Preservative Treatment of Timber. g 



penetration occurs in unruptured as well as ruptured bands of 

 "summer-wood" cells the phenomenon must be due to some un- 

 determined factor. 



F. Certain Structures Easily Confused With Drying Cracks. 



In those portions of the stem and branches of coniferous trees 

 which are called upon to resist heavy stresses in compression a 

 highly specialized type of tissue is laid down by the cambium. 

 This tissue is structurally designed to resist compression. The 

 inner and thicker part of the secondary wall of its component 

 cells or tracheids is composed of very fine more or less closely 

 approximated spiral bands. In longitudinal or cross sections 

 of dry wood these cells might easily be mistaken by the unsuspect- 

 ing observer for cells whose walls had developed numerous fine 

 cracks' or "slits" in drying. However, as is shown in Fig. 5, a 

 longitudinal section of freshly cut green white pine sapwood 

 taken from the immediate vicinity of the cambium, these struc- 

 tures are not a concomitant of drying or seasoning, but are fine 

 screw like bands deposited upon the inside of the cells by the 

 protoplasm. In a subsequent article of this series, the writer 

 will consider in detail this specialized tissue or "Rothholz" as it 

 is called by European investigators. 



Fig. 6 is a photomicrograph of a longitudinal section of air- 

 dried loblolly pine sapwood, and illustrates a condition which 

 occurs in air-dried hard pine from the Southern United States. 

 The striated or cracked appearance has not been produced by 

 shrinkage of the cell walls in drying, but is the result of incipi- 

 ent stages of decay. The minute mycelium of the fungus travels 

 spirally within the thick secondary walls of the "summer-wood," 

 gradually dissolving the wall substance by its enzymes or fer- 

 ments. Seen under magnifications such as are commonly used 

 in studying woody tissues these spiral cavities, in longitudinal 

 sections of the wood, might easily be mistaken for drying cracks. 

 However, by using thin sections and high-power, oil-immersion 

 lenses the structures are seen to be produced by a wood destroy- 

 ing fungus. In cross section the secondary wall is seen to be not 

 cracked as in Fig. 4, but drilled by numerous small circular bur- 

 rows which contain the dried mycelium of the fungus. 



