76 TIMBER AND SOME OF ITS DISEASES. [CHAP. 



pressure of one atmosphere. The aspirator-action of 

 the cells in the leaves reduces the pressure, and the 

 bubbles separate and experience relatively enormous 

 friction. Morever, the oxygen of the air will be 

 absorbed by the cells, and an equal volume of carbon- 

 dioxide will be returned : this is soluble in the water, 

 and is carried in solution to the transpiring surfaces. 

 Hence is seen a further cause for the reduction of 

 pressure in the bubbles, and an important aid in the 

 sucking action : the bubbles at length consist entirely 

 or almost entirely of nitrogen under a pressure of 

 considerably less than an atmosphere. 



It will be seen that the fatal defect in Boehm's hypo- 

 thesis is the assumption that water can be raised to 

 such enormous heights, as it must be in tall trees, 

 simply by differences of atmospheric pressure, when 

 we know that the pressure of one atmosphere is 

 balanced by a column of water a little over thirty feet 

 in height. However, as the faults in the above views 

 will come out best in the controversy which follows, I 

 will not dwell further upon the matter here. 



The next important contribution to the subject, in 

 order of publication, was a paper by Fr. Elfving 1 

 which appeared in 1882. 



Elfving classifies the adherents of the two hypo- 

 1 Uberdic Wasserkitiing im ffolz, Bot. Zeit. 1882, October. 



