468 Scientific Intelligence. 



volume is based upon the course of lectures which the author has 

 delivered at the University in Rome. The subject, however, has 

 been much expanded and developed, and now presents the Crys- 

 tallography of the present day as viewed by the author, from a 

 standpoint at once advanced and theoretical. The student whose 

 interests lie in this, rather than in the strictly practical side of the 

 subject, will find this work worthy of careful study. 



III. Miscellaneous Scientific Intelligence. 



i. 2 he Ascent of Water in Trees ; by Alfred J. Ewaet (ab- 

 stract of a paper read before the Royal Society of London). — 

 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 to the presence of 

 irregular internal thickenings in the vessels, and to local con- 

 strictions or deviations from the circular outline. 



Hence the velocity of flow is directly proportional to the pres- 

 sure and to the square of the radius of the tube, inversely pro- 

 portional 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 con- 

 tain air-bubbles, and these introduce a resistance to flow which is 

 inversely proportional to the radius of the tube, when the air- 

 bubbles and the 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 pres- 

 sures are proportional to a power of the radius lying between 1 

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

 2 and 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. 



