8CHAPTER 1~--,FIGURE1.5Epiphytic air plants growhigh above theground on the limbs of trees in tropical rain forests.PhotobyR. E. Ricklefs.Plants use the energy ofsunlight toproduce organic matterAll ecologicalsystemsdependon transformationsofenergy. For most systems, the ultimate source of thatenergy is sunlight. Plants and other photosyntheticorgan-isms use the energy of sunlight to synthesize organic mol-ecules from carbon dioxide and water. On land, mostplants have structures with large exposed surfaces-theirleaves-tocapture the energy of sunlight. Their leavesare thin because surface area for light capture is moreimportantthan bulk. Rigid stems support their aboveg-round parts. To obtain carbon, terrestrial plants take upgaseous carbon dioxide from the atmosphere. At the sametime, they lose prodigious amounts of water by evapora-tion from their leaf tissues to the atmosphere. Thus, plantsneed a steady supply-to replace water lost during photo-synthesis. Not surprisingly, most plants are firmly rooted
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