Ultrastructure and Development of the Sexine in the Pollen Wall of Silene alba (Caryophyllaceae)

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Abstract
Sexine development in S. alba (Caryophyllaceae) was investigated by EM and light microscopy. Earliest indications of wall pattern formation occur in the tetrad when a fibrous primexine arises near the microspore plasma membrane. The protectum elaborated by the primexine seems to serve as a scaffold upon which, after tetrad dissolution, electron-dense particles from the intralocular cavity are deposited, forming the mature sexine. Sporopollenin synthesis is associated with lamellae of unit membrane dimensions at the apertural pore regions. White-line lamellae probably contribute to further sporopollenin synthesis during final maturation stages. Pattern control mechanisms may reside in microspore mother cells.