Leaf Anatomy - University of Wisconsin–Stevens Point：叶片解剖结构-威斯康星大学–史蒂文斯点.doc

Leaf AnatomyModified Leaves Leaf Anatomy Introduction In studying the characteristics of stems and roots in previous laboratories, you have seen structures specialized for physical support and for absorption and conduction of water and minerals. This lab deals with the third vegetative plant organ typical of land plants: the leaf. Leaves have evolved as plant organs that are (1) obviously adapted for light interception and (2) subtly adapted for carbon dioxide uptake from the atmosphere for photosynthesis. Because of their large surface area and because they have pores for gas exchange, leaves are very vulnerable to water loss. This vulnerability results in a final adaptation found in all terrestrial vascular plant leaves: (3) features that prevent excess water loss but at the same time reduce CO2 uptake. What is contradictory about a leaf preventing water loss? If the leaf were completely water tight, it would also be completely air tight, and the carbon dioxide supply would be cut off. That would be fatal! An ideal leaf would allow carbon dioxide to enter without allowing any water to escape. The best scheme that evolution has produced, a system of controllable pores, stomates, falls far short of that ideal. Stomate control of water loss will be dealt with extensively in a later lab, but even now it is valuable to know a little about stomate response. Stomates tend to open when conditions are good for photosynthesis. They tend to close when drying conditions are extreme. The balance between photosynthetic opportunity and drought liability determines: (1) Whether stomates are open or closed. (2) How much water is lost from the plant. (3) How much photosynthesis can occur. An average plant is likely to take up at least 20 times as much water as it needs to supply its cells, losing the excess to the atmosphere without any major benefit [minor benefits might include cooling and increased mineral uptake with the water]. Exercises Gross Leaf Anatomy and Pl