Cladoxylopsids and iridopterids

Plants range up to moderate sized trees (Berry and Fairon-Demaret, 2002; Stein et al., 2007), and often have a trunk that is anchored by adventitious roots basally, and that forms a crown of branches distally (Figure 13.3a). Individual branches display one of several distinctive branching patterns (Skog and Banks, 1973; Stein, 1982), many being three dimensional, while others appear more-or-less planar (Figures 13.3a-c). Distal units fork more-or-less equally and are often considered to be the equivalent of pinnules, even though there is no laminar tissue interconnecting the terete terminal segments (Figure 13.3b). In contrast to modern ferns, the branching systems of cladoxylopsids and iridopterids do not display clear stem/leaf organography. Fertile species are typically assigned to one of several genera that consist of branching systems with terminal units that are more-or-less similar to the vegetative systems, except that each axis terminates as a sporangium (cf., Figures. 13.2 and 13.3). In some genera the sporangia are erect (Figure 13.3c) whereas in others they are reflexed or even recurved (Stewart and Rothwell, 1993; Taylor and Taylor, 1993). Those with recurved sporangia are more likely to be allied to modern equisetophytes, while those with erect sporangia tend to be related to modern ferns.

Large anatomically preserved axes often have a stele formed by a complex system of anastomosing xylem segments (Soria et al., 2001; Figure 13.3e) that divide radially to produce traces to branches. In somewhat smaller axes the xylem segments tend to be connected centrally, forming radiating ribs of a proto-stele (Figure 13.3d). Xylem of the smallest axes consists of a trilobed, bilobed, or terete bundle. The largest axes typically have an orthotropic branching pattern, whereas successively smaller axes often branch in a four-ranked (i.e., quadri-seriate) and then a two-ranked pattern. Protoxylem occurs near the periphery of the stele, and in some species there may be additional strands toward the center. Xylem maturation is mesarch, and often there is a hollow channel or parenchyma bundle associated with each protoxylem strand (Figure 13.3d). In

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