Pollen wall development in Sciadopitys verticillata (Sciadopityaceae)

Pollen wall development of Sciadopitys verticillata was observed by transmission electron microscopy. The pollen of S. verticillata is non-saccate and spherical, and the exine consists of the outer thick, sculptured ectexine and the inner lamellated endexine. At the early tetrad stage, the initial ectexine and lamellae of the initial endexine begin to form on the microspore plasma membrane. The ectexine granules gradually swell. Deposition of sporopollenin materials on the ectexine granules then results it their becoming partially connected to each other. Identification of the original small ectexine granules then becomes difficult, and, finally, the ectexine appears as a homogeneous, partially discontinuous layer. The granules of the early ectexine cannot be identified. At maturity, there are four to five endexine lamellae. Recent molecular data have shown that Sciadopitys first branches off from the Cupressaceae plus Taxaceae clade, which is characterized by granular exine. Although the ectexine of Sciadopitys is similar to that of the Cupressaceae during initial development, the morphology of the ectexine is significantly different in the mature pollen. The initial stage of pollen development clearly shows the structural homology of the granular ectexine. Divergence of the exine structure occurs in the later stages.