hughesii, from the Potomac Group of Maryland, extend to the SEM and TEM level, such as the presence of a thick nexine made up of foot layer (plus laminated
endexine under the sulcus), supratectal spinules, and a sculptured sulcus membrane.
The exine is 2.69 m thick and the ectexine is thicker than the
endexine. Intine thickness is 0.53 m.
Endexine was in reticulated shape, and ectexine was intectate.
Three different electron- dense layers form the pollen grain wall: ectexine with interrupted tectum, columellae and foot layer;
endexine and intine still in formation.
(1991) also noted that the
endexine thickness 2-3 times around the colpus and the porus and form a large costae formation.
4), no secondary bracts in the staminate cyme, pollen with operculum and
endexine without thickening.
A highly interesting observation concerns Sauromatum venosum (Araceae), which has inaperturate pollen grains with an
endexine and spines, both polysaccharidic in nature (Weber et al., 1998).
Using transmission electron microscopy (TEM), Nowicke and Skvarla (1983: 181) noted that "[t]he
endexine appears loose and fragmented near the apertures" in extant Itea pollen; if the same is true of the fossil grains, this may explain the textured appearance of the exine surrounding the pores under light microscopy.
A number of characters vary at too low a level to be useful (are highly homoplastic at tribal level), for instance pollen grain size and shape, number of infractectum layers and their thicknesses, extent and depth of cavea, spine shape and internal structure, number and size of abporal, interporal, paraporal and poral lacunae, and internal
endexine surface.
Endexine: (0) thick, under apertures only; (1)
endexine throughout, thickened under apertures; (2)
endexine throughout, not thickened under apertures; (3)
endexine absent under apertures.
endoaperture in the
endexine and/or membranous granular layer MGL (see