A large amount of time exists between the origin of land plants and the evolution of leaves with a flat blade (Kenrick 2001). For at least the first 40 million years of their existence, land plants were leafless or had only small spine-like appendages. The widespread appearance of megaphyll leaves, with their branched veins and planate form, did not occur until the close of the Devonian period about 360 million years ago (Beerling et al. 2001). (See Figure 9.10 for a figure of the geological time scale, which shows the Devonian period.) Beerling et al. (2001) proposed an explanation for this long delay. Drawing on anatomical and environmental data from the fossil record, they concluded that leaves with a broad lamina evolved in response to a massive drop in atmospheric CO2 during the Late Devonian period. The evolution of the megaphylls coincides with a drop of around 90% in atmospheric CO2 concentration (Beerling et al. 2001; Kenrick 2001). The fall was driven by plants. The evolution of land flora and, in particular, the advent of large plants, had a major impact on the carbon cycle by drawing CO2 out of the atmosphere and locking it up in trunks and leaves (Kenrick 2001). Also important was the evolution of roots. The physical and chemical effects of root systems on rocks and soils increase rates of weathering, which is thought to have been responsible for removing large quantities of CO2 from the Devonian atmosphere (Kenrick 2001).
Early Devonian land plants possessed few stomata. They occurred on leafless plants that possessed short, cylindrical, aerial stems (called "axes") or were on only minute leaves (microphylls) (Beerling et al. 2001). A planate leaf with the same stomatal characteristics would have suffered lethal overheating, because of greater interception of solar energy and low transpiration. When pla-nate leaves first appeared in the Late Devonian and subsequently diversified in the Carboniferous
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The use of dumbbells gives you a much more comprehensive strengthening effect because the workout engages your stabilizer muscles, in addition to the muscle you may be pin-pointing. Without all of the belts and artificial stabilizers of a machine, you also engage your core muscles, which are your body's natural stabilizers.