PITS 11 



cells is rarely coiiliiiuous (wcv ilie entire surface of tlie adjacent 

 })riinai'y wail. In certain Iraciieary elements of llie i)riniary 

 xyleni for example, the secondai-y wall is developed as discrete 

 rings, si)irals, bars or as a complex network oi- mesh, while in 

 other cell types, well-defiiied thin areas or pits occur. 



Secure prepared slides of transverse sections of the stem of 

 basswood (Tilia) and geranium {Pelargonium) and examine the 

 parenchyma tissue of pith and cortex under high magni^cation. 

 Note carefully the thin, apparently unstratified "compound mid- 

 dle lamella" of each cell and the prominent intercellular air 

 spa-ces. Tlie very slightly thickened "walls" of these cells appear 

 to represent the original intercellular substance and the two 

 adjacent primary walls of the terminal meristem cells from which 

 they originated. 



Examine under both low and high tnagnification the ex- 

 tremely thick-walled hast fibers of Tibia noting the very thin, 

 continuous compound middle lamella, the thick, obscurely strati- 

 fied secondary wall and the much reduced lumen of each cell. 

 Occasionally, in both transverse as Avell as longisections, very 

 small canal-like pits will be visible in the secondary w-all. 



IV. Pits. — With few exceptions, the secondary wall of plant 

 cells is interrupted by small cavities or recesses which are termed 

 pits. These thin areas in the secondary wall vary widely in size, 

 structure and arrangement and, since they exhibit some con- 

 stancy depending upon the type of cell, they provide significant 

 criteria in comparative studies, especially of xylem cells. Pits 

 typically occur in pairs; i.e., a thin area in the secondary wall 

 of a given cell normally lies opposite a similar recess in the ad- 

 jacent cell. Hence the term "pit-pair" designates the usual 

 condition and is contrasted in meaning with "blind pit" which 

 is a pit "without a complement opposite to an intercellular 

 space" (cf. Glossary of Terms Used in Describing Woods, p. 5). 

 Each member of a simple pit-pair consists of (1) the pit cavity, 

 which is the actual space within the secondary wall, and (2) the 

 2nt aperture or opening into the cavity. The members of a pit- 

 pair are separated from one another by a common piit membrane 

 w^hich represents a discrete portion of the presumably modified 

 intercellular substance and the two primary walls. Compara- 



