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figure 8.84 Branch of Pertica quadrifaria (Devonian). Bar = 2.0 cm.

Yunia is a Siegenian (Pragian, Lower Devonian) plant with spiny axes that shares some features with Pertica (Hao and Beck, 1991). The axes dichotomize in a cruciate arrangement and each contains a conducting strand with one or two protoxylem strands, depending on the level of the axis. Conducting elements have annual scalariform sculpture patterns. Closely associated sporangia are elongate and bear tri-lete spores with a relatively smooth sculptural pattern.

Dawsonites is a Devonian morphogenus that has been used by some workers for sporangia that are not organically connected to an axis or aggregated into large clusters (Halle, 1916b). The sporangia are fusiform and typically measure 3.0-5.0 mm long. The genus is now restricted to terminal sporangia that are believed to have been produced by Psilophyton . As spores are recovered from additional fertile specimens so that comparisons can be made, many of the Dawsonites species no doubt will be placed in synonymy.

Another morphogenus that has been used for naked, dichotomously branched axes of Siluro-Devonian age is Hostinella. Structurally preserved specimens consist of

Dawsonites
figure 8.85 Suggested reconstruction if Trimerophytonrobus-tius (Devonian). (From Taylor and Taylor, 1993.)

small fragments up to 3.0mm wide. The conducting strand is protostelic, centrarch, and composed of tracheids with scalariform to bordered pits. Some specimens of Hostinella are thought to represent an isolated branching fragment of Margophyton (Psilophyton) goldschmidtii, now believed to be a Lower Devonian member of the zosterophylls (Zakharova, 1981). A Late Silurian (Pridoli) specimen from New York State includes preserved apices and a rootlike structure (Edwards et al., 2004).

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