  The water ascent in tall trees is subject to controversy: the vegetal
biologists debate on the validity of the cohesion-tension theory which
considers strong negative pressures in microtubes of xylem carrying the crude
sap. This article aims to point out that liquids are submitted at the walls to
intermolecular forces inferring density gradients making heterogeneous liquid
layers and therefore disqualifying the Navier-Stokes equations for nanofilms.
The crude sap motion takes the disjoining pressure gradient into account and
the sap flow dramatically increases such that the watering of nanolayers may be
analogous to a microscopic flow. Application to microtubes of xylem avoids the
problem of cavitation and enables us to understand why the ascent of sap is
possible for very high trees.
