Preservative Treatment of Wood. 



13 



thick and heavily lignified wall, the so-called secondary wall, 

 which is completely enveloped by the thin primary wall. These 

 layers or walls surround an elongated cavity or lumen through 

 which fluids pass on their way to the leaves. It is quite obvious, 

 however, that provision must be made for enabling fluids to 

 pass easily and rapidly from the cavity of one cell into that of 

 adjoining cells. The dense secondary wall, although hygroscopic, 

 is not easily permeable to solutions and is therefore provided 

 with numerous pores or minute openings called bordered pits. 

 Since these structures are of fundamental importance in the pene- 

 tration of liquids through woody tissues it is essential to examine 

 them with considerable care. 



A, surface view of bordered pit. B and C, sectional views of bordered 

 pits. Br, embossed or bordered area of secondary wall. Oe, pit or ori- 

 fice in secondary wall. Me, membrane. Ts, thickened area of membrane 

 or torus. 



A circular area of the secondary wall surrounding the minute 

 opening or pit orifice is embossed or pulled away from the pri- 

 mary wall forming a saucer shaped blister which projects into 

 the cavity of the cell In surface view (see text figure A) this 

 area forms a halo or border CBr) about the opening (Oe) in the 

 secondary wall. The bordered pits of adjacent cells are exactly 



