Seed-bearing plants are divided into two groups. Those with exposed (naked) seeds, the gymnosperms, and plants that flower and produce seeds within a fruit, the angiosperms. Plants with seeds first appeared in late Devonian times and proliferated during the Upper Palaeozoic. Gymnosperm development peaked in the Mesozoic. Seeds have four main reproductive advantages over spores:
1 A multicellular embryonic plant is held within the seed, whereas a spore is a single cell.
2 Seeds contain a food supply that nourishes the plant until it is self-sufficient.
3 Seeds have a resistant, protective coat.
4 Fertilization and pollination are independent of free water.
With the evolution of seeds, plants were no longer restricted to damp habitats for reproduction and the potential for dispersal became much greater.
Seed ferns flourished in the Carboniferous and Permian. Although superficially similar to spore-bearing ferns their internal structure is very different. Seed ferns probably developed from progymnosperms (Fig. 12.9).
The origin of conifers is uncertain but they may have evolved from seed ferns or cordaitean plants. Occupying mainly dry environments, conifers were important plants in the Carboniferous and the Permian. At 30 m in height and with long, strap-like leaves, Cordaites was a very distinctive Upper Palaeozoic coniferophyte (Fig. 12.10).
Modern conifers can be traced back to the Triassic when the group underwent a major radiation. Cycads and bennettitaleans were important components of the Mesozoic vegetation although there are only a few living cycad genera. Similar to ferns and palms, cycads have large, compound leaves. The stems or trunks are usually unbranched and covered with scale-like leaf bases. Each "scale" represents a former leaf attachment site. Cycadeoids are very similar to cycads but they have different cone structures and the leaf traces are not preserved on the trunk (Fig. 12.11).
Presumably originating in the Permian, ginkgoes diversified in the Mesozoic and were widely distributed. They declined in the Cenozoic and only one ginkgo species survives to the present day. Ginkgoes are large, densely branched trees with entire or bilobed leaves. Fossil ginkgo leaves closely resemble the modern foliage. As living ginkgoes are deciduous, fossil species may also have seasonally shed their leaves.
Gnetales are a diverse group of gymnosperms with an increasing fossil record. The presence of flower-like cone clusters suggests that they might share a common ancestor with the angiosperms (Fig. 12.12).
Fig. 12.9 Seed ferns: (a) Callistophyton, a scrambling understorey fern (the fronds are approximately 50 cm in length), and (b) Medullosa, a tree fern with primitive compound leaves (height 10 m).
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