SCIENCE

Why do plants’ leaves shrink the further from the Equator they grow? It may all have to do with maintaining a comfortable temperature. (New Scientist)

Sure, they can get up and leaf. But can plants hear?

Teachers, scroll down for our quick list of key resources in our Teachers Toolkit.

Discussion Ideas

• New research hints that climate contributes to the size of a plant’s leaves. What are leaves?
  • Leaves are important organs of vascular plants. Most leaves are specialized for photosynthesis, the amazing process by which plants convert light energy to chemical energy.
    • In most plants, leaves are broad, flat, and attached to the plant’s stem to form the shoot. In some aquatic plants, leaves are submerged underwater. Some plants don’t have leaves at all.
    • “Leaves vary greatly in size, from less than 1 square millimeter to almost 1 square meter. Large-leaved plants like bananas and palms tend to live in the tropics, while small-leaved plants like heather and clover are found closer to the poles.”

• Why are leaves of tropical plants generally much larger than the leaves of plants in temperate or polar regions?
  • Climate control. Leaves are susceptible to both night-time freezing and daytime overheating.
    • Balancing these vulnerabilities depends on two major factors: “The first is how much water the leaf has available to cool itself down via transpiration … The second is the boundary layer: a pocket of still air that surrounds each leaf and acts as an insulator.”
      • Large leaves have thicker boundary layers, which makes it “hard to stay warm during cold nights, increasing the risk of frost damage.” This makes large leaves an unfavorable feature in chilly climates.
      • Perhaps just as interesting, the new research provides evidence that large leaves may be an unfavorable feature for desert plants as well as Arctic ones. Large leaves’ thick boundary layer often retains too much heat during the day. Plants in humid climates (such as rain forests) can counteract this by using transpiration, but desert plants cannot afford to lose that moisture.

• Why is the new research surprising to many people?
  • Many botanists overestimated the importance of water availability to leaf size. Much “conventional wisdom is that leaf size is limited by the balance between how much water is available to a plant and the risk of overheating.”
  • Many people also overestimated the risk of overheating to leaf size. “What we’ve been able to show is over perhaps as much as half the world the overall limits to leaf size are much more set by the risk of freezing at night than the risk of overheating during the day.“

• How might a better understanding of leaf size help predict how climate change may alter plant distribution?
  • The current period of climate change is associated with much warmer climates and arid conditions. “[I]f a particular region becomes warmer and drier, you may see smaller-leaf species replacing larger-leaf species because they’re better adapted to the new climatic conditions.”

• Get outside and do a plant survey! What is the average leaf size of plants in your neighborhood?
  • Are these plants native or introduced species?
  • How might human activity have impacted plant distribution in your neighborhood?
  • Is there a difference in leaf size between outdoor plants and indoor plants? If so, what might be some reasons for the difference?

TEACHERS TOOLKIT

New Scientist: We may finally understand why tropical plants have huge leaves

BBC: Clues to why leaves come in many sizes

Nat Geo: Can Plants Hear?