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.
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
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
: (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