Common Mode of Deposition for the Sporopollenin of Sexine and Nexine

Abstract

IN the formation of the exine of the pollen grain, the highly resistant wall polymer, sporopollenin, is laid down in a series of layers. Of these, the outer, the sexine of Erdtman's terminology¹, is often strongly sculptured. Sexine pattern may have high taxonomic specificity, and because the sculpturing is executed in so refractory a material, the morphogenetic processes concerned are of considerable interest. Rowley and Southworth² have shown that in the formation of the inner exine layer near the germinal aperture of the microspore of Anthurium sp., sporopollenin is deposited on lamellae of “unit membrane dimensions”. The superimposition and compaction of these lamellae as the coating of sporopollenin thickens produce the layer corresponding to the nexine 2. Essentially the same process has been described and illustrated by Godwin et al.³ in the growth of this stratum in Ipomoea purpurea. It is clear from various earlier reports^4–7 that the inner exine layer may often reveal lamellation even at maturity, indicating this kind of origin. The sexine and nexine 1, formed earlier than the nexine 2, usually reveal no lamellation, and Godwin et al.³ were forced to conclude that these parts of the exine have a different mode of origin. We show in this communication that all strata of the exine of Lilium longiflorum originate through a process involving the association or apposition of lamellae. Because the lamellae are generated near the plasmalemma, the observation re-emphasizes the role of this membrane and the cortical cytoplasm in the determination of exine pattern.