Physics mysteries

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Andriy BlokhinAndriy Blokhin (Source: ©Andriy Blokhin/ stock.adobe.com)

How can a tall spruce pull water from the roots to the top; does it have a pump inside?

Jaroslav Kores, Ph.D.

A tall giraffe pumps blood into its head by its heart. But what draws water to the top of tens of meters high spruce? Trees have no hearts or pumps... 

Capillary elevation is responsible for the nutrition of trees (and other plants)... 

You may think that you will not encounter this phenomenon anywhere else but (like all physics) this phenomenon has a wide use. You could ask in the same way why a cloth or a sponge absorbs water. 

Liquid molecules (e.g. water) act on each other by magnetic forces - that's why they also hold the liquid together in a container. These forces are quite large but not as large as in its solid state. Therefore, compared to its solid state, liquid molecules may move a little bit. As a result, liquids do not have a permanent shape - they always take up the shape according to the container. 

If we pour water into a container, all the surrounding molecules will have a gravitational force on the water molecules. 

Inside the water, this does not mean anything extraordinary, but at the walls of the container, the molecules of the container also enter into interaction. If the gravitational force of the container molecules is bigger than the gravitational force of the water molecules, the water molecules will try to be as close as possible to the container. We can notice this if we take a closer look at the water in a glass - the water level at the edges is a bit higher. The narrower the container, the higher the water rises. 

If the container is very thin (like a very thin straw or a so-called capillary), the water rises very high. The capillary does not have to be a container or a straw. By capillary we mean any empty thin place into which water can get. In the case of a cloth, these are the gaps between the individual fibres of the textile. In the case of a tree, these are the gaps in the tissues through which the water rises. 

The same principle causes masonry to get wet (water rises upwards through the gaps in the brick material) or, for example, a candle wick.