554 Dr. A. J. Ewart. [Nov. 2, 



" The Ascent of Water in Trees." By ALFRED J. EWART, D.Sc. r 

 Ph.D., F.L.S., Lecturer on Botany in the University of 

 Birmingham. Communicated by FRANCIS DARWIN, For. 

 Sec. R.S. Received November 2, Read December 1, 1904. 



(Abstract.) 



As the result of a series of experimental observations bearing upon 

 this problem, the author has been led to the conclusions stated in brief 

 below. 



The flow of water through open vessels filled with sap takes place 

 in accordance with Poiseuille's formula for the flow through rigid 

 cylindrical tubes, divergences being due jto the presence of irregular 

 internal thickenings in the vessels, and to local constrictions or 

 deviations from the circular outline. 



Hence the velocity of flow is directly proportional to the pressure 

 and to the square of the radius of the tube, inversely proportional to 

 the length of the tube and to the viscosity of the liquid. A small 

 number of large vessels, therefore, offer very much less resistance to 

 flow than a large number of narrow ones having the same length, and 

 the same total internal area of cross-section. Since viscosity is largely 

 dependent upon temperature, the latter forms an important factor in 

 regulating the flow, the viscosity and the resistance falling with a rise 

 of temperature. 



With an average rate of flow the total resistance due to the viscosity 

 of the water flowing through the vessels is always less, and in climbing 

 plants with large vessels is considerably less, than a head of water 

 equal in height to the stem. The adult vessels of actively transpiring 

 Angiospermous trees always contain air-bubbles, and these introduce a 

 resistance to flow which is inversely proportional to the radius of the 

 tube, when the air-bubbles and water-columns move together. When 

 the air-bubbles are comparatively stationary, as in most vessels, the 

 resistance is still further increased, and it becomes very great when 

 the vessels are small and the air-bubbles numerous. In intact vessels 

 containing air the rates of flow under similar pressures are propor- 

 tional to a power of the radius lying between 1 and 2, the volume 

 passing to a power of the radius lying between 2 arid 4. 



Estimations of the amount of flow, made from the rate of flow and 

 the diameters and number of the vessels, showed that the actual flow 

 takes place in the wood of Dicotyledons almost entirely through the 

 cavities of the vessels and hardly at all through the tracheides. In 

 young stems saturated with water under pressure, a considerable flow 

 takes place through the pith, but practically none in intact transpiring 

 stems. 



