Exudates were observed in the exine
(external surface) of the pollen grain of the castor bean; these exudates were most likely a lipophilic substance known as pollenkitt (Figure 2f), which is very common in members of the Euphorbiaceae family (Vargas et al.
After exocytosis the pro-orbicules nest on the tapetal plasmalemma and get a sporopollenin coat synchronously with the developing pollen exine (Christensen et al.
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).
Pollen is a rich source of protein (Faegri and van der Pijl 1971, Stanley and Linskens 1974:154), but the extent to which it can be used as a nitrogen source by many animals is debatable because the exine, or outer coat of the pollen grain, is highly resistant to degradation by digestive enzymes (Heslop-Harrison 1971).
Digestion occurs when enzymes in the bird's gut gain access to the pollen cytoplasm, either by mechanically penetrating the exine or by passing through the germination pore or a pore in the exine weakened by changes in osmotic pressure (Simpson and Neff 1983).
Saccate pollen involves an enlargement and alveolation of the exine of some complexity and diversity (e.
Tomlinson, 2000), but this seems inappropriate with the demonstration that the exine is hydrophilic (Bohne et al.
triaperturate, has a clavate exine
deriving from reduced columellae and
Altingia based on thicker exine
and fewer (8-15) pores, occurs during
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.
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.
most cases, however, the difference between the exine