Notes on Dipterocarps. {No. 7} On the fruit and germination of Isoptera borneensis
I.H. Burkill’s 1922 contribution to his “Notes on Dipterocarps” series examines the fruit morphology, water-dispersal mechanism, and germination behaviour of Isoptera borneensis, a dipterocarp whose corky sepals provide buoyancy for riverine seed distribution. The article uses detailed morphological observation to argue that the fruit-wall splitting observed during germination is not a vestige of ancestral dehiscence but a mechanical response to continuous embryonic growth, and that water-distribution in this genus represents a secondary adaptation linked to early fruiting at low canopy height.
Summary
Burkill’s central concern is the mechanism by which Isoptera borneensis disperses its seeds and what this reveals about the evolutionary history of the Dipterocarpaceae. The tree, which yields the commercially important Tangkawang fat, grows along the margins of moderate rivers and drops its fruits directly into the water, where the corky sepals keep them buoyant. Burkill demonstrates that without these sepals the fruits sink within sixty hours, and he contrasts this strategy with that of Vatica Wallichii (Pachynocarpus Wallichii), which achieves buoyancy through the fruit-wall itself rather than through modified sepals. He argues that water-distribution cannot be regarded as ancestral in the order but appears as an ultimate modification in lineages that have lost the height advantage necessary for wind dispersal, and that in Isoptera it is specifically connected with the tree’s habit of fruiting at the early age of six years, when it has not yet attained sufficient height to access the wind (p. 281).
The germination section is the article’s most original contribution. Burkill shows that the fruit-wall ruptures not along pre-formed lines of weakness but in response to the mechanical pressure of the still-growing embryo. The outer cotyledon, which occupies 240 to 280 degrees of the seed’s equatorial circumference, flattens as it expands, generating pressure primarily in the transverse direction and secondarily toward the placental attachment. This produces a characteristic pattern of one to three cracks, the most common being a three-part split with the third crack near the placental attachment (pp. 282–283). Burkill measures the angular divisions of six fruits split into two and five split into three, showing that the pattern is a direct mechanical consequence of the embryo’s geometry rather than a developmental programme.
From this, Burkill draws a broader phylogenetic argument. Because the embryo in Dipterocarps never ceases growing from fertilisation until the plant dies, the fruit-wall splitting is not dehiscence in any classical sense. He contends that the constancy of six ovules per ovary (of which only one matures) suggests an ancestry that lacked winged fruits, since six seeds carried together on the wind would be too heavy for efficient dispersal. Wind-distribution is therefore a later acquisition, and its loss in genera such as Isoptera, Vatica, Balanocarpus, and Pachynocarpus is a secondary reversion to water or ground dispersal (p. 284).
Key Findings
- Isoptera borneensis fruits sink within 60 hours when deprived of their corky sepals, confirming the sepals as the sole buoyant structure (p. 281).
- The tree commences fruiting at the early age of six years, a fact Burkill cites from van Romburgh and Ridley as the likely selective pressure favouring water-distribution (p. 281).
- The outer cotyledon occupies 240–280 degrees of the seed’s equatorial circumference, with the inner (placental) cotyledon occupying the remainder (p. 281).
- Angular measurements of six two-part splits and five three-part splits demonstrate that crack positions are mechanically determined by embryonic pressure rather than following fixed lines (pp. 282–283).
- The ovary universally possesses three chambers and six ovules in Dipterocarps, yet only one ovule matures in normal cases; Burkill estimates that roughly one flower in 10,000 matures fruit (p. 284).
- The cotyledons contain abundant chlorophyll before germination and are held on short petioles upon exposure to light, a condition shared with most Shorea species (p. 284).
Conclusion
Burkill’s definitive takeaway is that the fruit-wall splitting in Isoptera borneensis—and by extension in most Dipterocarps—is a purely mechanical phenomenon produced by the continuous growth of a non-quiescent embryo, not a residual dehiscent mechanism inherited from a six-seeded ancestor. This observation, he argues, undermines any hypothesis that the order evolved from a dehiscent, multi-seeded condition, and instead supports a model in which wind-distribution is a derived character whose loss in water-dispersing genera represents a secondary adaptation to low-canopy fruiting.
Context
- The article draws on Burkill’s own morphological measurements and dissections, supplemented by observations attributed to van Romburgh and Ridley on the age at first fruiting and on vivipary under abnormal conditions.
- It is the seventh in a series of “Notes on Dipterocarps” published in the Journal of the Straits Branch of the Royal Asiatic Society, and it corrects errors in a preceding note (No. 6) concerning Motley’s Labuan herbarium and the locality of Pachynocarpus umbonatus (p. 284).