1902] RISE OF 7 HE TRANSPIRATION STREAM 263 



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Gasspannung, die in solchen (zeitweise entleerten) Bahnen 

 herrscht, zu deren Wiederfiillung beitragen. Ihr Einfluss diirfte 

 stets dominirend sein/' 



That transpiration and absorption are not parallel has always 

 been inexplicable except on the basis of a storing up of the 

 "suction" (rather, reduced tension) by the rarefied gas in the 

 tracheae. And this involves the assumption that the differences 

 in tension suffice, in practice if not in theory, to lift the water. 



The one great objection to this theory is that the pressure of 

 one atmosphere at sea level is equal to that of a column of water 

 only lo"' high, which is no considerable part of the height of 

 large trees. The bare dogma ''it cannot be" is not instructive 

 nor stimulating, but has been much too effective in discrediting 

 this theory. The atmospheric pressure certainly cannot cause 

 an upward ^ow of a continuous column of water more than lo"" 

 high ; but except when root pressure is active and can act with 

 the atmospheric pressure, it may well be doubted if such a 

 column of water ever occurs in trees. The atmospheric pressure 

 would be ineffective against it solely because its downward 

 pressure would be greater. But it is well known that pressures 

 in excess of an atmosphere never are found in actively trans- 

 piring trees.' At whatever height the pressure in the tree may 

 be tested, it is always found such that the local application of a 

 counter pressure of an atmosphere would move the water present 

 upward. The problem is to explain how a pressure of one atmos- 

 phere decreases so gradually that it does not become zero in 

 several times lo" height of water, air, and wood. Denying the 

 possibility has distracted attention from the real question. The 

 conclusion of this paper will be occupied with this problem. 



IX. The belief that the forces operating below — atmos- 

 pheric and root pressure — and the tension of whatever kind 

 arising from evaporation by the leaves were altogether inade- 



'For the explanation of Figdor's (1898) apparent discovery that high pressures 

 are the rule in the tropics, see MoJlsch (1902 ; 53-55 ; also Gregor Kraus, 1895 - 216). 

 A. N. Prentiss is referred to by Clark (1874: 190) as telling of trees in Brazil which, if 

 cut during the dry season, suck in air " so forcibly as to cause a sound, amounting 

 almost to a groan." 



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