Many theories have been formed to account for the ascent of sap in high trees, when root pressure is not acting. All have been found, on careful examination, unsatisfactory. Our attention was particularly directed to the problem as we were together in Bonn, in the Summer of 1893, when Professor E. Strasburger was kind enough to show us some of his experiments on the question, and since then we have, at intervals, occupied ourselves with some considerations as to the cause of the ascent of liquids in trees. It was not, however, till late last Spring that we had leisure to enter definitely on the research. We wish to acknowledge the kindness of Professor E. Perceval Wright in giving us the benefit of his advice on all occasions, and also the advantage we derived from Professor G. F. FitzGerald’s suggestive ideas.
Strasburger’s experiments have eliminated the direct action of living protoplasm from the problem of the ascent of sap, and have left only the tracheal tissue, as an organised structure, and the transpiration-activity of the leaf, wherein to seek an explanation of the phenomenon. The authors investigate the capability of the leaf to transpire against excessive atmospheric pressures.
Normal urine is always grossly super-saturated in regard to the stone-forming salts, which are kept in solution by the action of the colloids. This action is best explained by the theory of adsorption. The amount of the stone-forming salts which can be held in solution depends on the surface area of the colloid, and therefore on its state of subdivision. Precipitation of these salts is due to failure of the colloid to hold them in solution. It may be due to an insufficient quantity, but is more probably due to coagulation of the colloid. When precipitation occurs in the urinary passages, the crystals are usually retained in the lower calyx of the kidney. A crystalline deposit tends to grow into crystalline concretions under the action of surface energy, thus forming true primary calculi. Stones of this type soon irritate the wall of the cavity in which they are contained and cause a reactionary exudate. The laminated stone is formed by continued deposition of crystals, coupled with rhythmic precipitation of a foreign colloid derived from the exudate. Stones originating in infected media are formed in a similar manner.
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