Preservative Treatment of Wood. 17 



carbon passing from one tracheid to the next, through the 

 raembranes of the bordered pits. On the left two tangential 

 pits filled with carbon are seen in surface view. Fig. 8 illustrates 

 a radial section of the same wood under high magnification. 

 The carbon particles are seen to penetrate the membrane in a 

 rim about the torus and in lines radiating outward from it. Thus, 

 the writer was forced to the conclusion that the pit membranes 

 in the tracheids of living coniferous trees are not entire as has 

 previously been supposed, hut are perforated by extremely minute 

 openings, which are located in the thinner radii of the membrane. 



The next step was to subject coniferous woods to a microscopic 

 examination to determine if these perforations were visible under 

 the highest powers of the microscope. This proved a difficult 

 undertaking owing to the minute size of the structures under 

 observation. Special methods of sectioning and staining were 

 essential owing to the peculiar structure of the bordered pits. 

 However, fairly satisfactory results were finally secured by 

 cutting exceedingly thin sections (2 micra). In this way it was 

 possible to cut away one of the embossed areas of the secondary 

 wall exposing to view the membrane and torus. By careful 

 staining and the use of very high magnification the detailed 

 structure of the membrane could in many cases be successfully 

 studied. Fig. 7 illustrates in a diagrammatic manner the struc- 

 ture of the pit membrane. In the center is the thickened por- 

 tion of the membrane or torus. Surrounding the torus is the 

 thin, permeable portion of the pit membrane. This is seen to be 

 composed of alternating bands of denser and thinner membrane 

 substance. The perforations are located in the thin, lighter 

 colored radii of the membrane. They are extremely variable in 

 size and may in certain species increase in size or coalesce to 

 form larger and more conspicuous openings. Fig. 9 illustrates 

 an unusually coarse type of pe'-foration which sometimes occurs 

 in the wood of the larch. 



It may well be asked in view of the perforated structure of 

 the pit membranes, why it is that gases cannot be forced easily 

 through long pieces of green wood and through many specimens 

 of thoroughly re-soaked, dry wood. This is, I believe, due to 

 capillary or surface tension phenomena combined with the 

 valve-like structure of the bordered pits. If we have an aqueous 



