7 2o SCIENCE PROGRESS 



investigations made by Professor Dixon and those who have worked along the 

 lines indicated by him have added materially to our knowledge of plant physiology, 

 and that the explanation of the ascent of sap here advanced is supported by 

 a remarkably convincing body of experimental data. It may be noted that the 

 author, doubtless because the ascent of sap is the main question at issue, hardly 

 deals with the other part of the subject as represented in the title of the book — 

 namely, transpiration itself. Recent researches have greatly extended and 

 modified our knowledge of this process, though it is doubtful whether they can as 

 yet be brought to bear directly on the question of the rise of sap. Still, a con- 

 sideration of modern work on transpiration would not have been out of place in a 

 book which, as it is, should more accurately have been entitled simply " The 

 Ascent of Sap in Plants," leaving room in the series for a " monograph " on 

 modern physiological and ecological researches in transpiration, a critical resume 

 and discussion of which would form an extremely useful publication from many 

 points of view. 



The early workers in plant physiology recognised the connection between the 

 flow of water upwards in the stems of plants and the absorption of water from the 

 soil by the roots on one hand and its escape as water vapour into the atmosphere 

 from the leaves on the other ; but though Hales and others made experiments on 

 the pressure set up by this flow and the amount of water exhaled by the leaves, 

 the ascent of the sap was ascribed by them to the vital activity of the plant. 

 Vitalistic explanations of this phenomenon persisted long after it had been shown 

 that water still rises in cut shoots and in entire plants whose roots had been killed 

 by heat, that the current still flowed when a considerable length of the stem was 

 killed by heat, and that poisonous solutions were carried up for a length of time 

 more than necessary to ensure the death of all the living cells of the wood. These 

 experiments are generally considered to have disposed finally of the vitalistic 

 explanations that the rise of sap might be due to the osmotic pressure set up in 

 the absorbing root-hairs, or to the " pumping" action of the living cells in the 

 wood, which were supposed to repeat the root-pressure effect at different levels in 

 the stem by absorbing water and afterwards returning it under pressure into the 

 vessels. Some writers, however, still regard these "killing'' experiments as 

 unconvincing, explaining the stoppage of the water flow as due to the blocking of 

 the vessels by products of the dead cells, and so on. Meanwhile, various physical 

 explanations were suggested, such as capillarity, differences between the pressure 

 of the outer atmosphere and the lower pressure of the gases within the plant, the 

 passage of the water within the walls and not in the free lumina of the vessels, 

 etc., but these were easily disposed of by experiment or by simple resort to 

 established physical facts which had been overlooked by the theorists. 



The problem was attacked from a quite new point of view in 1894 by Professor 

 Dixon and Dr. Joly, and the present work gives an account of the theory then 

 outlined and of the earlier and more recent experiments carried out to test its 

 validity and to illustrate its applicability. This theory rests essentially upon the 

 discovery made in 1846 by Donny that a column of water possesses great tensile 

 strength ; Donny himself considered that the presence of dissolved air reduces 

 the cohesion considerably, but Dixon and Joly found that this was not the case, 

 and that the tensile strength of the sap of plants is even greater than that of 

 water, so great that, according to the most recent determinations, it amounts to 

 more than 200 atmospheres. Since resistance to a current of water moving 

 through the wood of a plant at the velocity of the transpiration stream is 

 approximately equal to a head of water equal in length to the wood traversed, the 



