Meristem organization and organ diversity

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

“…In sporophytes, which are predominant generation, one to two AC forming regularly segmented shoot meristems are present in the Selaginellales [17][18][19][20], whereas the Lycopodiales have multiple superficial initial cells (Fig. 2e) [19,21,22], with four transient initial cells having been reported [23].…”

“…Moreover, regular shoot dichotomous branching in these groups results in repetitive reorganization of the SAM structure and the exchange of functioning initial cells. In Isoëtales, the SAM contains one (or two) AC or, alternatively, one group of superficial initials [17,19,24], depending on the age of the plant [25]. Unique to the lycophytes is the presence of microphylls, an evolutionary distinct type of leaf [16,19,24,26] that is initiated by one or two superficial cells in the peripheral zone of the meristem [18,22,27].…”

“…Evolutionary changes in the structure of the lycophyte SAM, reflected in the increasing numbers of initial cells (e.g., [19,24]) and relatively small microphyll primordia [32], seem to enlarge the diversity of phyllotaxis in these taxa; these factors may also be driving the frequent phyllotactic transitions caused by dichotomous branching in this group of plants. On the other hand, in Selaginella, where the meristem is segmented, phyllotaxis is stable and no phyllotactic transitions have been observed, even during dichotomous branching.…”

Specific intercalary growth of rhizophores and roots in Selaginella kraussiana (Selaginellaceae) is related to unique dichotomous branching

“…In shoot apical meristems (SAM), with multiple initial cells, such as in lycopods and a few seed plants that may branch dichotomously, new apices for twin axes are selected from the pool of meristematic cells formed prior to division. In meristems with a single initial cell (IC), such as in Selaginella or in ferns, formation of new meristematic centers is rarely associated with direct division of the original IC (Bierhorst, 1977); more often it is related to its disappearance due to segmentation or inactivation (Hagemann, 1980;Hagemann and Schulz, 1978;Hébant-Mauri, 1993;Imaichi, 2008;Imaichi and Kato, 1991;Jernstedt et al, 1994;Mueller, 1982). Cells, which are located between two new meristematic centers, undergo vacuolization and differentiation, giving rise to a furrow separating the twin axes (Hagemann, 1980;Nolan, 1969;Piche et al, 1982;Tomlinson, 1970Tomlinson, , 1971.…”

Free Axial Lobes: An Important Diagnostic Character in Pteridium (Dennstaedtiaceae)

“…Unlike seed plants, fern leaves typically show determinate (finite) growth (Imaichi 2008) with prolonged acroscopic extension due to meristematic activity of a cell at the leaf tip. More proximal elements of the leaf thus mature first.…”