Megaphyll development in ferns euphyllous

At inception, the leaf primordium arises from the apical flank of the SAM as a small mound (leaf buttress) comprising a group of surface (prismatic) and subsurface cells. Soon after, a single surface cell becomes enlarged and undergoes oblique divisions to cut off a single leaf apical cell in the leaf buttress (Figure 3.2A, B; Imaichi, 1988). The leaf apical cell is commonly lenticular with two cutting faces (Figure 3.5B; Hebant-Mauri, 1975; Bierhorst, 1977; Imaichi, 1982; Lee, 1989), and rarely with three cutting faces in some genera (i.e., Angiopteris, Osmunda, Botrychium) (Guttenberg, 1966; Imaichi and Nishida,

1986). Leaf apices with the single leaf apical cell and its immediate derivatives are comparable to the SAM in organization, so they are sometimes designated leaf apical meristems (LAMs). The LAM also resembles the SAM in having high plasmodesmatal densities equivalent to the SAM (cf. Figure 3.3C in this chapter with Figure 3A in Imaichi and Hiratsuka, 2007). The long retention of the active LAM in the leaf primordium is responsible for the prolonged leaf apical growth, resulting in the coiled crosier (Figure 3.2D). Surprisingly, in climbing leaves of some ferns, e.g., Lygodium japonicum, the leaf apical cell is maintained permanently at the apex of indeterminate adult leaves several meters long (Figure 3.5A, C; Mueller, 1982b). Long retention of LAMs causes acropetal tissue differentiation in leaves, as shown in the stem terminated by the SAM.

During or after the period of LAM activity, the marginal meristem or the marginal blastozone (sensu Hagemann & Gleissberg, 1996) is formed next to the LAM (Figure 3.5; Mueller, 1982b; Hagemann, 1984). In many ferns, lamina growth is restricted to the marginal meristem with the youngest lamina portion in the margins. The marginal meristem is fractionated to form the pinnae and pinnules, and at the final stage, the marginal meristem continues to grow without fractionation (Hagemann, 1984). Although it is still unclear what underlying mechanisms lead to fractionation of the marginal meristem, when the lamina is bilobed as in Lygodium japĆ³nica, the marginal meristem is divided by the cessation of cell proliferation in its middle portion (Figure 3.5D, E; Mueller, 1982b).

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