Soundings: Wind is linked to trees
March 16, 2007
Even as the oceans teemed with life, a bit less than half a billion years ago, no plants grew on the land. The light was better (brighter), herbivores were nonexistent, but hauling out onto dry land was too big a challenge – it was too dry.
Desiccation is death.
The leaves of plants today are covered with a water-proof barrier – a polyester-and-wax cuticle. This evolutionary “invention” allowed the first plants to leave the sea and colonize the land.
Walls work both ways. The cuticle kept water in, but it also kept essential carbon dioxide out.
Plants today have breathing holes – stomata (singular: stoma) – that they open to bring carbon dioxide into the leaf, and close if they lose too much water.
Most of a plant’s nutrients arrive dissolved in the water culled from the soil by its roots. Roots can’t push this nutrient-laden water up to the leaves, however. The leaves must pull it up from the roots.
Touch a drop of water, and draw your finger away slowly. The water stretches to follow – water is “sticky.”
As water molecules evaporate from a leaf’s stomata, they pull on the molecules below, which pull on molecules further back, which pull on molecules below them. An unbroken chain of water molecules runs from the stomata to the roots.
The whole process starts with water evaporating from the stomata.
Early in the 20th century, biologists noticed that the leaves of trees growing in warm climates were likely to have smooth edges, while leaves in cooler climates were likely to have edges that were rough, or toothed. They quantified the relationship in the 1970s. By counting how many plants had toothed leaves, and how many had smooth leaves, biologists found an amazingly close correlation with a region’s average temperature. The more plants with smooth-edged leaves, the higher the region’s average temperature.
Dana Royer (Wesleyan) and colleagues believe their data explain why this correlation may have evolved: Trees growing in cooler climates are limited by the length of their growing season. Toothed leaves allow them to make their growing season longer.
On a hot, still day, it’s hard to cool off. A bit of your sweat evaporates, but the air closest to your body – the boundary layer – soon holds as much moisture as it can. The air feels muggy.
A slight breeze – say, from a fan – sends fresh, less-moist air over your skin. Evaporating your sweat, it cools you off.
In regions that experience winter – regions outside the tropics – early spring can bring sunny days, perfect for photosynthesis … but the cool, moist air can’t evaporate enough water to pull nutrient-rich sap from the roots.
A slight breeze might help, but not enough. The boundary layer still becomes saturated, unable to hold any more moisture.
Designers of cars and planes want air to flow smoothly past their vehicles – the less air the vehicle feels, the better. To generate this smooth flow, they make edges as smooth as possible.
Same trick backwards: As air flows past the teeth on the edge of a leaf, it’s diverted this way and that. By generating turbulent – unsmooth – flow, leaves pull more air past their stomata, to evaporate more water.
Forty million years ago, the same streams that dropped nuggets of gold in what are now the Sierra foothills dropped leaves that had fallen from trees on their banks. The leaves fossilized.
Applying the leaf-edge/temperature correlation to fossil leaves, we’ve got a paleothermometer that can tell us a region’s average temperature millions of years ago.
Judging by its leaves, Nevada County, 40 million years ago, was sub-tropical – rather like Mexico today.
Alan Stahler trained as a biologist and is an amateur astronomer. He teaches private enrichment classes for students of all ages. His science programs can be heard at noon on alternate Tuesdays on KVMR-FM (89.5).