The Ascent of Water in Trees.

Abstract

In the earlier discussions of this problem, it has been tacitly assumed that it was only necessary to find forces sufficient to balance a head of water equal to the height of the loftiest tree to explain the ascent of sap in it. The problem is, however, rather one of dynamics than of statics, for we have to find forces sufficient not only to balance the head of water, but also to keep this water moving upwards in narrow tubes with a velocity varying from a few centimetres to as much as 6 metres per hour. Janse has in fact shown empirically, and his results have been confirmed by Strasburger, that to drive water through the stem of a Conifer at the transpiration rate requires a head of water several times the length of the stem. Water is a liquid of definite viscosity, and the resistance offered even to its slow flow through tubes of small diameter and considerable length is a factor of great importance. The purpose of the following research has been to find the amount of this resistance in definite cases, the forces required to overcome it, and hence the total force required to raise the water with sufficient rapidity to the summit of an actively transpiring tall tree. Furthermore, an attempt has been made to determine the possible means by which this force could be generated, and propagated in the conducting wood.