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Both internal anatomy andexternal morphology ofthe mature pollen grain of Nicotiana alata have been studied, together with pollen activation and tube emergence and its cytological zonation.A peculiar feature of mature pollen is the occurrence in the vegetative cell cytoplasm of a large quantity ofstacked cisterns of rough endoplasmic reticulum, which thendissociate during pollen activation.Pollen tubeemergence generally occurs 2hr 30minafterhydration, while cytological zonation is complete 4 hr 30 min after hydration when the first callosic plug is formed.

The conventional hypothesis of gametophytic self-incompatibility in the angiosperms involves one to four multiallelic incompatibility loci and the positive inhibition of incompatible pollen tubes. However, this concept does not accommodate recent experimental data indicating that there may be many loci. An alternative hypothesis which incorporates many loci and complementary pollen-style interactions suggests that there may be no S gene, as previously thought, and that gametophytic self-incompatibility is perhaps merely one aspect of extensive pollen-style interactions.