Abstract
The anatomy and morphology of the seed ofSobralia dichotoma was studied by means of light-(LM), scanning-(SEM) and transmission electron microscopy (TEM). The seedcoat ofS. dichotoma consists of three different cell types. Two of them form a coiled wick ending in a closed hilum. Imbibition leads to uncoiling and stretching of this pipe, thus shaping a central capillary. The reversible movement of the helical tracheoidal cells is interpreted as a mechanism of water uptake (uncoiling) and -storage (coiling). Moreover, a multilayered envelope surrounds the embryo. Its inward layer shows all features of a thick cuticle while the following layers are collapsed cells with walls rich in pectin. This semipermeable envelope may function in water storage by adjoining a water sheet to the embryo and thereby protecting the mature embryo against desiccation. It is suggested that this unique seed type is a derived condition and has evolved in adaptation to the specialised habitat in tropical montane cloud forests.
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Prutsch, J., Schardt, A. & Schill, R. Adaptations of an orchid seed to water uptake and -storage. Pl Syst Evol 220, 69–75 (2000). https://doi.org/10.1007/BF00985371
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DOI: https://doi.org/10.1007/BF00985371