Pollen wall development in Olea europaea L.

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Abstract
The ontogeny of the olive pollen grain (Olea europaea L.) wall is related to the structural changes which take place during the various stages of pollen grain development. The tetrad stage sees the deposition of the primexine, with probacules forming by deposition of material adjacent to the plasmalemma. Endexine development commences subsequently. The bacules and foot-layer are formed mainly from dense material found within the special callose wall. By the young free microspore stage the definitive structure of the exine has been determined. Throughout development microchannel-like structures are present in both bacules and the foot-layer. An electron-lucent zone appears between the plasmalemma and the endexine. This is the future site of intine deposition which occurs during the vacuolated microspore stage before postmeiotic mitosis. A fibrillar layer lines the arcade spaces of the exine. Between this stratum and the exine lies a thin layer of polysaccharides probably representing the remains of the primexine or glycocalyx. In the mature pollen grain the exine arcades contain numerous lipid drops (or pollenkitt), originating from the degenerating tapetum.