Ascent Of Sap In Trees Research Articles

The Theory of the Rise of Sap in Trees: Some Historical and Conceptual Remarks

The ability of trees to suck water from roots to leaves, sometimes to heights of over a hundred meters, is remarkable given the absence of any mechanical pump. This study deals with a number of issues, of both an historical and conceptual nature, in the orthodox Cohesion Tension theory of the ascent of sap in trees. The theory relies chiefly on the exceptional cohesive and adhesive properties of water, the structural properties of trees, and the role of evaporation (transpiration) from leaves. But it is not the whole story. Plant scientists have been aware since the inception of the theory in the late 19th century that further processes are at work in order to prime the trees, the main such process (growth itself) being so obvious to them that it is often omitted from the story. Other factors depend largely on the type of tree, and are not always fully understood. For physicists, in particular, it may be helpful to see the fuller picture, which is what this study attempts to provide in nontechnical terms.

An attempt to explain the ascent of sap in defoliated trees

The biologist Zimmermann has criticized in his book [12] that the effect of capillarity has been neglected in the past to explain the ascent of sap in trees. We want to propose a method how the sap ascent can be calculated, based on generalized solutions of the capillary equation. Although the theory of such solutions is not simple, the extracted method of calculating the sap storage in given cross sections of xylems can be applied also without having a large mathematical knowledge.

Ascent of Sap in Trees

Experimental results concerning the ascent of sap in the xylem are usually interpreted in terms of gradients of hydrostatic pressure in the xylem conduits. In this report an alternative model is proposed that is equally consistent with the experimental results: under static conditions the water column is supported by a gradient in the chemical activity of the water, and the hydrostatic pressure is constant throughout. Observations that support the new model are cited, and experiments are suggested that would permit a choice between the two models.

Ascent of Sap in Trees: Effect of Negative Absolute Pressure on Flow Resistance

IN considering the ascent of sap in trees, it is generally accepted1 that the transpiration process at the leaves somehow ‘pulls’ the sap up through the xylem vessels in the tree trunk during active transpiration periods. This ‘cohesion theory’ postulates that negative absolute pressure (liquid tension) must be present in trees more than 34 ft. high, or in any plant where the combined effects of hydrostatic head and flow resistance produce a pressure drop of more than one atmosphere. Investigators have had little success, however, in applying traditional resistance laws to the calculation of viscous pressure drop. Theoretical estimates of flow resistance to sap movement have varied by several orders of magnitude2 and experimental checks of flow resistance through sections of trees frequently show the flow rate decreasing with time at constant pressure3. It appears that the present resistance ‘law’ is incorrect, is being misinterpreted, or the viscosity of sap is not what it is assumed to be from measurements made at positive pressures. This communication is relevant to the latter possibility and reports, in particular, on whether negative absolute pressures have any detectable effect on flow resistance.

The Effect of Low Temperature on Ascent of Sap in Trees

Although the so-called cohesion theory is widely accepted as the most reasonable explanation of how water is lifted up to the leaves of tall trees, there are investigators who question or even reject it. Among the objections to the cohesion theory, Handley's experiments (5) have been cited repeatedly during recent years (e.g. 4, 8). In 1939 Handley published a paper describing experiments in which he chilled the stems of 4-foot-tall saplings of Acer Pseudo-Platanus L. and Fraxinus excelsior L. He reported that if the stem was chilled to between 0 and 20, the leaves wilted; if the temperature was raised again to slightly above 20, the leaves recovered, provided the low-temperature treatment had not lasted too long. This observation is rather remarkable since no known property of water changes drastically enough at a temperature above freezing so that the wilting could be explained in terms of a physical interference with water movement. Handley concluded therefore that metabolic forces are somehow involved in sap ascent. Handley's experiments are by no means the only ones in the literature dealing with the effect of temperature on the ascent of sap, but since instrumentation of the older experiments was rather crude when compared to modern standards, it is difficult to evaluate them today. It may suffice here to refer to Handley's own review of these older papers (5). During the summer of 1959 a study was begun on the effect of locally applied temperature on phloem transport (11). In the course of these experiments the question arose as to the temperature at which xylem sap freezes in the tree.

High Root-Pressures in Palms

DESPITE some impressive demonstrations of root-pressure1–3, ‘pumping’ or positive pressure is generally thought to be of little importance for the ascent of sap in trees. This is probably true for dicotyledons, but in ten species of palms at Calcutta I was able to detect and measure positive root-pressures throughout the year, sometimes enough to lift sap to heights exceeding those of the trees.

Ascent of Sap in Trees

WITH reference to an article on the above subject which appeared in your issue of October 26, 1905, the following extract from a paper—which your contributor has doubtless not seen, published nearly ten years ago—will probably interest some of your readers.

The Mechanics of the Ascent of Sap in Trees 1

THE following remarks, relating to one of the most powerful and universal of the mechanical operations of organic nature, are based mainly on the numerous experimental results reported in Dr. A. J. Ewart's recent memoir.2 Their chief object is to assert the view that we are not compelled to suppose the sap, in the column of vessels through which it rises, to be subject to the great actual pressure, amounting in high trees to many atmospheres, that is sometimes postulated. It is hardly necessary to remark that the problem of the rise of sap is one of mechanics, in so far as concerns the mode of the flow and the propelling power.

Note on the mechanics of the ascent of sap in trees

The following remarks, relating to one of the most powerful and universal of the mechanical operations of organic, are based mainly on the numerous experimental results reported in Dr. A. J. Ewart’s recent memoir. Their chief object is to assert the view that we are not compelled to subject to the great actual pressure, amounting in high trees to many atmospheres, that is sometimes postulated. It is hardly necessary to remark that the problem of the rise of sap is one of mechanics, in so far as concerns the mode of the flow and the propelling power. Contrary to the view above referred to, it seems not unreasonable to consider that the weight of the sap in each vessel is sustained in the main by the walls and base of that vessel, instead of being transmitted through its osmotically porous base to the vessels beneath it, and thus accumulated as hydrostatic pressure.