Microphyll development in lycophytes

Like fern leaves, lycophyte microphylls arise from the apical flank of SAMs, but from a lower site than the leaf primordium in fern SAMs (Figures 3.6A, 3.7A; Freeberg and Wetmore, 1967; Dengler, 1983). The most remarkable feature of microphyll development is the lack of both the LAM and the marginal meristem," resulting in basipetal tissue differentiation (Dengler, 1983). Replica SEM observations on a growing Selaginella leaf clearly show features common to microphylls (Figure 3.6): (1) the leaf initiation involves several dermal cells arranged in a horizontal line to form a plate-like protrusion, (2) there are neither apical meristems nor marginal meristems, and (3) lamina expansion is attributed to cell proliferation over the entire leaf primordium. Leaf development of Isoetaceae and Lycopodiaceae has received little attention since the review by Guttenberg (1966). Preliminary examination shows that the leaf pri-mordia of Lycopodium (Figure 3.7B, C) and Isoetes species with needle-like leaves are also plate-like at inception (R. Imaichi, unpublished data). The leaf primordia of Selaginellaceae have lower PD densities than SAMs, without high PD densities in the apical portions (cf. Figure 3.3A, D). Lycopodiaceae leaf primordia have very low PD densities identical to SAMs (Figure 3.3B, E).

Figure 3.6 SEM images of microphyll development of Selaginella martensii. (A) Dorsal view of SAM with growing dorsal leaf primordia (d0-d8). Small white rectangles indicate given positions of growing leaves to help show cell proliferation. (B), (C) Development of first dorsal leaf (d1). (C) Image taken 8 days after (B). (D), (E) Development of fifth youngest leaf primordium (d5). (E) Image taken 7 days after (D). Scale bar 50 |im for (A), 20 |im for (B)-(E).

Figure 3.6 SEM images of microphyll development of Selaginella martensii. (A) Dorsal view of SAM with growing dorsal leaf primordia (d0-d8). Small white rectangles indicate given positions of growing leaves to help show cell proliferation. (B), (C) Development of first dorsal leaf (d1). (C) Image taken 8 days after (B). (D), (E) Development of fifth youngest leaf primordium (d5). (E) Image taken 7 days after (D). Scale bar 50 |im for (A), 20 |im for (B)-(E).

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