Notes on Dipterocarps. {No. 2} The seedling and the seed-production in some species of Shorea
This note by I. H. Burkill describes the embryology, germination, and early seedling development of several Shorea species, with observations on flowering and seed-production cycles recorded at the Singapore Botanic Gardens (p. 161).
Summary
Burkill details the structure of the Shorea embryo, noting that of six ovules per flower only one matures, and that the fleshy inner cotyledon wraps around the central placenta and the two sterilised loculi (p. 161). He traces the germination sequence: the seed falls with the radicle directed upward, the cotyledons rupture the ovary wall, the hypocotyl elongates and curves to direct the radicle into the soil, and the cotyledons become nearly horizontal (pp. 162–163). A consistent feature across S. leprosula, S. rigida, S. macroptera, and S. bracteolata is that the outer cotyledon stands slightly higher than the inner, with the degree of bulk varying among species (pp. 163–164). The Indian S. robusta is noted as exceptional in having hypogeal cotyledons with greatly elongating petioles, approaching Dipterocarpus (p. 166).
Burkill also records abnormal seedlings of S. macroptera observed in 1916, including individuals with three cotyledons followed by whorls of three or more leaves, and abnormally split leaves (pp. 166–167). On seed production, the period from flowering to seed fall is about ten weeks in most species and nine weeks in S. bracteolata and S. gibbosa; flowering in Singapore appears to occur in the first suitable season after three years from the last heavy seed-crop, while in India S. robusta flowers almost annually but produces a good seed crop only about once in three years (p. 167).