Origin and evolution of shoot apex in vascular plants.

利用透射电镜观察了蕨类植物和种子植物茎尖中胞间连丝（plasmodesmata，PD）的分布和频率，并对这3种类型的茎尖进行了比较，以探讨单丛型、单形型和单形型3种维管植物茎尖的演化。在蕨纲、木贼纲和裸蕨纲的单丛茎尖中，顶端细胞的细胞壁具有最多的PD，而远离顶端细胞的细胞壁具有最少的PD。PD频率从顶细胞向下有梯度。蕨类植物体内不存在次生PD，因此植物体内的PD网络具有细胞系特异性。与此相反，典型的裸子植物的单芽顶端表现出非常低的PD频率，在芽顶端没有梯度。这与被子植物典型的双芽顶相同。裸子植物也有形成次级PD并构建界面特定PD网络的能力。这一事实与目前接受的故事一致，即叶（大叶）和茎的进化独立地发生在两个不同的系统发育线中。在微叶石松两种类型的PD网络，谱系特异性和接口特异性，被确认。前者是卷柏科植物茎尖单细胞的特征，后者是石松属植物多细胞的特征。石松科植物可能是在从其他维管植物中分化出来后才获得形成次生PD的能力。值得注意的是，单个顶端细胞或多个顶端原始细胞的存在导致细胞间PD网络的茎尖组织完全不同。

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.