28 TRANSPIRATION AND ASCENT OF SAP ch. 



and Hales, a little later, attributed the rise of water in 

 the vessels to capillarity. These forces have ever since 

 been called in again and again to play their part in the 

 various theories formed to account for the ' circulation 

 of juices," or as it is called in more recent times, the Ascent 

 of Sap in Plants. 



Gas pressure theory. Direct descendants of these 

 theories were the air-pressure and the gas-pressure theories 

 of Bohm and Hartig, and these, like their predecessors, 

 were soon rendered untenable by quantitative examination. 



Jamin's chain. Nor did the comparison of the dis- 

 tribution of the water and gas in the plant to a Jamin's 

 chain avail to save these doomed hypotheses, for it was 

 early recognised that any support rendered to the water 

 columns by this configuration must at the same time act 

 as a resistance to upward motion. 



A recent attempt to rehabilitate the Jamin's chain hypo- 

 thesis, or rather a modification of it, was not more happy. 

 It was suggested that the peculiar intermixture of air and 

 water in the conducting tracts brought into play obscure 

 physical forces which lead to the elevation of the water. 

 It has been shown, however, that the experiments upon 

 which the view was based, were completely vitiated by 

 the neglect of a property of plaster of Paris which enables 

 it to continue to absorb water long after it has set. This 

 absorption gave rise to an upward movement of water in 

 the experiment, which was erroneously believed to demon- 

 strate that the ' suction ' of less than one atmosphere 

 applied to the top of a continuous column of water sus- 

 pended in a porous substance with air bubbles intermingled 

 could still operate as a suction more than 12 metres lower 

 down. 



Boehm's views. It will be understood that all that 

 was needed to refute the foregoing theories was a clear 

 statement of the theories themselves and a quantitative 

 estimate of the forces they asserted were adequate to 



