Notes on Dipterocarps. {No. 6} On the genus Pachynocarpus. I.H
Burkill and Foxworthy (1922) present a taxonomic revision of the dipterocarp genus Pachynocarpus, arguing that decades of accumulated species additions by Hooker, Burck, Heim, King, and Brandis have inflated the genus to the point of confusion, and that a return to a narrower circumscription—two or possibly three species—is warranted. Working from the original Calcutta herbarium sheets and their own field collections across the Malay Peninsula, they demonstrate that most names previously placed in Pachynocarpus belong properly to Vatica, and that the critical diagnostic character (degree of calyx adnation) is invisible in flowering specimens, which explains the recurring misidentifications.
Summary
The article addresses a persistent taxonomic dilemma in Dipterocarpaceae: the boundary between Pachynocarpus and the polymorphic Vatica. Hooker’s 1860 definition rested on a single character—calyx adnation in fruit—but subsequent authors, working largely from flowering material in which that character is not yet expressed, repeatedly conflated the two genera. Burkill and Foxworthy trace the genealogy of these errors through the writings of Burck, Heim, King, and Brandis, showing how each successive author compounded the confusion by transferring species on the basis of incomplete evidence. Their central argument is that the six (or seven) names then standing under Pachynocarpus should be reduced to two or three, with the remainder returned to Vatica.
A substantial portion of the article is devoted to Vatica Wallichii, which the authors studied extensively in the wild and in cultivation at the Singapore Economic Garden. They document its ecology (flood-prone lowland habitat), its phenology (flowering most commonly in April–May, fruit ripening six months later), and its water-dispersal mechanism, having observed fruits stranded in lines at the flood limit in the Tasek Gelugor Forest Reserve. They also describe Pachynocarpus Stapfianus as a tree of rising land, ecologically distinct from V. Wallichii, and Vatica Ridleyana as a Vatica whose large fruits sink rather than float, distributing instead by animal transport or rolling.
The authors are careful to flag residual uncertainty. They note that the original Motley specimens from Borneo may contain a Vatica in flower mixed with a Pachynocarpus in fruit, gathered months apart, and that P. umbonatus requires re-collection to confirm the identity of its flowering material. They also record a discrepancy in Hooker’s original description of the wood (soft and white versus the hard, resinous wood noted on the attached field label), suggesting possible misattribution of the label to the wrong sheet.
Key Findings
- The genus Pachynocarpus is reduced from six or seven accepted names to two or three species: P. umbonatus Hook. f., P. Stapfianus King, and possibly P. verrucosus (Burck) Heim; the remainder (P. Wallichii, P. ruminatus, P. Ridleyanus) are returned to Vatica (pp. 272–273).
- Vatica Wallichii fruits float for an average of 22 days before sinking, a figure derived from a controlled experiment reported in a 1920 note (p. 275).
- Vatica Wallichii attains a height of approximately 60 feet with a breast-height diameter of 20–30 inches; in high forest the trunk is branchless to 30 feet or more (p. 275).
- Pachynocarpus Stapfianus at Bangi, Selangor, measures 38 ft. 2 in. in height with a girth of 3 ft. 1 in. at breast height; King’s Collector recorded individuals 80–100 ft. high with girths up to 3 ft. (p. 279).
- Vatica Ridleyana tree No. 795 in the Singapore Botanic Gardens stands approximately 100 feet tall with a trunk circumference of 62 inches at breast height; its fruits sink immediately in water, unlike the floating fruits of V. Wallichii (p. 278).
- The original Pachynocarpus umbonatus type material in the Calcutta herbarium bears a field note (attributed to Motley) describing the wood as “very hard close and lasting, when cut yellowish brown, turning nearly black on drying,” which contradicts Hooker’s published description of the wood as “soft and white” (p. 273).
Conclusion
Burkill and Foxworthy conclude that Pachynocarpus is best maintained as a small genus of two or three species, sharply distinguished from Vatica by the degree of calyx adnation in fruit, but that the boundary remains porous in flowering material and requires further field work in Borneo to resolve. Their revision is a corrective exercise: by returning the majority of previously assigned species to Vatica and identifying the ecological and morphological characters that genuinely separate the two genera, they restore clarity to a classification that had become unwieldy through successive, poorly grounded transfers.
Context
- The authors examined the original Herbarium Hookerianum sheets in the Royal Botanic Gardens, Calcutta, left there by Sir George King, alongside their own accumulated material from the Malay Peninsula (p. 273).
- The article contributes to the broader 19th- and early-20th-century effort to resolve the taxonomy of Dipterocarpaceae, a group in which Vatica had accumulated 55 species by 1922 with a distribution from Ceylon to New Guinea (p. 271).