An Irish botanist solved the mystery of how trees supply themselves withwater, a finding with wide implications, writes Mary Mulvihill
It was a deceptively simple question, and, as so often happens with such questions, the answer turned out to be important. The answer came from a fine Irish botanist. And the question? How do tall trees drink?
It is a question not only about how plants function but also about how water moves around our planet. Plants shift huge volumes of water every day and therefore play a crucial role in the water cycle and climate. Water from the soil moves through plants, evaporates from their leaves into the air, forms clouds, then falls as rain to the soil, when the cycle starts again. Take away the plants and, as places find when they lose their forests, you also take away the rain.
But how does water rise to the top of a tall tree? The highest one in Ireland, a Douglas fir in Powerscourt, Co Wicklow, stands 57.5 metres tall. There are sequoia trees in California that are twice that, and the tallest tree ever measured, an Australian gum tree, stood at 132.6 metres high in 1872. That's a long way from any water in the soil.
A mechanical pump can suck water up only about 10 metres before the water column breaks. This is clearly not enough, so 19th-century botanists believed that cells in the tree trunk must pump the water up. In the 1880s, however, a German botanist showed that even when cells in the trunk die, a tree can still draw water. Clearly, a new theory was needed.
Enter two scientists from Trinity College in Dublin: the pioneering geophysicist John Joly and the young Dublin botanist Henry Horatio Dixon (1869-1953). They saw the German experiments in 1892 and, combining their knowledge of physics and botany, three years later proposed a new theory.
They suggested that evaporation at the leaves generates enough suction to draw a continuous column of water through the very narrow xylem cells that form the tree's internal plumbing. The plant essentially acts as a wick, with the energy it needs coming from the sun.
Dixon performed elegant experiments to prove that a thin column of water could withstand being stretched to the top of a tall tree. It works because of the stickiness of water molecules, and also how they behave in the tiny xylem bore holes.
In 1914 he published a book, Transformation And The Ascent Of Sap In Plants, and his "cohesion theory", as it is called, became the accepted explanation of how tall trees drink.
It is now also accepted that trees can tolerate momentary breaks in the water column: sensitive listening equipment detects these as clicks, and listening to the clicks is how water stress is monitored in valuable crops such as vines.
Dixon, who became professor of botany at Trinity, worked on other aspects of plant physiology, but according to his successor Prof Mike Jones he is best remembered for his work on the rise of sap. This perceptive botanist died 50 years ago this month, on December 20th, 1953.