Seed Morphology in Relation to Dispersal, Evolution, and Propagation of Cycas L.

Abstract

A comparative morphological study of mature seeds of C. revoluta, C. media, C. normanbyana, C. taiwaniana and C. wadei revealed a 3-layered seed coat consisting of sarcotesta, sclerotesta, and a thin membranous jacket. Seeds of C. circinalis, C. rumphii, and C. thouarsii have an additional layer of spongy tissue, which causes flotation in water. It is speculated that long-distance dispersal of buoyant seeds occurs by the Indian Ocean currents, while nutritionally valuable starch in sarcotesta provides incentive for mammals to affect local dispersal across land. Floating seed taxa differ only in minor morphological characters, despite their wide distribution, suggesting a single polymorphic species. Two groups of Cycas may be recognized: the C. revoluta group with relatively small viable seeds that sink in water and float only when inviable, and the C. circinalis-rumphii group with large seeds that float regardless of viability.