Ascent of Sap

The problem of elucidating the mechanisms of transport in the various parts of the transport network has been treated in the past in a compartmentalized manner. As regards the question how considerable volumes of water with dissolved nutrients reach the leaves, the cohesion theory postulates that the evaporation of water from the surface of leaves (transpiration) provides the driving force. As the radius of the xylem conduits is too large for the capillary action to be of much help negative 

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Could capillary rise account for the ascent of sap This question can be easily answered by formulating the Bond number as a fraction of the weight force of the lifted water column, to the force exerted by the surface tension ... 

To induce the ascent of sap in tree, the menisci present in the leaf stomata pull up water (Zimmerman, 1983). Because most trees are more than 10 m high, one can deduce that the absolute liquid pressure in the sap column is negative. No gaseous phase can exist in such conditions. The vascular system developed in trees has many other implications for the drying process ... 

Zimmerman, M.H., 1983. Xylem Structure arul the Ascent of Sap, Springer, Berlin, Germany. 

M. H. Zimmermann, Xylem Structure and the Ascent of Sap, Springer-Verlag, Berlin (1983). 

M. Amin, "Ascent of Sap in Plants Means of Electrical Double Layers, J. Biol Phys. 10, 103-109 (1982). 

We mourn for a friend, who as long ago as 1974 entrusted a book to us, (Zimmermann and Brown Trees ). On the recent founding of a new book series ( Springer Series in Wood Science ), Martin H. Zimmermann willingly contributed his own work as the initial volume ( Xylem Structure and the Ascent of Sap ). This was to be his last book the moving epilog — placed at his own wish at the last moment at the conclusion of the volume — became also his farewell to active publishing. 

Next, we will use Poiseuille s law to estimate the pressure gradient necessary to cause a specified volume flux density in the conducting cells of the xylem.4 The speed of sap ascent in the xylem of a transpiring tree can be about 1 mm s-1, which is 3.6 m hour-1. We note that vw equals the volume flux density of water, Jyw (see Chapter 2, Section 2.4F) the average speed of the solution equals JVy the volume flux density of the solution, which for a dilute aqueous solution such as occurs in the xylem is about the same as Jvw (see Chapter 3, Section 3.5C). Thus Jvin the xylem of a transpiring tree can be 1 mm s-1. For a viscosity of 1.0 x 10-3 Pa s and a xylem element with a lumen radius of 20 pm, the pressure gradient required to satisfy Equation 9.11b then is... 

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