Lycophytes

Lycophyte shoots with microphylls branch either equally or unequally depending on plant group, like fern shoots. Interestingly, in some Lycopodiaceae species, equal and unequal branching occurs in one individual (Figure 3.10A; e.g., Lycopodium complanatum, L. tristachyum) (Guttenberg, 1966). This is a remarkable contrast to the fern shoots that show dichotomous or lateral branching depending on species. Since Troll's (1937) definition, equal and unequal branching modes in lycophytes have been traditionally called isotomous or anisotomous dichotomy, respectively. Isotomy occurs by equal division of the shoot apex, and anisotomy results from unequal growth of a once isotomously bifurcated shoot apex (Troll, 1941).

The developmental anatomy of shoot branching in lycophytes has been poorly examined, but available data are not in accordance with Troll's anisotomy; unequal branching is not produced by unequal growth of once equally divided SAMs, but by unequal division of SAMs. In unequal branching in Selaginellaceae shoots with the apical cell based SAM, lateral shoot primordia arise from the apical flank of the SAM retaining the original apical cell, and the lateral apical cell forms from one of the primordium surface cells (Siegert, 1974; Imaichi and Kato, 1989). This developmental manner is identical to lateral shoot formation in ferns (Figure 3.9A). In unequal branching of Lycopodiaceae SAMs with plural initial cells, the original (main) SAM appear to be retained and new SAMs for lateral shoots form from the apical flank (Figures 3.9C, 3.10B). In equal branching of Lycopodiaceae shoots, two new SAMs appear to be formed after the cessation of the original SAM in equal branching (Figures 3.9D, 3.10C; Guttenberg, 1966). Such unequal and equal bifurcation in lycophyte SAMs are similar morphologically to the lateral and dichotomous branching in fern SAMs, respectively, regardless of the different meristem organizations (Figure 3.9). In the Isoetaceae, Stylites shows apparently equal branching, but Isoetes does not branch (Bierhorst, 1977). There are no developmental data on Stylites. In conclusion, there appears to be no difference in meristem branching behavior between the megaphyllous fern and microphyllous lycophyte SAMs. Stems of both the plant groups may be comparable to each other.

3.5 Roots

The root is an important organ for anchoring the aerial shoot in the soil and absorbing inorganic nutrients from it. Comparative anatomy and fossil evidence combined with phylogenetic mapping suggest that the root evolved at least twice - once each within the lycophytes and euphyllophytes (ferns and seed plants) (Friedman et al., 2004, and references therein). Roots of ferns and lycophytes have similar adventitious and endogenous origins in stems or special root-producing organs, with poorly developed embryonic roots. The most prominent trait characterizing fern and lycophyte roots is whether the root branches laterally (monopodially) or dichotomously (Figure 3.12 below; Gifford and Foster, 1989).

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