When Plants Defy Science

For a long time, plant scientists or botanists were quite puzzled by an ability of many plants that seemed to defy the laws of science. At question was how any plant could grow to over 33 feet tall. Understanding the wonder of it requires a little background of the laws that seem to be violated.

Capillary action

The air pressure at sea level and at room temperature is somewhat less than 15 pounds per square inch. This pressure pushes against all objects equally. Certain liquids, though, have a strong surface tension. The molecules stick to each other. Because of this, if the end of a small tube is put in a container of water, the air pressure pressing down on the water in the container causes the water to travel up the tube. The narrower the tube, the farther up the water will travel.

This is the basis of how the original barometer worked.

The action is called capillary action and it was believed at one time that capillary action was how plants got water from the roots to the top of the plant. Botanists knew at once that there was a problem with this idea. Even with a tube with a tiny diameter, water will only travel up to a certain point, under the force of gravity. That natural limit is about 33 feet.

What this means is that capillary action easily explains how a plant like a tomato gets water from the roots to the top leaves, it doesn’t explain how many trees that are over 33 feet tall get water to the top. If capillary action was the explanation for how water was transported to the top of all plants, no tree should be able to grow taller than 33 feet. It would be a violation of natural laws.

Transpiration solves the problem

Capillary action operates very much as a gravity pump, forcing water up a tube. It simply didn’t explain why any plant could be more than 33 feet tall. However, water doesn’t just flow to the leaves and then back down to the roots. Plants have tiny pores on the leaves that allow water to evaporate from the leaf surface. This is a process called transpiration. In a way, it functions in a similar way that perspiration works, yet it is totally different than perspiration.

As it happens, though, it explains how a plant can grow to more than 33 feet; the limit at which water can be forced up the tiny tubes in a plant by the gravity pump. Over-simplifying, instead of pumping or pushing the water to the top, the plant pulls the water to the top, due to transpiration.

Part of this goes back to the tremendous surface tension of water. The other part of it has to do with gravity, but in a different way that gravity is used in capillary action.

As water evaporates from the leaves at the top of a tall tree, it creates a partial vacuum in the transport system of the plant. Since water tries to stick together, this drop in gravity within the transport tubes of the plant draws water up from below. The amount of pressure differential is tremendous, too. The drawing force of water up the transport tubes is many times higher than the force of gravity that operates the capillary pumps. It is equivalent to over 1,000 pounds per square inch!

Although the mechanism of transpiration is tremendously simple, it allows plants to grow well in excess of 33 feet. For instance, coastal redwood trees can grow well well over 300 feet tall. Even more interesting is the fact that plants have been using this system for a very long time, far longer than it took for mankind to figure out how to do the same thing.

We often refer to a person who is brainless as being a ‘vegetable’. It seems that when we say this, we are actually insulting vegetables.