28 Henry H. Dixon. 



this attitude of other investigators; and his peculiar physical views 

 and methods of argument rendered his papers very hard to discuss. 

 In these, published over a period of 30 years, are to be found records 

 of much patient experimental work. 



We have already seen that he was the first to bring into pro- 

 minence the air pressure theory. He rendered valuable service in the 

 discussion of the problem of the ascent of sap by withstanding the 

 Unger -Sachs view that the Transpiration Stream moves in the 

 imbibed walls, but on the other hand, by introducing the comparison 

 of the Jamin's chain, he was responsible for much of the obscurity 

 which has long beset the discussion. 



Later he became convinced that his earlier explanation was 

 incorrect, and he evolved a new point of view. But owing to his 

 peculiar method of treatment it is very difficult to obtain a clear 

 conception of his new standpoint. 



The first published account ^) of what seem to be his final views 

 was written in 1889. In this paper he begins by stating, as his custom 

 was when dealing with the subject, that there are only three forces 

 which can eifect the absorption and the raising of the water in high 

 trees. These three forces are (1) Endosmotic pressure, (2) Differences 

 of air pressure, and (3) Capillarity. 



He rejects the first owing to the slowness with which it can 

 act — It may be pointed out that, as the area of the surfaces, over 

 which the osmotic action is supposed to take place, is unknown, but 

 may be assumed to be enormous, this argument is without weight — 

 He also discards osmosis because plants continue to draw up water 

 after their roots and stems have been killed either by heat or by 

 poisons. 



He concludes that differences of air-pressure can only in part be 

 responsible for the entry of water into the roots and the rise of 

 water in the stems of plants, because the full atmospheric pressure 

 can support a column of water 10 m high only. He also states that, 

 if differences of air-pressure were responsible for the upward motion, 

 rhythmical changes in pressure would be necessary. 



In dealing with capillarity he describes an experiment in which 

 a rooted cutting of Salix, sealed hermetically into a reservoir com- 

 pletely filled with a dilute nutrient solution, drew up mercury in a 

 tube leading into the reservoir to a height of 64 cms. He further 

 found that, when he had previously boiled the lower half of the stem 

 and the roots of the cutting, it always drew up the mercury to the 

 contemporaneous barometric height. This experiment leads him, 

 apparently, as Strasburger says, by a process of exclusion, to 



^) J. Böhm, Ursache des Saftsteigens. Ber. d. Deutsch. Bot. Gesell., 7. 1^89 



