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Green Corner




An A-Z of the environment & issues

by Peter Hatswell



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X is for Xylem

Xylem is one of those incredible building blocks of nature without which we would have no tall plants or trees and life today would be totally different.

The word is derived from the Greek for wood xylon which we also see in xylophones that were originally manufactured with wooden keys.

Flowering plants, which have been around since the Mesozoic period, outnumber other species by a factor of 7 to 1 and rely on xylem to transfer water and minerals from the roots to the leaves. They form hollow tubes each up to several feet in length but die after a few months after being encased by cellulose and lignum to provide a strong, flexible and vital conduit for life. Cellulose and lignum are the first and second most abundant organisms on the earth.

You can see xylem in the annual rings of a felled tree or branch and the way they work is quite miraculous. Even the best mechanical vacuum pump could only lift water by about 10 metres but some of our trees grow to 125m (Douglas fir) and can live for up to 4000 years (Bristlecone pine). So how does xylem do this? The answer is a combination of capillary action and the vacuum caused by evaporation at the leaves, the molecular bond between the oxygen and hydrogen in water making the composite xylem tubes stronger than steel wire. This process operates unhindered even if tree bark is girdled (ring of bark removed) but the plant will eventually die because the food manufactured in the leaves cannot pass down to the roots.Xylem phloem MC.  Photographer: Michael Clayton

Leonardo da Vinci thought that the cross-section of a tree trunk was equal to the sum of the cross-sectional area of all the main branches but in practice this is not quite true, the branches having a slightly greater area and proportional shares of xylem.

Coal, oil and gas all have their origins in decayed and compressed vegetable matter which have taken many thousands of years to form and may have absorbed less desirable elements from the surrounding rock. However, fresh dry wood is made largely of carbon and can be used as a clean sustainable fuel or as a structural material, trapping the carbon dioxide it has absorbed from the atmosphere throughout its growing life.

It is thus important that any wooden materials that we buy come from sustainable sources (look for the FSC mark) so that they are replaced for the benefit of future generations. A low price can indicate that the wood has been cut from a poorly managed forest. If you are lucky enough to have access to the internet, you can help protect rain forests for free by clicking daily on



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