Ultrastructure and development of the gametophyte vaginula-sporophyte foot complex in the liverwort Targionia hypophylla L.

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The development of the placental complex including the gametophyte vaginula and the bulbous foot of the sporophyte in the liverwort Targionia hypophylla L. (Marchantiales) was studied by transmission electron microscopy. The vaginula and foot are separated by an intervening space and each consist of parenchymatous cells without intercellular spaces. Transfer cells begin to differentiate at the gametophyte-sporophyte interface just prior the onset of meiosis. While a single epidermal transfer-cell layer has developed in the foot by the end of meiosis, a multilayered pattern of transfer cells is formed in the vaginula. Gametophyte transfer cells have wall labyrinths which decrease in complexity with distance from the foot, lack plasmodesmata, and show signs of degeneration in the proximity of the foot. During meiosis, amyloplasts of both vaginula and foot lack starch and develop some thylakoid grana. In the subsequent stage of spore maturation, obliteration of the wall labyrinth occurs in both gametophyte and sporophyte transfer cells. The developmental pattern of the placental complex in Targionia is discussed in relation to that of mosses.