Origin and evolution of shoot apex in vascular plants.

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The frequency and distribution of plasmodesmata (PD), intercellular channels, in the shoot apices was calculated using TEM sections, and compared between Pteridphytes and Spermatophytes to discuss the evolution of the three types of shoot apices in the vascular plants: the monoplex-, the simplex- and the duplex-types. In the monoplex shoot apices of the Filicopsida, the Equisetopsida, and the Psilopsida, the cell walls of apical cell has the greatest number of PD, and those of cells apart from the apical cell the lower number of PD. There is a gradient in PD frequency from the apical cell downward. The Pteridophytes are believed to lack secondary PD, and then PD network in the plant body is cell llneage-specific. In contrast, the simplex shoot apex typical of the Gymnosperms exhibits very low PD frequency with no gradient in a shoot apex. This is the same as the duplex shoot apex typical of Angiosperms. It is plausible that the Gymnosperms also have the ability to form secondary PD and construct interface-specific PD network. This fact is consistent with the currently accepted story that the evolution of the leaf (megaphyll) and stem occurred independently in the two different phylogenetic lines. In the micorphyllous Lycopsida two types of PD network, the lineage-specific and the interface-specific, are recognized. The former is characteristic for the Selaginellaceae shoot apices with the single apical cell, and the latter for the Lycopodiacae with plural apical cells. It seems likely that the Lycopodiaceae gains the ability to form the secondary PD after they branched off phylogenetically from the rest of the vascular plants. It is noteworthy that presence of single apical cell or plural apical initials results in drastically different shoot apex organizations with respect to cell-to-cell PD network.