442 
Notes. 
THE RESISTANCE TO FLOW IN WOOD VESSELS.— In vessels filled with 
water, the resistance to flow depends upon the rate of flow, the square of the radius 
of the tube, the length of the vessel and the viscosity of water. Although the vessels 
are relatively narrow tubes, the rate of flow is not very great, so that the total 
viscosity resistance to flow in an ordinary tree during active transpiration is consider- 
ably less than the height of the tree 1 . As air appears in the vessels, however, two 
new factors are introduced. In intact vessels these bubbles when large and pressed 
against the sides of the vessels remain approximately stationary, the water flowing 
past them. Under these circumstances the surface tension film around each bubble 
forms an additional limidng surface to the ascending stream. If the latter exercises 
any friction upon the sides of the bubble, and it is difficult to see how friction against 
a stationary surface tension film around the air-bubble can be avoided, then the 
bubble must be subjected to some downwardly directed force which counterbalances 
the frictional and gravitational upthrust upon it. 
The explanation of this peculiarity seems to lie in the fact that a true Jamin’s 
chain action is exercised in wood vessels containing air-bubbles and water-columns, 
in spite of the fact that the walls of the wood vessels are lined internally by a more or 
less continuous adherent film of water. No Jamin’s chain action can be exercised 
when the liquid wets the smooth walls of a containing vessel equally at all points, 
since in this case the boundaries of the surface tension films move as readily as the 
central regions instead of being dragged back, so that the front boundary of each 
drop becomes convex, and the hinder one concave. As a matter of fact, however, 
a true Jamin’s chain resistance does appear to be shown in vessels lined by a film 
of water and whose walls are wetted by water. Possibly this may be due to the 
internal localized thickenings of the walls, the surface tension films coming within the 
limit of adhesion opposite to these but moving freely between them. In this way 
a valvular mechanism could be maintained in the vessels, by the aid of which water 
could be passed from point to point in a vessel by slight local forces, without the 
total head of water in the tree requiring to be maintained by any pronounced 
pressure or tension applied at base or apex. Thus in Fig. 8 a slight pressure 
generated between b b would be prevented from acting downwards by the bubble 
beneath adhering at a a , so that the surface tension film became more convex, and 
would act upwards only. A wave of actions of this kind running up along the vessels 
would raise the water upwards. 
Evidence as to the existence of a true Jamin’s chain resistance in the conducting 
wood of actively transpiring trees is afforded by the fact that the resistance to flow 
is much greater than would be expected from a viscosity calculation. In addition 
direct evidence was obtained in the following way. Living branches of the plants 
previously examined (1. c.) were shaved down at one end until thin enough to 
examine the uncut vessels under the microscope. The other end was fixed to 
a pressure apparatus containing a coloured liquid which could be driven slowly 
1 See Ewart on the Ascent of Water in Trees. Phil. Trans. 1905, p. 44 seq. 
