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Extensible helical wall thickenings of the testa of chiloschista lunifera orchidaceae as seed attachment mechanisms

, : Extensible helical wall thickenings of the testa of chiloschista lunifera orchidaceae as seed attachment mechanisms. Berichte der Deutschen Botanischen Gesellschaft 93(2): 391-404

The seed coat micromorphology of the tropical Asian epiphytic orchid C. lunifera (Rchb. f.) J.J. Smith is described, and illustrated by SEM micrographs. The spherical embryo is covered by a 0.6 mm long seed coat of thin-walled, dead cells. The testa exhibits a microstructure characteristic of the subtribe Sarcanthinae. At the micropylar end of the seed the testa is composed of 2 different cell types, both characterized by secondary helical wall thickenings. One cell type (E-cells) is identifiable by a thick banded helix; these tracheoid cells resemble tracheal elements in vascular tissue. The 2nd cell type (F-cells) is characterized by thinner walls and a very fine thread like wall helix. After wetting the seed with water, within a few seconds the E-cells extend slightly and show an active hygroscopic movement. The F-cells swell and the thin helical wall thickenings become unravelled, producing threads up to 4 mm long. This positive mechanism enables the seed of the epiphyte to attach to a suitable substrate such as moist bark. From .apprx. 1100 spp. in 210 orchidaceous genera studied, C. lunifera is the only taxon to have this special type of dispersal mechanism. This is the only known example within the Monocotyledons of an attachment mechanism operated by means of hygroscopic movement and unravelling of helical wall thickenings. Comparison with similar mechanisms in certain Dicotyledons (Cobaea, Blepharis) indicate that the mechanism of Chiloschista could be among the most complicated attachment mechanisms in Angiosperms.

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