A classification of land plants is given in Table 12.1. Informal groupings are used in this scheme. As an artificial classification it provides a working description of plant diversity rather than an explanation of the evolutionary relationships. Some of the groups are unnatural, for example "seed ferns" incorporates the seed plants that are not included in the other groups.
Land plants are separated into those with a vascular system and those without. There are three important groups of non-vascular plants, all of which are extant: the hornworts, liverworts, and mosses. Vascular plants are separated into seedless and seeded plants. Seedless vascular plants include three extinct groups, and three groups with living relatives: club mosses, ferns, and horsetails. Vascular seeded plants are divided into plants with naked seeds, the gymnosperms, and plants with seeds enclosed by a fruit, the angiosperms. The most important groups of gymnosperms are the conifers, cycads, and ginkgoes. Angiosperms are the flowering plants, the dominant living flora.
Table 12.1 Plant classification, based on common morphological characteristics and not common lineage. These are the most useful groupings for fossil plants.
Similar to liverworts but sporangia are able grow continuously
Small plants with a flattened leaf-like body (thallus)
Filamentous mat with simple leaves and root-like structures (rhizoids)
Vascular seedless plants
Extinct plants with dichotomously branched, simple axes. Sporangia are terminal
Lycophytes (club mosses)
Leafy plants with sporangia on the upper surface of leaves or at the leaf-stem intersect
Extinct dichotomously branched plants, sometimes with spiny axes. Sporangia on the side of axes
Extinct fern-like plants with woody tissue
Vascular seedless plants (cont'd)
Large-leaved plants with sporangia on the lower surface
Plants with leaves and branches fused in whorls. Fertile branches have terminal cones
Vascular seeded plants: gymnosperms
Seed ferns Fern-like plants with spores on the leaves
Woody trees with needle or scale-like leaves. Seeds in cones
Woody, stemmed plants with palm- or fern-like leaves and cones
Extinct plants that resemble cycads but have flower-like cones
Woody trees with fan-shaped leaves. Cones absent
Unusual group with cone clusters resembling flowers
Vascular seeded plants: angiosperms
Angiosperms Plants with flowers. Seeds enclosed
(flowering plants) within a fruit
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