Angiosperms are the most diverse and widespread group of living plants (Fig. 12.13). Leaf impressions of angiosperm-like plants are known from the Triassic. The first true angiosperm fossils are from the Cretaceous (Fig. 12.14). During this period angiosperms diversified rapidly, particularly in low latitudes, and dominated most habitats by the end of the Cretaceous. Paralleling the rise of the angiosperms, spore-bearing plants and gymnosperms declined through the Cretaceous. Abundance and diversity decreased in most other plant groups; although conifer diversity remained relatively stable they were increasingly polarized, geographically, into marginal (high altitude) environments.
Angiosperms reproduce sexually using flowers. Flowers are essentially clusters of modified leaves, some with a reproductive role. There are over 235,000 living angiosperm species compared with 720 living species of gymnosperms. Angiosperm success is attributed to the following evolutionary developments:
1 A vascular tissue with vessels that provides support and transports water more effectively.
2 Diverse pollination mechanisms including the pollination of flowers by insects. These are more efficient systems than random wind pollination.
3 An enhanced reproductive process, including double fertilization, resulting in a plant embryo and a food source contained within the seed.
4 The enclosure of developing seeds within a fruit that protects the seeds and also aids seed dispersal.
5 The enclosure of the gameophyte generation within the sporophyte seed enabling angiosperms to colonize a much wider range of habitats.
Catamites (a sphenophyte) is the stem remains of the extinct giant horsetail, although it is commonly used to describe the entire plant. Catamites is usually preserved as a cast. The outer part of the stem was made of resistant wood but the center was formed of softer tissue, pith. After death, the pith decayed rapidly leaving a hollow, wooden cylinder. Sediment then infilled the cavity, hardened, and formed a "pith cast". The texture of the outer surface of the fossil reveals the structure of the plant's vascular system on the inside of its stem. The figured stem fragment is 18 cm in length.
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