14 Forestry Quarterly. 



opposite to one another and form a row of lens shaped cavities 

 (See Figs. 5 and 6) which communicate with the. cavities of the 

 two cells by means of the two openings (Oe) in the adjacent 

 walls. Communication between the two cells is interrupted, 

 however, by the primary walls which in this region become very 

 thin and form a delicate membrane (Me) which divides the lens 

 shaped cavity into two plano-convex cavities (text figure B). 

 The dividing membrane possesses a circular thickened area (Ts) 

 called the torus which acts as a valve. Before sufficient pressure 

 can be brought to bear upon the delicate membrane to rupture it, 

 the torus is forced to one side or the other sealing more or less 

 effectively one of the openings in the secondary walls. (See 

 text figure C, also Figs, i and 3.) 



The necessity for these delicate and complicated valve-like 

 structures is seen when it is taken into consideration that water 

 in passing from the roots to the leaves of a tall tree must pass 

 through a large number of minute cells which are less than a cen- 

 timeter in length. Not only must the aqueous solutions pass 

 through a large number of cell walls, but at times they must do 

 so comparatively rapidly. The bordered pit is designed, there- 

 fore, to expose a relatively large surface of very thin, permeable 

 membrane without seriously impairing the strength and rigidity 

 of the secondary wall. 



Detailed Structure of the Pit Membrane. 



It is a commonly accepted fact among physiologists and anato- 

 mists that in conifers the wood cells or tracheids are entirely 

 enclosed in the thin primary membrane. In the region of the 

 bordered pits the "primary wall" may be greatly reduced in 

 thickness but, although very permeable to water containing salts 

 and gases in solution, it is supposed to be impervious to, undis- 

 solved gases and finely divided solids, and more or less imper- 

 vious to heavy oils and other heavy or viscous liquids. This 

 conclusion, that the primary wall or membrane is unperforated 

 even in the regi Jn of the bordered pits, is based upon two lines of 

 evidence. Careful microscopic examinations have failed to reveal 

 perforations in the pit membranes. (Russow, Nageli, Strassbur- 

 ger and others discovered that the pit membrane was not ho- 

 mogenous throughout, but frequently composed of fine alter- 



