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The text below is taken direct from the last edition of Bean (Bean 1981a) where this species was discussed under the name Kalopanax pictus. As alluded to in the final paragraph below, the name Acer pictum has since been conserved, making K. septemlobus (Thunb.) Koidz. the correct name for the Castor Oil Tree. We have amended the accepted name and synonymy accodingly as a stopgap until this account is revised fully for TSO.
A large, deciduous tree 80 to 90 ft high in the wild, with a trunk as much as 4 ft in diameter. In cultivation it is a very elegant tree, the branches armed with stout, broad-based, yellowish prickles. Leaves palmate, measuring in young plants as much as 14 in. in width, scarcely so much in length, deeply five- or seven-lobed, becoming smaller as the trees increase in age, and then from 7 to 10 in. wide, the shallow lobes ovate-triangular, long-pointed, toothed, reaching about one-third or less towards the centre; upper surface dark shining green, lower one paler and covered with grey down when young, which falls away afterwards except from the vein-axils. Flowers small and white, and produced in numerous umbels forming a large flattish inflorescence up to 2 ft across.
Native of Japan, where it attains a large size in the forests of Hokkaido and is also found on the other islands. It is also native to Sakhalin, the Russian Far East, Korea, and China. It is said to have been introduced by Maximowicz in 1865, though some of the seeds he sent may have yielded the var. maximowiczii (see below). It is one of the most remarkable of cool temperate trees, for its foliage is of a type very sparsely represented in the open air, though common enough in greenhouses and stoves. It is quite hardy, though the shoots may decay back if not fully ripened.
The species is variable in foliage. In western China the lobing is often very shallow, as shown on trees at Kew raised from seeds sent by Wilson, in which the lateral lobes are reduced to cusps. At the other extreme is var. maximowiczii (Van Houtte) Li, which in its typical state has lanceolate lobes reaching two-thirds of the way or more towards the middle of the leaf. The type plant of this variety was raised by Van Houtte from a stray seed sent from St Petersburg and was figured in 1874 in Flore des Serres, t. 2067. It may have represented a juvenile phase of the species, subsequently perpetuated by vegetative propagation. On the other hand, Nakai (Journ. Arn. Arb., Vol. 5, pp. 11-14) recognises the var. maximowiczii as a wild-occurring variety and points out that trees with the deeply lobed leaves of this variety may bear flowers. Certainly the specimen at Nymans in Sussex, sixty or slightly more years old, bears leaves as deeply cut as in the seedling plant from which the var. maximowiczii was first described, but flowers and bears fruits. The following specimens are not differentiated into ‘type’ and var. maximowiczii as the degree of lobing has not been noted in every case: Kew 40 × 31⁄4 ft and 45 × 3 ft (1967); Lythe Hill, Haslemere, Surrey, 48 × 31⁄4 ft (1969); Nymans, Sussex, 36 × 5 ft (1966); Wakehurst Place, Sussex, 40 × 5 ft (1964) and 45 × 41⁄2 ft (1965); Exbury, Hants, 46 × 6 ft at 3 ft (1970); Hergest Croft, Heref., 35 × 53⁄4 ft (1969); Endsleigh, Devon, 45 × 51⁄2 ft (1957); Dawyck, Peebl., 50 × 4 ft (1966); Kelburn, Ayrs., 49 × 31⁄4 ft (1970).
specimens: Kew, pl. 1913, 46 × 41⁄2 ft, 49 × 4 ft and 46 × 41⁄4 ft (1984); Greenwich Park, London, 52 × 51⁄2 ft (1984); Emmetts, Ide Hill, Kent, 68 × 101⁄4 ft (1984); Wakehurst Place, Sussex, 70 × 51⁄2 ft (1974) and 52 × 51⁄2 ft (1976); Nymans, Sussex, 36 × 51⁄2 ft (1985); Brockenhurst Park, Hants, 56 × 6 ft (1979); Exbury, Hants, 46 × 6 ft at 3 ft (1970); Endsleigh, Devon, 60 × 8 ft at 3 ft (1974); Dyffryn Gardens, near Cardiff, 48 × 61⁄4 ft (1984); Dawyck, Peebl., 59 × 51⁄2 ft (1984); Stobo, Peebl., 59 × 51⁄4 ft (1984); Leckmeim, W. Ross, 50 × 61⁄2 ft (1983).
Of the above specimens some would be referable to var. maximowiczii, but the difference between the typical state and this variety is not worth maintaining (Bot. Mag., n.s., t.737).
In the reprints it was added that the correct name for this species is Kalopanax septemlobus (Thunb.) Koidz. The case for the adoption, or rather re-adoption, of this name is argued in the article accompanying the plate cited above, but the matter is a contentious one, the essential problem being how the name Acer pictum should be typified. It is, however, certain that if K. septemlobus is indeed the name that should be adopted, the maple now known as Acer mono would have to revert to its earlier name of A. pictum Thunb.