Exine ultrastructure of in situ pollen from the cycadalean cone Androstrobus manis Harris, 1941 from the Jurassic of England

Abstract

Pollen grains extracted from the cycad pollen cone Androstrobus manis Harris, 1941 from the Bajocian of Yorkshire were studied by means of LM, SEM and TEM. Boat-shaped pollen grains of Cycadopites-type were found, with a distinct sulcus and a typically cycadalean foveolate-fossulate surface. The exine is two-layered, with a more electron-dense endexine. The tectum is continuous. There is no distinct boundary between the tectum and infratectum. The infratectum is alveolate, with numerous, irregularly distributed rounded alveolae in the majority of the sections. Occasionally, rounded alveolae are organized in several rows. Even more rarely, elongated alveolae are arranged perpendicular to the surface of the exine. Such a variable ectexine appearance has been already described in modern cycads and was explained by not strictly perpendicular orientation of pollen walls towards the plane of the sections. Distally, the exine becomes thinner at the expense of the ectexine. The alveolae disappear, and the ectexine becomes thin and homogeneous in the apertural region. Pollen grains of A. manis show many similarities to pollen of modern cycads: a distinct sulcus, a foveolate-fossulate surface pattern, and an infratectum consisting of elongated alveolae. They differ in the ultrastructure of the apertural region, since pollen grains of modern cycads retain the infratectal layer even in the apertural region. A. manis shows differences with earlier studied species of Androstrobus, that point to the heterogeneity of this cycadalean genus.