Notes on the exine ultrastructure of Onagraceae and Rhododendroideae (Ericaceae)

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Abstract
There are only three unrelated families (Onagraceae, Ericaceae and Cacsalpiniaceae) known so far with viscin thread bearing pollen grains. In this work members of the Onagraceae and Ericaceae were investigated. Supposing that the viscin threads represent a specialized part of the exine, it is assumed that they are deduced from a single exine architecture principle. This can be confirmed especially for the investigated species. Similarities are also recognizable in early developmental stages. These similarities are the massive nexine, the endosexinel layer, consisting of sporopolleninous elements arranged in a paracristalline configuration and furthermore the tectal layer which can be identified by a higher gradation of sporopollenin polymerisation.