January 21, 1915] 



NATURE 



559 



much as his interesting- experiments on transpira- 

 tion in oxygen, air, COg, ether, and chloroform 

 give values differing inter se in practically the 

 same way as the evaporation of a surface of water 

 under like conditions, it must be allowed that there 

 is here no error of the above-mentioned kind. 



The second chapter is devoted to the discussion 

 of those hypotheses of the ascent of water in trees 

 which depend on the co-operation of living- cells 

 in the water-way, views that may be said to 

 begin with Godlewski's theory (1884), which was 

 followed by the cognate hypotheses of Wester- 

 maier, Janse, and others. The whole chapter is 

 good reading, but the most interesting part is the 

 description of Dixon's experiments bearing on the 

 effect of killing the sections of the water-way by 

 heat. In this way it has been shown by Janse 

 and Ursprung that the transpiration current is 

 checked and that the leaves above the heated 

 region wither. This fact was used as an argument 

 for the "vital" theories of the ascent of water. 

 But Dr. Dixon shows clearly that by heat, poison- 

 ous substances are developed, and that withering 

 occurring above the injured region may undoubt- 

 edly be produced by this cause, and therefore that 

 the argument for vital theories loses its validity. 



With chapter iv. begins the kernel of the book, 

 namely, the re-statement of the cohesion theory of 

 the ascent of water. The most interesting features 

 of this section are the discussions on the effect of 

 air bubbles in the water conduits, and the evi- 

 dence in favour of cohesion theon,' which is derived 

 from a general discussion on the structure of 

 wood. 



This is followed in the next chapter by a discus- 

 sion of the purely physical problem of cohesion. 

 He confirms and extends Berthelot's estimation of 

 the tensile strength of water containing air, a 

 point of great value. He also works out the 

 tensile strength of sap — a necessary precaution — 

 and records the astonishing tensions of about 132 

 and 207 atmosphere. The sap used in these ex- 

 periments was ingeniously obtained by centri- 

 fuging, a method likely to be useful in other 

 botanical researches. 



Having shown how great is the cohesion of the 

 water columns, Dr. Dixon goes on to the other 

 half of the problem — the resistance offered by 

 wood to the passage of sap. He makes it prob- 

 able that Ewart's estimate of this element in the 

 problem is much too high, and points out (p. 132) 

 "that if Ewart had obtained my results, the diffi- 

 culty of resistance, which he finds to be fatal to 

 the cohesion theory of the ascent of sap, would 

 not have presented itself to him." 



The rest of the book is mainly devoted to the 

 question whether the leaves can develop traction 

 NO. 2360, VOL. 94] 



sufficient for raising the sap, a force which he 

 says {p. 139) "must at least be equal to the pres- 

 sure produced at the base of a column of water 

 which is twice the height of the transpiring tree." 

 The problem in fact comes to be whether the 

 osmotic pressure in the cells of the leaves is suffi- 

 ciently great to account for the raising of water 

 in very tall trees. One method employed was to 

 expose leaves to compressed air in a closed vessel ; 

 in this way it was shown pressures of 26-38 atmo- 

 spheres must exist in the leaves. Fin-ally he pro- 

 ceeded to ascertain the osmotic pressure in the 

 leaves by determining the freezing point of the 

 cell sap. This was effected by employing a 

 thermocouple in place of the thermometer used in 

 Bcckman's method of cryoscopy. In this way 

 Dixon is able to determine the freezing point of 

 small quantities of fluid — a practical point of con- 

 siderable importance. The osmotic pressure in the 

 leaves deduced by this method are finally shown 

 to be more than sufficient to account for the ascent 

 of water. F. D. 



THE SCHUMANN REGION OF THE 

 SPECTRUM. 

 The Spectroscopy of the Extreme Ultra-Violet. 

 By Dr. T. Lyman. Monographs on Physics. 

 Pp. V+135. (London: Longmans, Green and 

 Co., 1914-) Price 55. net. 



THE remote ultra-violet part of the spectrum, 

 first investigated by Victor Schumann and 

 since known as the "Schumann region," has in 

 recent years become of distinct importance in 

 several directions. A knowledge of the vibrations 

 of short wave-lengths is indispensable in the study 

 of spectral series in relation to theories of emission 

 spectra, and it has further been found that photo- 

 electric, photo-chemical, and photo-abiotic pro- 

 cesses become much more pronounced in this 

 region. In the present monograph full accounts 

 of the methods and results of observations are 

 given in an interesting and convenient form by 

 Prof. Lyman, who was among the first to take up 

 and extend the work of Schumann. The first part 

 of the book, referring to the ordinary ultra-violet 

 spectrum included between wave-lengths 4000 and 

 2000, is a valuable introduction to the larger 

 second part in which the Schumann region is dealt 

 with. 



As is now well known, the opacity of air, glass, 

 and quartz to the Schumann rays renders it neces- 

 sary that the entire spectrograph and source of light 

 should be enclosed in a vacuum, and that special 

 materials should be used for the construction of 

 lenses and prisms; specially prepared photo- 

 graphic plates are also necessary. Schumann's 



