Exine stratification, ectexine structure and angiosperm evolution

The exine of phanerogams is considered to consist of an endexine covered by an extexine. These are defined morphologically and ontogenetically. The endexine is deposited on tangential membranes, probably originated by the plasmalemma. The ectexine is deposited in a primexine without the mediation of membranes (the latter, however, are sometimes present, but not tangential). The endexine may or may not remain laminated in the mature state, may or may not be separated from the ectexine by a thin clear zone, and may or may not be distinguishable from the ectexine in stainability and solubility. In this perspective, laminated inner layers of annonaceous and myristicaceous pollen, for example, are considered endexinous, and homologous with endexines of gymnosperms and other angiosperms, as is the inner basically laminated zone of the exine of pteridophytes. Only when it can be ontogenetically demonstrated that no tangential membranes support the inner exinous zone can a pollen be considered to lack an endexine. With respect to ectexine structure in angiosperms, it is stressed that in view of fossil and comparative data, the columellar condition may well be the most primitive one. The possibility exists that granular or massive ectexines are primitive in at least some groups, but in the author's opinion this has not actually been demonstrated. In the Annonaceae and Myristicaceae, for instance, it seems more straightforward to consider the columellar condition to be primitive. Putatively primitive families such as the latter are not very old, and are advanced in many of their characters. They may have lost their columellae in some of their genera and species, as undoubtedly have specialized groups such as saprophytic or hemiparasitic Gentianaceae and Olacaceae.