Some deciduous trees of the Combretaceae family, such as Terminalia tomentosa, show a remarkable ability to store water in their stems during the dry season in India. A small cut in the tree shows the amount they can carry. How do they do this and do any other plants or trees store water in a similar way?
鈥� The picture printed in your 1 October edition shows a stem section of Terminalia tomentosa, which when cut produces a steady stream of fluid. The plant does this by acquiring water during the rainy seasons and holding onto it in specialised cells, known as parenchyma cells. These are undifferentiated and can expand to accommodate fluid throughout the stem and roots. The parenchyma cells are immediately adjacent to the xylem of the tree, so making contact with this fluid-carrying material is easy. The tree does this to cope with water shortages during the dry season. The tree will always take up water when it is available.
Other trees do this too, as do some vines. They all use the same mechanism. In fact some vines in South America and Africa store enough water to supply native people when surface water is not readily at hand.
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Trees in temperate regions also collect water. Members of the white oak family store water from autumn to spring and use it during the dry summer months. Other trees that do this are the famed sugar maples of north-eastern America, black birch, negundo maples, Norway maples, and many species of grape vine. If any of these trees are tapped in early spring, the water flow with affiliated nutrients and sugars can be quite prominent, and is the starting point for maple and birch syrup. If grape vines are cut during spring or early summer they will often bleed so copiously that the plant can dehydrate and die. That鈥檚 why there are special times of year for vine pruning.
鈥淚f grape vines are cut during spring they can bleed so copiously the plant can die鈥�
The temperate trees inherited this trait from tropical species, although they use it differently. For them the aim is to withstand the rigours of cold weather rather than to avoid dry-season dormancy. It is the tree version of being a cactus.
H. William Barnes, Warrington, Pennsylvania, US