98 TIMBER AND SOME OF ITS DISEASES, [chap. 



was thus completely severed, although (the two cuts 

 being at different levels) the continuity of the woody 

 substance of the cell-walls and their imbibed water 

 was not broken. In many of the cases Dufour 

 found he could not drive water through such cut 

 branches, and yet transpiration was not prevented 

 and water passed the wounds. 



Dufour rejects the view that this could occur by 

 means of the tracheides, although he is not prepared 

 to decide how far concurrent movements in the 

 lumina may affect the matter. 



Dufour points out that the " air-pressure theory '* 

 (then being eagerly discussed on all sides) presents 

 the one insuperable obstacle that it will not account 

 for the ascent of the water to a greater height than 

 about 10 meters ; and that it is of no use to invoke 

 the aid of capillarity, as supporting the columns of 

 water, for in just so far as it supports them it prevents 

 their being driven upwards, or moved at all. 



In a short paper published in Berlin in 1883,^ 

 R. Hartig offers his hypothesis in an amended form. 

 In answer to Dufour, Hartig insists that the move- 

 ment is usually and in the main from lumen to lumen, 

 but admits the possibility that when transpiration is 



^ Die Gasdrucktheorie unci die SaMsche InuHHHonstheone, Berlin, 



