Student 1

Plant woody, evergreen; true roots present, endomycorrhizal; vascular cambium + [xylem differentiating internally, phloem externally]; wood homoxylous, tracheids +; tracheid/tracheid pits circular, bordered; sieve tube/cell plastids with starch grains; stem cork cambium superficial, root cork cambium deep seated; nodes ?; leaves spiral, simple, axillary buds?, prophylls two, lateral; plant heterosporous [with microspores (pollen) in stamens and megaspores in ovules]; sporangia eusporangiate [i.e. developing from a group of superficial cells and with walls two or more cells across], on sporophylls, aggregated in indeterminate cones/strobili; pollen mono[ana]sulcate, germinating distally and lacking proximal marks where they were attached to other members of the tetrad, pollen exine and intine homogeneous, ovules unitegmic, crassinucellate, megasporangium indehiscent, monosporic [only one megaspore develops from meiotic tetrad and germinates to form female gametophyte]; male gametophyte development endosporic [with first divisions inside the pollen grain], gametes two, with cell walls; female gametophyte endosporic, initially syncytial, walls produced later surrounding individual nuclei; seeds "large", first cell wall of zygote transverse, embryo straight, short-minute, with morphological dormancy, cotyledons 2; plastid transmission maternal. [Back to Index]

Estimates of the age for crown-group seed plants, i.e., extant seed plants, range from 348-285 million years before present (e.g. Becker et al. 2000; Leebens-Mack et al. 2005).

SEED PLANT EVOLUTION (still being developed)

Sister to the monilophytes, ferns and their relatives, are the lignophytes, and the split between the two is old, occuring mid- to later Devonian, some 375-400 million years before present (Pryer et al. 1995, 2000, 2001a, 2004; Schneider et al. 2002). Lignophytes have distinctive secondary thickening, a vascular cambium which is bifacial, producing secondary phoem externally and secondary xylem internally (e.g. see Donoghue 2005). Thus lignophytes often have stems with large amounts of wood, and there is also bark produced by the separate cork cambium. The origins of seed plants, the focus of this site, are to be sought in the mid-Devonian progymnosperms, often homosporous plants with complex leaves and well-developed secondary thickening with much parenchyma mixed in with the tracheids (see below). The leaves proper may have been small, although the branch systems as a whole may have been flattened; there are no obvious leaf traces. The origin of a stele more like that of extant spermatophytes can be seen in taxa in which the solid central vascular tissue became ridged and dissected into vertical columns, pith developing (Stewart & Rothwell 1993 for a good summary).

Seed plants or spermatophytes in general are characterised by heterospory, i.e. the plant produces a number of microspores ("pollen") per microsporangium and a single megaspore per megasporangium (Kenrick & Crane 1997). (Heterospory has evolved several times in land plants; in seed plants, the dividing megaspore receives its nutrition from the parental sporophyte, whereas in other heterosporous land plants megaspore development is independent of the sporophyte.) The ovule is the megasporangium and enveloping integument, and the seed is an ovule when it drops from the plant; ovules are known from the Devonian (Stewart & Rothwell 1993). Some plants with the kind of vegetative morphology described in the previous paragraph may have been heterosporous, stems having been found in the same fossil beds as fossilised seeds (see e.g. Beck 1962, 1981; Carluccio et al. 1966; Namboodiri and Beck 1968). The progymnosperm Archaeopteridales and Aneurophytales are fossil orders of plants of this kind; they have a relatively very rich fossil record, and may have morphologies very unlike those of any living plant, whether angiosperm or gymnosperm. In the Carboniferous in particular there was a considerable variety of plants with fern-like leaves and ovules, the seed ferns or pteridosperms (the Carboniferous has sometimes been called the age of ferns, rather, it is the age of tree ferns). Recent studies are helping to clarify their morphology (Taylor et al. 2006 and references, also other papers in J. Torrey Bot. Soc. 133(1). 2006), and this will help us to understand the phylogeny of seed plants as a whole. Seeds of Mesozoic seed plants are very diverse in their morphology (e.g. Anderson & Anderson 2004), and Bennettitales (perhaps close to Gnetales - see below), Corystospermales (pteridosperms), which seem to have survived the end-Cretaceous mass extinction in Tasmania (McLoughlin et al. 2008), and conifers were all radiating in the Triassic. Indeed, by the lower Carboniferous there were to be found the rather conifer-like Cordiatales, which had compound pollen-bearing structures and saccate pollen, and slightly later, the still more conifer-like ("ancestral") Voltziales.

Note that a major distinction has been drawn between manoxylic and pycnoxylic groups, secondary xylem of the former has much parenchyma mixed in with the tracheids, in secondary xylem of the latter, there is much less parenchyma. The cycadophytes, which include seed ferns, cycads, and the immediately unrelated cycadeoids (e.g. Bennettitales), have the former kind of wood, while the coniferophytes, which include