i8 Forestry Quarterly. 



solution on both sides of the pit membrane shown in Fig. 2 and 

 the molecules or miraite particles of water are set in motion by a 

 .'-light pressure they are free to pass easily and rapidly from one 

 cell to another through the perforations in the pit membrane. Now 

 let us suppose that we force air into either of the cells. The 

 water will be gradually driven out until the air comes in contact 

 with the pit membrane. As soon as this happens the surface 

 tension of the water in the minute openings of the membrane 

 resist the further penetration of the air. When additional pres- 

 sure is applied to force the water from the perforations this mem- 

 brane is forced to the side and the torus seals, more or less 

 effectively, the opening in the secondar}- wall. (See Figs, i and 

 3.) If a very short piece of wood is used so that the air has to 

 penetrate only one or two membranes it is sometimes possible to 

 pump air through green wood. On the other hand, if there are 

 numerous membranes to be encountered, capillarity or surface 

 tension and the valve-like structure of the tori effectively pre- 

 vent the penetration of air, even under heavy pressures. The 

 fact that re-soaked dry woods is, in many cases, somewhat less 

 impervious to air than unseasoned material is probably due to 

 the rupturing of the pit membrane during the process of drying. 



Summary and Conclusions. 



1. Wood is a highly specialized and complex plant tissue, de- 

 signed primarily to conduct aqueous solutions and to give 

 strength and rigidity to stem and branches. 



2. It is extremely variable in different species and even in 

 different parts of the same individual due to variations in the 

 functions which it is called upon to perform. 



3. Coniferous woods or "softwoods" are composed largely of 

 minute cells or tubes with closed ends. Liquids in passing 

 through this tissue travel primarily in the cavities of the cells 

 and pass from cell to cell by means of delicately constructed 

 valves or bordered pits in the cell walls. 



4. The membranes of the bordered pits are not always entire 

 as has been previously supposed, but possess (in all species 

 examined) numerous minute perforations whose presence may be 

 demonstrated by careful microscopic examination and by experi- 

 mental means. 



