Initiation and branching of fern roots

The root apical meristem (RAM) of ferns originates in shoot tips near the procambium, grows through the stem cortex, at last exiting through the

Figure 3.11 Longitudinal section of a shoot tip of Hypolepis punctata, showing root initiation. White arrows indicate endogenous root primordia. The inset shows an enlarged figure of the root apical meristem (RAM). s, SAM; ra, root apical cell; rc, root cap. Scale bar 200 |im.

Figure 3.12 Diagrams showing two patterns of root initiation and branching.

(A) Endogenous origin and lateral branching in megaphyllous euphyllophytes.

(B) Exogenous and terminal branching in microphyll lycophytes. (Modified from Kato and Imaichi 1997.)

Figure 3.12 Diagrams showing two patterns of root initiation and branching.

(A) Endogenous origin and lateral branching in megaphyllous euphyllophytes.

(B) Exogenous and terminal branching in microphyll lycophytes. (Modified from Kato and Imaichi 1997.)

stem epidermis (Figure 3.11; Stevenson, 1976c). The main root produces lateral roots below the root tip, with new RAMs initiating from endodermal cells of the vascular bundles (Figure 3.12A; Ogura, 1972; Barlow, 2002). Fern RAMs have tetra-hedral single apical cells that regularly cut off derivatives on four faces with the basal derivative (root cap cells) facing outwards. The RAMs of Marattiaceae and

Osmundaceae - in which the SAMs sometimes have no apparent apical cells - are distinctive in having no discernible apical cells (Guttenberg, 1966). Determining whether the root apical cell is mitotically active or inactive has been a controversial topic (Gifford, 1983). The central question is whether the root apical cell is equivalent to the mitotically inactive "quiescent center" (QC) found commonly in angiosperm roots (Clowes, 1961). The most prominent hypothesis at present is that fern RAMs have no QC, and the QC evolved later in the seed-plant clade (Barlow et al., 2004).

There have been few reports on PD networks only for fern RAMs. Azolla and Dryopteris have high PD densities (Cooke et al., 1996) comparable to those of fern SAMs, suggesting a strong correlation between RAM and SAM structures and PD networks in ferns.

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