The approximately 630 species of conifers are, for the most part, trees. Conifers include the tallest (over 100 m) and oldest (more than 5000 years) trees on Earth (Fig. 33.17). Well-known conifers include pines, junipers, and redwoods. Most conifers have strong apical dominance, and thus develop a single straight stem from which extend much smaller horizontal branches. Conifers have traditionally served as masts for sailing ships, and today are used for telephone poles. Conifer xylem consists almost entirely of tracheids, and so its wood is strong for its weight, making it well suited as a building material. Conifers supply much of the world’s timber, as well as the raw material for producing paper.

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Most conifers are evergreen, meaning that they retain their often needle-like leaves throughout the year. In some species, the leaves are remarkably long lived, remaining green and photosynthetic for decades. Many conifers produce resin canals in their wood, bark, and leaves that deter insects and fungi. Conifer resins are harvested and distilled to produce turpentine and other solvents. In addition, conifer resins are the source of a number of important chemicals, including the drug taxol used in the treatment of cancer. Amber, prized for its beauty, is fossilized resin.

Conifer reproduction tends to be slow: After pollination, it can take more than a year before the seeds are ready to be dispersed. Pollen is produced in small cones, and seeds in larger cones. The seed cones become woody, providing protection, as the fertilized ovules develop slowly into seeds. Conifers are wind pollinated, and most species also rely on wind for seed dispersal. However, some conifers, such as junipers and yews, produce seeds surrounded by fleshy and often brightly colored tissues that attract birds and other animals.

Conifers dominate the vast boreal forests of Canada, Alaska, Siberia, and northern Europe. Conifers tend to become more abundant as elevation increases, and they are common in dry areas, such as the western parts of North America and much of Australia. Only a small number of conifer species are found in tropical latitudes, most commonly at higher elevations. Before the rise of the angiosperms, however, conifers were more evenly distributed across the globe, including in the lowland tropics. One of the great questions in the evolutionary history of plants is why conifers were displaced from tropical latitudes by the angiosperms but continued to thrive in cold and dry environments.

An important difference between angiosperms and conifers is that angiosperms transport water in the xylem vessels, whereas conifers transport water in tracheids. In wet and warm environments, the xylem vessels of angiosperms can be both wide and long, increasing the efficiency of water transport through their stems. Thus, one hypothesis is that the evolution of xylem vessels gave angiosperms a competitive advantage in moist tropical environments. With more water flowing through their stems, angiosperms could have higher rates of photosynthesis and grow larger leaves. Once conifer populations began to dwindle, they could have disappeared completely because wind pollination requires large populations. In colder or drier regions, the dangers of cavitation due to drought or freeze followed by thaw (Chapter 29) constrain the size of angiosperm vessels. This helps level the playing field and explains the greater coexistence of conifers and angiosperms in cold or dry climates.

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