The average heights attained by various species of tree differ widely. The eucalyptus towers four hundred feet or more above the ground, twenty times as tall as the common dogwood. These characteristic heights are determined by the ability of the species to draw water from the reluctant soil and to raise it to great heights. The stature attained by the eucalyptus and the giant sequoias is possible only because of an unbelievable power to raise a column of water. The tremendous force which green plants exert in raising water from the soil was demonstrated by Detrouchet. Cutting a grapevine from its stem, he tied a bladder over the end. Within a few hours this bladder was filled with sap, and a little later the pressure became so great that it burst. Through this experiment Detrouchet determined that the force with which sap mounts the stem of a grapevine is five times as great as that of the blood stream in the large artery of a horse. For many years scientists have tried in vain to find out what causes the rise of sap.
Detrouchet, after years of patient investigation, wrote regretfully at the conclusion of his experiments, "The phenomenon of the rise of sap still remains without any plausible explanation." Many fine-spun theories have been offered. Some have thought that it is a kind of pumping action of the roots; others, a suction that these roots exert. Neither of these theories has been very definitely formulated. And even granting that the roots give the soil water its necessary start on the upward journey, what keeps it going? And when the sap water reaches the topmost leaf, what mysterious machinery reverses it and starts the stream with its burden of manufactured food down the twigs and the trunk?
Let us consider, therefore, some of the theories. Perhaps the explanation most commonly favored was the theory of capillarity, which assumed that the water rose in the trunk just as oil rises in a wick, through the tendency of liquids to ascend in narrow tubes and crevices. But the difficulty here was that capillary attraction couldn't begin to raise water high enough or fast enough to account for the ascent of sap in a plant.
Later, Dixon, a British scientist, found that by sealing a column of water in a glass tube he could make it carry considerable weight. Ordinarily a stream of water is extremely unstable; but with air excluded Dixon found that a water column would support a strain of several hundred pounds a square inch. The similarity of these conditions and those found in the stems of plants seemed suggestive, since the fine fibres of plants are really exceedingly slender tubes connecting with each other from the ends of the farthest root hair up to the leaf canopy.
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