Eudicots first appear in the fossil record about 125 million years ago and, by 90 to 80 million years ago, most of the major groups we see today were present. Today, there are estimated to be approximately 160,000 species of eudicots, nearly three-quarters of all angiosperm species (Fig. 33.23). Eudicots are well represented in the fossil record, in part because their pollen is easily distinguished. Each eudicot pollen grain has three openings from which the pollen tube can grow, whereas pollen from all other seed plants has only a single opening. Eudicots take their name from the fact that they produce two cotyledons, whereas monocots produce one. But, because the magnoliids and early angiosperm groups also have two cotyledons, this largest of all angiosperm groups is referred to as the “eu-” or “true” dicots.

Many eudicots produce highly conductive xylem. High rates of water transport, and thus high rates of photosynthesis, may explain why eudicot trees were able to replace magnoliid trees as the most ecologically important members of forest canopies. In addition, many eudicot trees lack strong apical dominance and produce crowns with many spreading branches. Today, tropical rain forest trees are an important component of the diversity of eudicots. Important eudicot trees in temperate regions include oaks, willows, and eucalyptus.

708

At the other end of the size spectrum are the herbaceous eudicots, which make up a substantial fraction of eudicot diversity. Herbaceous plants do not form woody stems. Instead, the aboveground shoot dies back each year rather than withstand a period of drought or cold. At the extreme are annuals, herbaceous plants that complete their life cycle in less than a year, persisting during the unfavorable period as seeds. Annuals are unique to angiosperms and the majority of these annuals are eudicots. Why might eudicots be successful as both trees and herbs? One possibility is that the ability to produce highly conductive xylem may allow herbaceous eudicots to grow quickly and to produce inexpensive and thus easily replaced stems. Examples of herbaceous eudicots include violets, buttercups, and sunflowers.

The herbaceous growth form appears to have evolved many times within eudicots as tropical groups represented by woody plants expanded into temperate regions. For example, the common pea is an herbaceous plant that resides in many vegetable gardens; its relatives include many tropical rain forest trees. Peas are members of the legume, or bean, family, some of which can form symbiotic interactions with nitrogen-fixing bacteria (Chapter 29). The ability to trade carbon for nitrogen may explain why legumes are important in many habitats. Many of the trees of the African savannas, as well as trees in seasonally dry regions throughout the world, are legumes.

Eudicots are diverse in other ways as well. Most parasitic plants and virtually all carnivorous plants are eudicots, as are water-storing cacti that grow in deserts. Some, such as roses and blueberry, are woody shrubs. Others grow as epiphytes in the canopy of rain forests, and still others, such as grapes and honeysuckle, are vines. Perhaps the most unusual are the strangler fig trees. These begin as epiphytes and then produce roots that descend to the forest floor and fuse to form a solid cylinder that “strangles” their host tree. Finally, eudicots make a significant contribution to the diversity of the dinner table. Apples, carrots, pumpkins, and potatoes are all eudicots, as are coffee, cacao (the source of chocolate), and tea. Many plants with oil-rich seeds, such as olives, walnuts, soybean, and canola, are eudicots, as are buckwheat and quinoa.

The reproduction, growth, and physiology of angiosperms are summarized in Fig. 33.25 on pages 710 and 711.