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This maple, which appears to be very rare in the wild state, was discovered by Joseph Rock in the valley of the Yalung, Szechwan, China, in July 1929, and introduced by him at the same time under his No. 17819. Plants were raised and distributed by Messrs Hillier but none have been traced, and the only known adult specimens outside China grow at the Strybing Arboretum, Golden Gate Park, San Francisco. Rock recorded it in his Field Notes as a tree to 30 ft high, with spreading branches and an ash-coloured bark. The leaves are digitately compound, with four to seven leaflets; main stalk reddish or brownish, 13⁄4 to 22⁄6 in. long; lowermost leaflets almost sessile, the upper ones on short stalks; they are glabrous, glaucous beneath, lanceolate, tapered at the base, blunt at the apex, the middle ones 3 in. long, 3⁄5 in. wide. Flowers not seen. Fruits in lax corymbs; wings spreading at a wide angle, 4⁄5 in. long; nutlets downy, 1⁄5 in. wide.
It has been placed provisionally in Section Trifoliata, but differs from the other members of the section (of which A. griseum and nikoense are the most important) in having four to seven leaflets in each leaf.
The relationship of this species is not with section Trifoliata, to which it was provisionally assigned, but with section Oblonga (Integrifolia), of which A. buergerianum and A. oblongum are the best-known species. But despite having several botanical characters in common with members of this section, it is quite distinct in its palmately compound leaves, and ranks as a monotypic series.
The original trees in Golden Gate Park, San Francisco, grow in shade and fruit infrequently. But a propagation at Marshall Olbrick’s Western Hills nursery near San Francisco produces good seed, and a specimen now in the Ventnor Botanic Garden, Isle of Wight, was raised from this by Messrs Hillier. It is about 15 ft high and suffered no damage from the frosty spells early in 1985 and 1986. In less favoured gardens it may prove to be less reliably hardy, and it is advisable to protect young plants, which are certainly tender.
A. pentaphyllum is now firmly established in cultivation and available from some nurseries in Britain. This is as well, since Mr Vertrees has been informed that only some two or three hundred trees remain in the wild. A portrait appears in his book on p. 155.