Orbicules provide an interesting model to study sporopollenin biosynthesis since they are a-cellular structures, independent of cytoplasmic control, contrary to the pollen exine (Clement & Audran, 1993).
Moreover, the mode of development seems to be similar between exine and orbicular wall (Christensen et al.
Tomlinson, 2000), but this seems inappropriate with the demonstration that the exine is hydrophilic (Bohne et al.
Exine stratification in extant gymnosperms: a review of published transmission electron micrographs.
Altingia based on thicker exine and fewer (8-15) pores, occurs during
The second group has thin exine with microspinate and/or
It also produces exine precursors, orbicules, and sporophytic recognition proteins and lipids which form the pollen coat (or pollenkitt or tryphine) (Echlin, 1971; Pacini et al.
A distinct membrane was found here in Hypoxis (Lilianae-Asparagales: see below) and has also been described in Arum (Araceae), where it adheres closely to the callose wall of the tetrads and to the developing exine, up to the late microspore stage (Pacini & Juniper, 1983).
El-Ghazaly and Jensen (1987) found for both orbicule walls and pollen exines of Triticure aestivum an intense staining for acidic and neutral polysaccharides and for unsaturated lipids, while the structures stained moderately for proteins.
They observed that orbicules become part of mature exines and thus actively take part in the formation and development of the pollen exine.
most cases, however, the difference between the exine of the
flowers; and the exine is twice as thick in longi-pollen ([+ or -] 2