PITS 13 



2, Bordered pit-pairs are typical of dead water-conducting 

 cells, notably tracheids and vessel elements. In contrast to the 

 previous type, the cavity of each member of the pit-pair is over- 

 arched by a rim-like development of the secondary wall which is 

 termed the border. As seen in face view, the pit aperture is cir- 

 cular or broadly elliptical. In median section view, the border 

 over-arching' each pit member is apparent and, an additional 

 structural peculiarity is observable, namely the to7-us. The latter 

 is a discoid, central, thicker portion of the pit membrane which 

 is slightly wider than the diameter of the pit aperture. The re- 

 mainder of the membrane is much thinner and sufficiently pliable 

 so that under certain conditions the torus may be pressed against 

 one or the other of the two pit apertures. To understand clearly 

 the structure of a bordered pit-pair, it must be visualized in both 

 face and sectional views. Reference to Eames and MacDaniels 

 (1925, pp. 27-30, Figs. 15, 16, 17 and 21) and to Jeffrey (1917, 

 pp. 5-6, Figs. 4 and 5) will prove helpful. 



Obtain a preparation of macerated xylem of the stem of Pinus 

 and examine it under low magnification. Bordered pit-pairs are 

 large and obvious in the tracheids, which are elongated cells with 

 acute or blunt tips. Select a suitable tracheid and study the ap- 

 pearance of the bordered pits in face view under high magnifica- 

 tion. Observe that each pit appears as three concentric outlines. 

 The outermost circle demarcates the edge of the pit cavity, the 

 intermediate circle represents the edge of the torus and the some- 

 what refractive innermost circle is the pit aperture. In many 

 tracheids, the successive bordered pits are more or less clearly 

 set apart from each other by "eye-brow" or rim-like ridges, 

 termed crassulae. These are interpreted as ** thicker portions of 

 the intercellular layer and primary walls betw^een primary pit 

 fields" and in the past have been designated as "Bars of Sanio" 

 and "Rims of Sanio" (cf. Glossary of Terms Used in Describing 

 Woods, p. 4). The structure of bordered pit-pairs in sectional 

 view can be effectively studied in macerated material if the edge 

 of the pitted walls are turned towards the observer. In order to 

 study critically the pit membrane and the torus, it is neces- 

 sary to examine thin, properly-stained radial sections of pine 

 xvlem. 



