An explosion of grasses and herbs As in Late Cretaceousand Paleogene times, flowering plant fossils representour best gauge of climatic shifts. In the world ofplants, the Neogene Period might be described as theAge of Grasses or the Age of Herbs. Whereas grasses arewidespread on Earth today, they occupied relatively littleland early in the Cenozoic. Grasses have exploded in diversityduring the latter part of Cenozoic time. Today theyinclude some 10,000 species (Figure 19-3). Grasses containsmall bodies of silica, termed phytoliths, that weardown the teeth of animals that graze on them, and particularkinds of silica bodies indicate that grasses adaptedto open habitats expanded between Late Oligocene andEarly Miocene time. Herbs, or herbaceous plants, aresmall, nonwoody flowering plants that die back to the ground after releasing their seeds. (Defi ned in this way,herbs include many more plants than the few we use toseason our food.) The Compositae, a family of herbs thatincludes such seemingly diverse members as daisies, asters,sunfl owers, and lettuces, appeared near the beginningof the Neogene Period, only 20 million or 25 millionyears ago. Today this family contains some 13,000 species,including nearly all the plants that ecologists refer to asweeds (see Figure 19-3). As any gardener knows, weedsare exceptionally good invaders of bare ground. They maynot compete successfully against other plants to retain thespace they invade, but they soon disperse their seeds toother bare areas that have been cleared by fires, floods, ordroughts and spring up anew.Grasses and herbs fl ourish in seasonally arid climates—grasses because they require substantial sunlightand the lack of summer moisture excludes shadegeneratingtrees, weeds because seasonal dryness causesother plants to die back, providing bare ground for themto invade. The recent success of grasses and herbs is primarilythe result of the worldwide deterioration of climatesduring this interval.
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