On the Stem Apex, Leaf Initiation and Early Leaf Ontogeny in Filicalean Ferns

Diversity of phyllotaxis in land plants in reference to the shoot apical meristem structure

“…Generally, the SAM of monilophytes contains a single AC, which is usually tetrahedral and distinct within the meristem, and derived segments [35][36][37]. The exceptions to this characteristic AC shape are members of the Salviniaceae family, which have a lenticular AC with two cutting surfaces, relating to the dorsiventral symmetry of the shoots [36,38].…”

“…Regardless of the shape, the AC divides parallel to the anticlinal sides and gives rise to segments (merophytes), which in species with relatively flat or concave apices are more rectangular and elongated, forming a distinct superficial layer of prismatic cells [36,37]. Interestingly, additional potential ACs, formed by the anticlinal oblique divisions can occur at the SAM surface in some filicalean ferns [35]. Leaves are formed in the merophytes by a constant sequence of divisions and can be in close clonal relationship to the successively cut segments, as, e.g., in Ceratopteris [36,37,39].…”

“…Leaves are formed in the merophytes by a constant sequence of divisions and can be in close clonal relationship to the successively cut segments, as, e.g., in Ceratopteris [36,37,39]. Typically, however, more than one merophyte is involved in leaf formation, and thus the impact of segmentation on the leaf spatial arrangement is less pronounced and strict, e.g., in Dicksonia, Osmunda [35][36][37]40].…”

Dichotomous branching: the plant form and integrity upon the apical meristem bifurcation

“…The dichotomy here is defined in a narrow sense as an equal longitudinal division of this single AC, where both derivative cells become the initials for twin apices (Goebel, 1928; Troll, 1937; Schoute, 1938; Bierhorst, 1977). Alternatively, the meristem comprises of one or more groups of morphologically similar initial cells and their youngest derivatives (Evert, 2006).…”

“…As a consequence, direct division of the AC is uncommon. It was reported to occur only in some ferns (Bierhorst, 1977). But even this interpretation was later criticized, mostly because the ACs for the new branches originated by formative divisions and not by a direct dissection of the AC (see Lyndon, 1998 for discussion).…”