73 TIMBER AND SOME OF ITS DISEASES. [CHAP. 



of gum through. He points out that this easy 

 permeability is not suggestive of imbibition. 



If a piece of the yew branch is taken fresh, fastened 

 to a tube, and the wood of the alburnum exposed 

 by removing a clean longitudinal slice, then it is 

 possible to see the air-bubbles in the tracheides ex- 

 pand and contract under the microscope, if the 

 observer alternately sucks and blows through the tube. 

 Similarly, by merely blowing down one leg of a U 

 tube containing eosin solution, the dye can be forced 

 through a piece of yew-branch fastened to the other 

 leg ; on afterwards splitting the wood, the alburnum 

 alone is dyed, and the dye is in the cavities of tJie 

 tracheides, between the air-bubbles, the substance of 

 the lignified walls not being stained except at the cut 

 ends. This and similar experiments proved that the 

 eosin solution did not traverse the lignified parts at all 

 it filtered from tracheide to tracheide through the 

 unlignified membranes of the bordered pits, the only 

 part stained. 



As is now well known, these .bordered pits of the 

 Conifers occur almost exclusively on the radial walls of 

 the tracheides, a very few being formed on the outer 

 tangential wall of the last rows of tracheides formed 

 in autumn. Elfving argued that if the fluid travels 

 via the bordered pits, then it ought to be possible 



