6o ANATOMY OF THE GYMNOSPERMS 



last instance, when the active protoplasm is withdrawn and the 

 cell passes into a permanent condition, the membrane disappears, 

 and the pit then forms a lenticular cavity in the line of the cell 

 wall, which opens into adjacent tracheids. This is the appearance 

 presented by all fully developed wood of the Coniferse. The 

 obvious purpose of such pits is to provide channels of communi- 

 cation between adjacent tracheids which would otherwise be com- 

 pletely isolated by the impervious nature of the secondary wall. 

 This fact serves to explain the situation in which such pits occur. 



Radial lualls. The characteristic situation of the bordered 

 pits is on the radial walls, where, as was shown many years since 

 (13, 160), "the pits of contiguous tracheids always correspond to 

 one another in such a way that on each limiting surface all the 

 cavities of the pits of one fit exactly over those of the other. 

 The plano-convex cavities are thus applied to one another in pairs 

 so as to form the lens-shaped pit cavities," as seen in tangential 

 section. But on surfaces abutting on elements of another order, 

 e.g. parenchyma cells, the bordered pits of the tracheids corre- 

 spond to nonbordered pits or else are opposite an unpitted wall. 

 Four typical variations of the bordered pits may be recognized, 

 -(i) the multiseriate, when they are disposed in any number 

 of rows more than two, (2) the two-seriate, (3) the one-seriate 

 with occasional pairs of pits, and (4) the strictly one-seriate. The 

 general sequence thus presented will be found to be in direct 

 accord with the evolution of higher types of structure and 

 organization. 



The most primitive type of Gymnosperm presenting a multi- 

 seriate arrangement is the genus Cordaites. Among eleven 

 species of this genus which have been critically studied within 

 recent years (45) there is a general agreement in the constancy 

 of this character which thereby becomes of generic value. In 

 all the species the pits are disposed in such a compact manner 

 throughout the entire extent of the tracheid as to present a 

 hexagonal outline. In Cordaites acadianum they are two- to five- 

 seriate (plate 7). In other species they vary from two-seriate 

 in C. hamiltonense and C. Newberryi (plate 8) to occasionally 



