad portum Essington legit Armstrong n. 380 herb. Hookeri.' John Armstrong was a collector for Kew Gardens appointed to establish a government garden at Port Essington in 1838. He then collected in Timor in 1839--45. His collecting ranged beyond the immediate environment of the Port Essington settlement, a number of the plants he collected not being known from that site (including Cycas spp.). C. armstrongii has a limited natural distribution, occurring only in the Darwin district, and the type must have originated from there, considerably to the west of the Port Essington settlement site. Three sheets are present in K, one labelled `No. 379/ Coll. in April 1839 at Port Essington', another labelled `Port Essington/ Australia/ Armstrong', and the third labelled `Port Essington/ (bis)'. The U specimen consists of two leaflets only, but with the rachis cut to match the second of the above K sheets. It appears that the second and third K sheets and the U sheet are all parts of Armstrong 380, and consequently parts of the holotype. Bentham (1873) included C. armstrongii in C. media, and his treatment was followed until Maconochie (1980) recognised the distinctive nature of C. armstrongii, although not on the basis of the unarmed petiole as Miquel had done originally (see below). Although Miquel in the protologue referred to previous publications of forma inermis (Miquel 1842: 7, 17; 1847: 412), this combination was not actually published in these papers, he merely discussed the unusual thornless specimen. This is also an indication that Miquel had seen more than just the U specimen, which has no petiole. Conservation status: locally extremely abundant, not considered to be at risk. Although not well reserved, the extreme abundance of this species would buffer it from any threat in the medium term. However, frequent fire effectively blocks reproduction, and uncontrolled development progressively alienates significant proportions of the habitat. Several of the Northern Territory species, including this one, are probably the most abundant of all cycads, with populations numbering into tens of millions.
Description:
Stems arborescent, to 3(-6) m tall, 5-11 cm diam. at narrowest point. Leaves bright green, semiglossy to highly glossy, 55-90 cm long, slightly keeled to flat (not keeled) in section (opposing leaflets inserted at 130-160° on rachis), with 100-220 leaflets, with white and orange tomentum shedding as leaf expands; rachis usually terminated by a spine. Petiole 10-25(-35) cm long, glabrous, spinescent (usually), for 20-60% of length. Basal leaflets not gradually reducing to spines. Median leaflets simple, weakly discolorous, 55-140 mm long, 4.5-8 mm wide, inserted at 70-90° to rachis, decurrent for 3-5 mm, narrowed to 3.5-5 mm at base (to 50-85% of maximum width), 6-14 mm apart on rachis; median leaflets section flat; margins flat; apex aristate, not spinescent; midrib raised above, raised below. Cataphylls linear, pungent, pilose, persistent. Pollen cones ovoid, orange, 11-20 cm long, 7.5-10 cm diam.; microsporophyll lamina firm, not dorsiventrally thickened, 25-35 mm long, 14-17 mm wide, fertile zone 24-28 mm long, sterile apex 4-6 mm long, level, apical spine prominent, sharply upturned, 7-10 mm long. Megasporophylls 13-22 cm long, grey-tomentose and brown-tomentose; ovules 2-4, glabrous; lamina lanceolate, 30-70 mm long, 18-35 mm wide, regularly dentate, with 20-28 pungent lateral spines 1-4 mm long, 1-2 mm wide, apical spine distinct from lateral spines, 10-25 mm long. Seeds flattened-ovoid, 34-37 mm long, 32-36 mm wide; sarcotesta orange-brown, not pruinose, 3-4 mm thick; fibrous layer absent; sclerotesta smooth. Spongy endocarp absent.
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