Cold weather compromises the ability of leaves on deciduous trees to create food through photosynthesis. Their thin cells, which are filled with water, would easily burst when it freezes, turning the leaves into dead weight no longer able to produce food. This unproductive excess surface area would catch more wind during winter and could lead to broken limbs. Leaves would also burden the tree by collecting snow and ice.
Plus, come autumn, many leaves are already injured by insects and disease.
To reduce these liabilities, the tree starts to grow a layer of cells between the leaf and the point at which it is attached to the branch. Eventually, this “wall” severs the leaf from the tree. The process is called abscission.
- Shape show-offs! The term for when a tree grows more than one leaf shape on the same plant is heterophylly.
- The old soft shoe: Do the insoles of your shoes wear out? Our ancestors inserted the soft, fuzzy leaf of the mullein plant into their footwear. It not only served as a cushion that reduced blisters, but medicinal properties in the leaf are said to prevent infection. Caveat: Some people are highly allergic to mullein.
- Are you a foliage foodie? Animals that love to lunch on leaves are called folivores.
- Malicious mulch: Don’t add black walnut leaves to your mulch or compost pile. They contain juglone, a chemical that is toxic to other plants.
- Re-leaf for what ails you: One superstition holds that if you catch a falling leaf on the first day of autumn, it protects you from getting sick that winter.
A leafy quiz
Leaves are a tree’s food factory. Here is a list of parts that keep them functioning. Can you match these parts to their job and place on the diagram? Answers are below.
- Leaf margin
- Secondary veins
A. I am the tip of the leaf that is farthest from where it is attached to the stem. My shape ranges from sharply pointed to rounded, depending on the plant. While I have no specific function that is common in all leaves, I do serve a purpose in some. If I’m sharp and pointed, I deter herbivores by making it uncomfortable for them to eat the leaf. If I’m curved, in wet climates I help to drain water off a leaf more quickly.
B. I am the central, supporting ridge running down the center of the leaf. I provide the strength that keeps a leaf facing the sun and upright during strong winds. I also help to transport nutrients produced in the leaf during photosynthesis to other parts of the plant.
C. I am not as thick as the midrib I emerge from, and I extend to other parts of the leaf. I am one of the many tubes that help the midrib support the leaf and transport nutrition.
D. I am the thin flat membrane that covers the leaf, and I am sometimes referred to as the blade. I am the surface in which photosynthesis occurs.
E. I am the edge of the leaf. I can be lobed, simple, smooth or toothed. Most often, I am the part that helps an observer identify a plant.
F. I am the tiny stalk that attaches the leaf to the stem. Because of me, the leaf is able to twist so that it faces the sun. I am not found on all leaves: These leaves are called sessile.
G. I am the part of the plant where a new leaf starts to form, usually in early spring.
H. I am one of the many pores in the leaf’s surface that regulates the intake or release of water (transpiration) by opening or closing as needed. I also help the leaf control the carbon dioxide needed for the plant to create food. When there is too much water or carbon dioxide, I close; when there is too little, I open up.
- A. Apex, 6
- B. Midrib, 4
- C. Secondary veins, 1
- D. Lamina, 7
- E. Leaf margin, 8
- F. Petiole, 3
- G. Bud, 5
- H. Stoma, 2