Etymology:
In reference to the centre of distribution and development of
this species on Bougainville Island.
Illustration: Hill 1994c, fig. 11. Historical notes:\ Recognised as a distinct taxon in 1994. Distinguishing features: C. bougainvilleana is distinguished by the very broad, falcate leaflets with a hard, glossy cuticle, continuous laminar hypodermis and mesophyll interrupted by the broad, low midrib, the relatively short, thick, usually partly spinescent petiole, and the large, heavy, narrowly triangular megasporophyll lamina with 16-32 lateral spines that are 4-9 mm long. It is nearest to C. seemannii, differing in being generally more robust in all features, together with the possession of continuous adaxial hypodermis and a usually partly spinescent petiole. Cataphylls are also longer and more pungent. This species reaches its best development on Bougainville, being slightly less robust in the New Britain Group. This is interpreted as a result of genetic inflow from mainland New Guinea introducing a different gene complement into New Britain, possibly from C. rumphii. Similar but even more highly variable forms occur in the islands off the eastern point of the New Guinea mainland (the Trobriand, D'Entrecasteax and Louisiade groups), possibly representing a higher degree of mixing and subsequent hybrid segregation of C. bougainvilleana and C. scratchleyana. Plants from the Solomon Islands are nearest to C. bougainvilleana, but are also very highly variable, with the range also approaching C. seemannii. Similarly, plants from Vanuatu are somewhat more robust than is usual in C. seemannii, but less so than in the Solomoms. There is also a suggestion that intergradation occurs with C. rumphii sens. strict. to the west through the Admiralty group (Manus Island) and the north-east coast of mainland New Guinea. The variability in the Solomon Islands, Vanuatu and the Admiralty Islands is interpreted as a product of genetic interchange resulting from successive waves of colonisation facilitated by the aquatic dispersal mechanism. Distribution and habitat: Confined to near-coastal sites in Bougainville, New Britain and the Solomom Islands. Sporadic but locally abundant, on stabilised calcareous coral-sand dune country and nearby headlands, often in more or less closed mesophyll forest. Conservation status: Vernacular: di di (Ambungi village, N. Brit.), diduna (Tonolei village, N. Brit.), pago (Matong and Pomio villages, N. Brit.), diriuna (Suvai village, Boug.), bai bai, bae bae (Rennel and Baga Is., Solomon Is.)
Description:
Stems arborescent, to 5 m tall. Leaves bright green, highly glossy, 240-270 cm long, flat (not keeled) in section (opposing leaflets inserted at 180° on rachis), with 130-260 leaflets, tomentum shedding as leaf expands; terminated by a spine or paired leaflets; 3-11 mm long. Petiole 30-65 cm long, pubescent, unarmed or spinescent. Basal leaflets not gradually reducing to spines. Median leaflets simple, strongly discolorous, 250-340 mm long, 13-18 mm wide, inserted at 60-80° to rachis; section flat; margins slightly recurved; not spinescent; midrib raised above, raised below. Cataphylls linear, pungent, pilose, persistent. Pollen cones fusiform, yellow to brown (pale); microsporophyll lamina firm, dorsiventrally thickened, apical spine rudimentary, sharply upturned. Megasporophyll lamina ovate to lanceolate, 70 mm long, 35-50 mm wide, regularly dentate, with pungent lateral spines, apical spine distinct from lateral spines; ovules 4-10, glabrous. Seeds flattened-ovoid; sarcotesta orange-brown; fibrous layer absent; sclerotesta apically crested. Spongy endocarp present.
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