264 BOTANICAL GAZEl TE [October 



quale to explain the ascent of sap left the one obvious alterna- 

 tive of some motive agent distributed along the stem. The only 

 modern theories along this line making any pretence at 

 completeness have sought this intermediate force in the activity 

 of living cells. There are two distinct theories of this kind, 



Westerm 



Accord- 



West 



path of the transpiration stream, and their osmotic activity 

 furnishes the motive power. The vessels, and even some inter- 

 cellular spaces (1884:1106-10), serve as reservoirs or landings 

 for the water, its running down in them being prevented by 

 capillarity. The limit on the height to which the water can well 

 be carried at a stretch in the parenchyma alone is placed by 

 Westermeier at 3'=°'. It is not shown in detail that the structure 

 of any tree is such as this theory demands ; still, the association ^ 



of tracheae and parenchyma is a striking phenomena, from 

 which, in fact, Westermeier got his idea. Such rapid movement 

 by osmosis in living cells as this theory provides for is known ^ 



I 



nowhere else. 



Godlewski's theory is in better accord with the generally 

 accepted fact that the tracheae are the chief path of the transpi- 

 ration stream ; he holding that the water passes through living j 

 cells, the medullary rays, only at occasional intervals, receivmg ; 

 each time an impulse like that of root pressure, which carries it f 

 some distance up in the tracheae. Godlewski's idea was that the 

 water moved upward instead of falling in the tracheae, when 

 forced into them by living cells, because of differences in the 

 tension above and below; which would make this theory subject 

 to the same limitations as if the living cell feature were omitted 

 (Zimmermann, 1885). Janse (1887:34-48) sought to obviate J | 

 this difficulty by ascribing the water's failure to sink to friction. 

 There is no escaping the fact that water will sink in wood (unless 

 hindered by too much air) by its own weight. For this explana- 

 tion to hold it would be necessary then for the living cells to 

 raise the water faster than it could run back, which is not to be 

 expected of osmosis, and would be very wasteful at best. 



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