Shown is a mockernut hickory (Cara tomentosa) leaf from a young tree in my back yard. I plucked the leaf so that my kids could feel that it is fuzzy underneath, which is how you know it’s a mockernut and not some other hickory species.
Some features of the leaf pointed out in the picture:
1. This is one leaf made up of five leaflets. As far as I can tell scientists are still puzzling over why some plants have developed compound leaves, while other plants have simple leaves. Here is one hypothesis. Are there other mathematical models that can help us understand compound leaves?
2. Many species of insects “mine” leaves. As larvae they live inside the leaf and eat away at the leaf’s insides. These mines may have been made by moth, sawfly, fly, beetle, or wasp larvae. What rules does the miner follow to determine the shape of the mine? Can we model it mathematically?
3. I’m not sure why leaf edges of some plants are toothed while those of others are smooth, but botanists have discovered that there is a correlation between leaf-margin toothedness and climate (particularly mean annual temperature). The more toothed leaf species present, the colder the climate. So fossilized leaves can be used as a sort of thermometer frozen in time.
See for example: leaf margins and climate.
4. Insects that eat leaves are an important link in the chain that passes energy from the Sun, to plants, and then to animals. When you see leaf damage like this think of it as energy being harvested to fuel bird flight.
5. This leaf is changing from green to yellow to brown for the Fall. As circulation to the leaf shuts down, chlorophyl breaks down and is not replenished. We can understand how and why leaves change color, but why is it so beautiful to us? Maybe what matters is that it is beautiful to us.
6. The stalk that attaches the leaf to the stem is called the petiole. In some plants leaves have no petiole. In some other plants the petioles have no leaves! Hickory has both a fairly long petiole and substantial leaf.
7. These leaf spots are probably caused by a fungus or a bacterium living in the leaf. The parasite probably doesn’t provide any benefit to the hickory tree, but it doesn’t cause much damage either. Parasites don’t usually engender warm fuzzy feelings in us. Arguably, there is good reason for that. Then again, they are fellow creatures trying to make a living. If they’re not doing much harm that’s probably more than we can say for ourselves. Who are we to deny them? Maybe the biodiversity they add provides something to the ecosystem that we’re not seeing yet. What probability distribution might the size and frequency of the leaf spots follow?