68 



THE ANATOMY OF WOODY PLANTS 



merely double contour of the more ancient group, are marked by 

 triple concentric outlines. The outer circle corresponds to the 

 boundary of the pit membrane, while the inner one outlines the 

 mouth or aperture of the pit. The intermediate circular outline 

 delimits the thickened central region of the torus, a structure, 

 which so far as is known, was not present in Paleozoic gymnosperms. 

 Its presence in the conifers is in all probability correlated in some 



A D 



FIG. 49. Radial and tangential sections of the wood of the white pine (Pinus 

 Strobtts). Explanation in the text. 



way with the large size of the bordered pits; it has, in fact, as 

 pointed out in an earlier chapter, been interpreted as a safety 

 device useful in preventing the rupture of the broad pit membranes 

 under extreme pressure. Not only are the pits of large size and 

 more complex organization in the pine than in cordaitean forms, 

 but the walls of the tracheary elements in their vicinity, partic- 

 ularly toward the ends of the tracheids, are distinguished by trans- 

 verse bands of cellulosic or pectocellulosic material, and these are 

 conveniently designated "bars of Sanio." They should not be 

 confused with the trabecttlae, radially directed lignified bars running 

 transversely through the lumina of the cells of many gymnosperms 

 living and extinct. (They are even found in some cases among the 



