lO 



The Irish Naturalist, 



January 



REVIEWS. 



THE ASCENT OF SAP. 



Transpiration and the Ascent of Sap in Plants. By Henry H. 

 Dixon, Sc.D., F.R.S. Macmillan & Co., 1914. Pp. viii. + 216. Price 

 5s. net. 



This book of eleven chapters is one of a series of science monographs 

 designed to " afford to authorities upon definite aspects of science a 

 means by wliich an adequate statement of their work may be made 

 available to the scientific world within a volume of reasonable dimensions 

 and at a moderate price." Although the earlier theories of the ascent 

 of sap are briefly reviewed and criticised in the book its main purport 

 is to give an account of the so-called " cohesion theory," first formulated 

 by the author and Dr. Joly in 1894, a-nd of the data on which it is based. 

 In a general way therefore, perhaps, the most hite resting part of the 

 book is chapter iv., where this theory is discussed. Briefly stated it 

 may be said that according to the older theories the ascent of sap was 

 presumed to result from pressure acting upwards from below on the 

 ascending column, assisted, according to certain authorities, by subsidiary 

 pressures developed locally and at intervals in the living cells distributed 

 in the woody tissues of the stem. According to the cohesion theory, on 

 the other hand, the ascent of sap during transpiration is due to a pulling 

 force developed in the leaves, acting from above downwards and trans- 

 mitted through the sap which is therefore in a state of tension in the 

 conduits of the wood. The later chapters of the book deal with the 

 tensile strength of the sap, the tension required to raise it, the osmotic 

 pressures of the cells of the leaves and finally the amount of energy 

 required and available for causing its rise. After studying these points 

 the reader cannot fail to recognise that this theory has very much to 

 recommend it, and, indeed, up to the present no substantial arguments 

 have successfully been advanced against it. It may be pointed out, 

 however, that the theory only explains the ascent of sap when the plant 

 is transpiring, and apparently would not account for its rise — if such 

 occurs — in a deciduous tree, for example, in spring before the foliage 

 buds have expanded, unless, indeed, transpiration from the bark of the 

 twigs be looked upon as capable of developing sufficient tension. 



On reading the book one cannot but be struck with the critical attitude 

 which the author adopts, not only towards the work of others, but also 

 to his own work, sources of possible error being looked for everywhere 

 and when found eliminated as far as possible by approaching the various 

 problems along all the available avenues of attack. The book also reveals 

 the author (as his published papers have already done to those who have 

 studied them) as possessing great originality and resource in dealing 

 with experimental problems especially such as demand delicate and 

 accurate quantitative measurement. In these respects attention may 

 be called to the particularly interesting experiments in chapter iii. dealing 



