Land plants are classified into two major groups according to the absence or presence of vascular tissue, as detailed in Figure 14.8. Plants that lack vascular tissue formed of specialized cells for the transport of water and nutrients are referred to as nonvascular plants. The bryophytes, liverworts, mosses, and hornworts are seedless and nonvascular, and likely appeared early in land plant evolution. Vascular plants developed a network of cells that conduct water and solutes through the plant body. The first vascular plants appeared in the late Ordovician (461–444 million years ago) and were probably similar to lycophytes, which include club mosses (not to be confused with the mosses) and the pterophytes (ferns, horsetails, and whisk ferns). Lycophytes and pterophytes are referred to as seedless vascular plants. They do not produce seeds, which are embryos with their stored food reserves protected by a hard casing. The seed plants form the largest group of all existing plants and, hence, dominate the landscape. Seed plants include gymnosperms, most notably conifers, which produce “naked seeds,” and the most successful plants, the flowering plants, or angiosperms, which protect their seeds inside chambers at the center of a flower. The walls of these chambers later develop into fruits.
To learn more about the evolution of plants and their impact on the development of our planet, watch the BBC show “How to Grow a Planet: Life from Light” found at this website (http://openstaxcollege.org/l/growing_planet2).
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