Dicots (170,000 spp.)
for the transport of water and nutrients to upper plant parts, specialized roots for absorption, a waxy cutin layer on leaf surfaces to reduce evaporation, and the production of lignin to provide structural strength for taller plants. Before the Cretaceous age, 144 million years ago, in the time of the dinosaurs, ferns dominated in tropical climates, forming large trees. Although the living plants are now gone, their carbonized remains are the basis of today's coal deposits. The familiar fern plant is a sporophyte that produces spores on the underside of the fronds. When the spores fall on a suitable surface, they produce a small gametophyte. This, in turn, produces eggs and motile sperm cells that fertilize and grow into a mature sporophyte. Ferns may also reproduce asexually from horizontal stems called rhizomes.
The third group is the gymnosperms, or "naked seed'' plants. The seed is formed after fertilization. It contains the embryo, which is a young sporophyte. It also contains a quantity of starch for use by the embryo during germination. An outer seed coat provides protection. In conifers, gnetophytes, and angiosperms, seeds are formed when the ovule, which contains the egg, is fertilized by the pollen grain, the male gametophyte. This arrangement eliminates the requirement that there be free water present in order for fertilization to happen, as is needed by all seedless plants.
Of the four gymnosperm divisions, by far the most familiar is that of the conifers. They include 50 genera with 550 species. One is the redwood (Sequoia sempervirens), the tallest terrestrial plant, at up to 11 m in diameter and 117 m high. Others are the pines, firs, spruces, hemlocks, cypresses, junipers, and yews. Most bear their seeds in cones, except for the yew, which contains them in a fleshy fruitlike structure. The bark of the Pacific yew (Taxus brevifolius) has been found to contain a substance, called taxol, that is used as a treatment for breast and ovarian cancer. Most conifers keep their leaves for several years and do not lose them all at once. A few conifers, however, are deciduous; that is, they lose their leaves at the end of the growing season. These include the European larch (Larix decidua) and the bald cypress (Taxodium distichum).
The cycads form tall trees that resemble palms, with the difference that the trunk is covered with the bases of leaves that have been shed as the tree grew taller. New leaves are grown only at the top of the trunk. (Palm trees are monocot angiosperms.) Cycads produce compounds that are neurotoxic and carcinogenic. They harbor cyanobacteria and thus contribute to nitrogen fixation. Gnetophytes include many unusual plants, such as Welwitschia, which grows mostly below the soil, as well as trees, vines, and shrubs. Some are very similar to dicotyledonous angiosperms. Ginkgo includes a single species of tree, Ginkgo biloba, that was known to the West from fossils but was thought to be extinct. It was subsequently found cultivated on temple grounds in China and Japan, but apparently was not found anywhere in the wild. It is now cultivated throughout the world and is easily identified by its unique fan-shaped leaves. It is exceptionally resistant to air pollution and thus has found use in urban parks and along roadsides.
The final group is that of the angiosperms, the flowering plants. Angiosperms have dominated the land for 100 million years. Their rise paralleled that of the mammals, both of which were relatively insignificant during the tenure of the dinosaurs. The single division of the angiosperms is called anthophyta, making these two terms synonymous. They are distinguished by the presence of flowers, fruit, and a distinctive life cycle. Most familiar plants are angiosperms, from the millimeter-sized duckweed (Lemmaceae) to 100-m-tall Eucalyptus trees; from the aquatic water lily (Nymphaea odorata) to the Saguaro cactus (Carnegiea gigantea). Although mostly autotrophic, it includes parasitic and saprophytic species. For example, the Indian pipe (Monotropa uniflora) lacks
chlorophyll but obtains nutrients from the roots of other plants through an association with fungi.
The angiosperms are divided into two classes: monocotyledones and dicotyledones (monocots and dicots, for short). The names refer to the embryonic leaves of the seed. Monocots have a single leaf (called a cotyledon), dicots have two. They can be distinguished by looking at the starchy food-storage part of the seed, called the endosperm. Rice and corn are monocots and have a single endosperm. Dicots, such as peanut or bean, have two. Even without seeing the seeds, the two are easily distinguished. Monocot leaves have parallel veins, as in grasses. Dicots have a network venation, such as in oak or maple leaves. In monocots the vascular bundles (described in the next section) are usually arranged throughout the cross section of the stem, as in celery. The vascular bundles of dicots are arranged in a ring (Figure 7.1).
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