lO Forestry Quarterly. 



Summary and Conclusions. 



1. Spiral cracks in the walls of tracheids and fibers occur in 

 only a small percentage of dry wood. 



2. Spiral cracks when present are confined in coniferous woods 

 to the heavy thick-walled tracheids at the end of the year's 

 growth. 



3. Spiral cracks are confined to the secondary and tertiary layers 

 of the cell wall and the primary wall remains unruptured. 



4. Air passes as easily through dry cells whose walls are un- 

 ruptured as through cells whose secondary walls have cracked in 

 drying. 



5. In many cases air cannot be passed through dried cells 

 whose secondary walls possess well developed "slits" or cracks. 



6. Although drying cracks or "slits" do not close when dry 

 wood is thoroughly re-soaked, re-soaked wood is in many cases 

 as impervious to air as unseasoned material. 



7. Although air cannot be forced through long pieces of green 

 coniferous wood even under heavy pressures, it passes in many 

 cases through short pieces of more than one fiber length. 



8. In long-leaf pine paving blocks, when the penetration was 

 confined largely to the dense bands of "summer-wood," the walls 

 of the latter were in the majority of cases found to be unrup- 

 tured. 



From this it is clear that Tiemann's "slit" hypothesis cannot 

 account for the penetration of gases and oils into seasoned wood. 

 Similarly, Weiss' theory cannot account for the greater penetra- 

 tion of preservatives into dense tissues. In both cases some un- 

 determined factor or factors are at work which control the in- 

 jection phenomena. 



