life11e_ch34

Various hypotheses have proposed how water moves in the xylem

Until the twentieth century, two mechanisms for moving water through the xylem were under consideration: upward pressure by living cells and capillary action. Both of these possibilities were largely ruled out by experiments:

A simple experiment in 1893 ruled out the hypothesis that root cells might initiate a pumping mechanism to propel water upward. A tree was cut at its base and the sawed-off part was placed in a vat containing a solution of poison that killed living cells. The poison rose up the trunk, killing any living cells it encountered along the way. The experiment demonstrated that a living pump in the root is not necessary to push the xylem sap up a tree. Because the roots were absent, it was clear that they are not involved in movement through the xylem. Furthermore, when the poison sap reached the leaves, they died and all upward movement of the solution stopped, showing that living leaves are necessary for water to move in the xylem.
Because of its surface tension (see Key Concept 2.4) and adhesive forces between water and its container, water will move up a narrow column by a mechanism called capillary action. You’ve seen this when you put a straw into a cup of water. Capillary action was ruled out as a primary mechanism for upward xylem sap transport when calculations showed that xylem vessels (at 100 micrometers [µm] in diameter) are too wide to get water to the top of a 15-meter tree in this fashion. In fact, the maximum height for a water column raised by capillary action alone in a 100-µm tube would be only 0.15 meters.