Lycopsids, small and large
The clubmosses, Class Lycopsida, arose at the same time as the rhyniopsids and other dichot-omously branching plants, but they are distinguished by having their sporangia arranged along the sides of vertical branches, instead of at the tips, and by having numerous small leaves attached closely around the stems.
Low herbaceous lycopsids existed throughout the Devonian and Carboniferous, and they showed considerable variation in leaf and sporangium shape, and in the nature of the spores. From the Late Devonian onwards, most lycopsids produced two kinds of spores, small and large (microspores and megaspores), that developed within terminal cones. Lycop-sids are represented today by some 1100 species, all small herbaceous forms.
During the Carboniferous, several lycopsid groups achieved giant size, and these are the dominant trees seen in reconstruction scenes of the great coal swamps of that period. The best known is Lepidodendron, a clubmoss that reached 35 m or more in height. Fossils of Lepidodendron have been known for 200 years because they are commonly found in association with commercial coalfields in North America and Europe. At first, the separate parts - roots, trunk, bark, branches, leaves, cones and spores - were given different names, but over the years they have been assembled to produce a clear picture of the whole plant (Fig. 18.9).
The giant lycopsids were adapted to the wet conditions of the coal swamps, but these habitats receded at the time of a major arid phase in the latest Pennsylvanian and Early Permian. Lepidodendron and its like died out. Medium-sized lycopsids, about 1 m high, existed during the Mesozoic, but truly arborescent ("tree-like") forms never evolved again.
The horsetails, or equisetopsids, are familiar to gardeners as small pernicious weeds. Their upright green shoots, with a characteristic jointed structure, are linked by underground rhizome systems. The sporangia are grouped into bunches of five or 10 below an umbrellalike structure arranged along the stem to form a sort of cone, a unique feature of the group. The horsetails are a small group today, consisting of a mere 15 species, most of them small, but one reaching a height of 4 m or more. The early history of the group shows much greater diversity.
The horsetails arose during the Devonian, and Carboniferous forms flourished in disturbed streamside settings where they could
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