The morphology and ultrastructure of Caytonanthus

Abstract

The morphology and ultrastructure of Caytonanthus arberi pollen organs and pollen, collected from the Middle Jurassic, Cayton Bay locality of England, are described. Pollen organs consist of flattened rachises with suboppositely branched pinnae, which bear terminal synangia. The synangium is covered with a two-zoned (fibrillar and amorphous) cuticle. The locule of each pollen sac is lined with a lamellated tapetal membrane and contains numerous pollen grains and orbicules. Grains are small, monosulcate, and bisaccate. Exine ornamentation of the proximal wall is psilate, while distally the apertural membrane is scabrate. The exine is composed of a thick, lightly staining sexine and a thin, darkly staining nexine. The sexine is tectate-alveolate laterally, becoming nearly homogeneous medially. The infratectal alveolae are robust and are continuous with the endoreticular units of the sacci. Sacci are eusaccate, with endoreticulations attached only to the outer walls. Several immature grains have also been identified and indicate that the nexine is lamellate throughout and that infratectal alveolae and nexine lamellae are well developed prior to complete tectum synthesis. Saccus size and ultrastructure of Caytonanthus pollen are compared with those of other Late Paleozoic and Mesozoic seed ferns that produced saccate pollen (Callistophytales, Glossopteridales, Corystospermales) and found to be smaller and more extensively filled with thicker endoreticulations. Documentation of the eusaccate character state in Caytonanthus is also discussed regarding its phylogenetic implications. Key words: Caytonanthus, Caytoniales, Mesozoic, pollen, seed fern, ultrastructure.