In the late spring and summer, many parts of the United States experience smog alert days - days when the concentration of pollutants in the air has been considered unhealthy. Many of these pollutants are produced by automobiles - particularly hydrocarbons, carbon monoxide, nitrogen oxides and, indirectly, ozone. Most discussion of the adverse effects of smog center on humans, and the effects on other forms of life, such as plants, are scarcely mentioned. So how does air pollution from automobiles affect plant growth? Let's think about how this hypothesis might be tested. Two problems must be addressed. The first is how to obtain two sets of plants, one of which exposed to more automobile pollution than the other set. One way to do this would be to look at plants that are close to a busy road (within 10 meters) and compare them to plants that are a distance from such a road. The second problem is how to assess the health of the plants. One way is to measure the size of the leaves or needles in plants (evergreen needles are simply a modified leaf). Most photosynthesis in plants occurs in the leaves, so they are the "food factory" of the plant. Do you think a healthier plant is more likely to have larger or smaller leaves than a less healthy one? Look at the leaves and at the measuring devices available at Station 3 and think about the characteristic (length, width, mass, etc.) you will use to assess leaf size.