Lycophyte root apical meristems and their branching

Lycophyte RAMs show a greater diversity of apical organization than fern RAMs. RAMs show layered (Isoetaceae, Figure 3.13C) or non-layered (Selaginel-laceae) structures (Guttenberg, 1966; Ogura, 1972; Yi and Kato, 2001). Lycopo-diaceae RAMs were interpreted as having the layered structure (Ogura, 1972), but some actually show a non-layered structure with a mass of initial cells (Figure 3.13B). In Selaginellaceae, RAMs possess no apparent apical cell (Guttenberg, 1966), or have single apical cell (Imaichi and Kato, 1989).

All Selaginellaceae and Isoetaceae roots branch dichotomously, whereas Lycopodiaceae roots superficially show both dichotomous and unequal branching (Figure 3.13D). RAM division in lycophytes has been poorly examined, with only one recent paper on Isoetacean roots (Yi and Kato, 2001). RAMs are divided into two because of a small intervening group of non-meristematic cells derived from apical initial cells of the outer layer of the RAM by periclinal divisions, and two new RAMs are formed on either side of these non-meristematic cells (Figure 3.12C, D). These non-meristematic cells are similar to "pavement cells" that intervene to split two dividing apical meristems of the shoot (Barlow et al., 2004). In this sense, RAMs have a similar branching mode to SAMs, that is, the apex ceases meristematic activity and is replaced by two new apices.

In contrast, information about RAM meristem behavior in unequal branching is totally lacking. The question is whether (1) the original RAM is retained as is, and lateral small RAMs newly form in the flank (comparable to the unequal branching of the shoot apex), or (2) the meristem is replaced by newly formed RAMs of unequal size. Research to clarify the meristem behavior in RAM branching is urgently needed.

Figure 3.13 Root branching and RAM of Lycopodium clavatum (A, B) and dichotomously branching roots of Isoetes asiatica (C, D, modified from Yi and Kato, 2001). (A) Equally and unequally branched roots arising from a stem segment. (B) Median longitudinal section of a RAM with a mass of lightly stained initial cells (asterisk). (C) Incipient root branching with two meristem groups separated by two files of three cells (demarcated by white lines). (D) Two newly formed RAMs (arrows) are still covered by the original root cap. c, root cap; s, stem segment. Scale bar 1 cm for (A), 200 |im for (B), 50 |im for (C) and (D).

Figure 3.13 Root branching and RAM of Lycopodium clavatum (A, B) and dichotomously branching roots of Isoetes asiatica (C, D, modified from Yi and Kato, 2001). (A) Equally and unequally branched roots arising from a stem segment. (B) Median longitudinal section of a RAM with a mass of lightly stained initial cells (asterisk). (C) Incipient root branching with two meristem groups separated by two files of three cells (demarcated by white lines). (D) Two newly formed RAMs (arrows) are still covered by the original root cap. c, root cap; s, stem segment. Scale bar 1 cm for (A), 200 |im for (B), 50 |im for (C) and (D).

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