origins of seed ferns Free Essay, Term Paper and Book Report
Seed ferns constitute a heterogeneous assemblage of fossil plants that are characterized by having fernlike leaves on which ovules (or preovules) and pollen-producing (or prepollen producing) structures are borne (Taylor 1981; Stewart 1983). Ovules and microsporangia were borne on unmodified or only slightly modified foliage, and in this characteristic were evidently very unlike the coniferophytes, whose reproductive structures are born in cones. Ovules of some of the seed ferns are like that of the modern cycads, but their eusporangiate microsporangia differ in usually being fused to form a synangiate organ, e.g. in the Mississippian Telangium of the Lyginopteridae (Taylor & Taylor 1993; Stewart & Rothwell 1993). The large fronds of the pteridosperms were usually spirally attached on the stem, and often the rachis or petiole dichotomised at least once proximal to the first basal pinnae.The wood of the pteridosperms is constructed of thin-walled trachieds and parenchyma giving it a spongy organisation, while the outer cortical region shows a supporting system of longitudinal bands o......
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